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BRAlr:!'
A MANUAL
OF IHB
ITOMY OF INVERTEBRATED. ANIMALS
•H0MA8 H. HUXLEY, LL.D., F.K.S.
LONBON fc A. CHURCHILL, KEW BUELINGION STREET
IS
"7 ■ i • f -^
PBEFAOK
Thb pTMent Tolnme oa the Aaatoaj of Invertebnted AnimiJii fnlflla an imdertakiiig to prodnM a IvwUm on compantiTe anatoiBf for itndemt^ into whidi I entered tw>-aikd-tw«n^ jean ago. A oonsideraUe inatalment of tlie work, rdating lAoUj to the Jawarfabrafa, ^tpeared in the 'Medical Ttmea and Gaaette ' for the jrean 1866 and 1857, nnder the title of "Lectnrea on General Natural Hiatorj." Bnt a TSrietr of circimuUiioea having con- apired, abont that time, to compel me to direct my attention more particnlarlj to the Fertebrfita, I was led to interrupt the publication of the "Lectures" and to complete the Vertebrate half of the propoaed work first. This appeared in 1871, aa a 'Uanoal of the Anatomy of Tertebratod
A period of incapacity for any aerious toil preveatad me from attempting, b«£)re 1671, to grapple with the immense mass of new and important information respecting the atmcture, and especially the derelopment, of Invertebrated animala, which the activi^ of a boat of inTestigators has accnmolated of late years.
That my pn^neas has been slow will not surprise any one who is acquainted with the growth of the literature of animal morphology, or with the expenditure of time involved in the attempt to veri^ for onesdf eren the cardinal facts of Uwt science ; bnt I ham endeavonred, in ibe Uak ohapter, to supply the moat important recent
IT PREFACE.
additions to our knowledge, respecting tbe gronpe treated of in those wliich have long been printed.
When I commenced this work, it was mj intention t continue the plan adopted in the ' Manaal of the Anatomy of Yertebrated Animals/ of grring a summary account of what appeared to me to be ascertained morphological facts, without referring to my sources of information. I soon found, howeyer, that it would be incouYenient to carry out this scheme consistently ; and some of my pages are, I am afraid, somewhat burdened with notes and references.
I am the more careful to mention this circumstance as, had it been my purpose to give any adequate Bibliography, the conspicuous absence of the titles of many important books and memoirs might appear onaccountable and indeed blameworthy.
My object, in writing the book, has been to make it useful to those who wish to become acquainted with the broad outlines of what is at present known of the morpho- logy of the Inveriehrata ; though I have not avoided the incidental mention of facts connected with their physiology and their distribution. On the other hand, I have abstained from discussing questions of etiology, not because I under- estimate their importance, or am insensible to the interest of the great problem of Erolution; but because, to my mind, the growing tendency to mix up SBtiological specula- tions with morphological generalisations will, if unchecked, throw Biology into confusion.
For the student, that which is essential is a knowledge of the facts of morphology ; and he should recollect that generalisations are empty formulas, unless there is some- thing in his personal experience whicH gives reality and substance io the tesmg of the proporitCons in whioh these generalisations are expressed.
A
CONTENTS.
INTRODUCTION.
PAOS
The General Principles of Biology 1
CHAPTER I. The Distinctive Characters of Animals 42
CHAPTER II. The Protozoa 76
CHAPTER III. The Porifera and the CoBlentcrata 110
CHAPTER IV. The Turbellaria, the Rotifera, the Trcmatoda, and the Cestoidea 176
CHAPTER V. The Uirudinea, the Oligocheta, the Polychccta, the Gcphyrea . 2U
CHAPTER VI. The Arthropoda 200
CHAPTER VII. be air-breathing Arthropoda 371
CHAPTER VHI. The Polysoa, the Brachiopoda, and the Mollusca .432
Vm CONTENTS.
CHAPTER IX.
PAG]
The Echinodermata 54J
CHAPTER X. The Tunicata or Asddioida 59!
CHAPTER XI.
The Peripatidea, the Myzostomata, the Enteropneusta, the Chse- tognatha, the Nematoidea, the Physemaria, the Acantho- cephala and the Dicyemida . . . . . 624
CHAPTER XII. The Taxonomy of Invertebrated Animals 056
Index 689
THE ANATOMY
or
INVEETEBEATED ANIMALS.
INTRODUCTION.
I. — THE OKKSRAL PRINCIPLES OF BIOLOOY.
The Biological sciences are those which deal with the phenomena manifested by living matter; and though it ii customaiy and convenient to group apart such of these phenomena as are termed mental, and such of them as are exhibited bj men in society, under the heads of Psychology and Sociology, yet it must be allowed that no natural boundary separates the subject matter of the latter sciences from that of Biology. Psychology is inseparably linked with Physiology; and the phases of social life exhibited by animals other than man, which sometimes curiously foreshadow humlkn policy, fall strictly within the province of the biologist.
On the other hand, the biological sciences are sharply marked off from the abiological, or those which treat of the phenomena manifested by not-living matter, in so far as the properties of living matter distinguish it absolutely from all other kinds of things, and as the present state of knowledge furnishes us with no link between the living and the not-living. These distinctive properties of living matter are — 1. Its ekemieal eompontion — containing, as it invariably does, one or more forms of a complex comx>ound of carbon, hydrogen, oxygen, and nitrogen, the so-called protein (which lum never yet been obtained except as a product of living
B
2 THE ANATOMY OF IKYEBTEBBATSD ANIMALS.
bodies) united with a large proportion of water, and forming the chief constituent of a substance which, in its primai} unmodified state, is known as protoplasm,
2. Its wniversal dinntegration and toasie hy ooeidaHon, and its eoneomiiant reinieffredion hy the intus-suaception oj new matter.
A process of waste resulting from the decomposition oi the molecules of the protoplasm, in virtue of which thej break up into more highly oxidated products, which cease to form anj part of the living bodj, is a constant con< comitant of life. There is reason to believe that carbonic acid is always one of these waste products, while the othere contain the remainder of the carbon, the nitrogen, the hydrogen, and the other elements which may enter intc the composition of the protoplasm.
The new matter taken in to make good this constant loss is either a ready-formed protoplasmic material, supplied by some other living being, or it consists of the elementa of protoplasm, united together in simpler combinations, which consequently have to be built up into protoplasm by the agency of the living matter itself. In either case, the addition of molecules to those which already existed takes place, not at the surface of the living mass, but by interposition between the existing molecules of the latter. If the processes of disintegration and of reconstruction which characterise life balance one another, the size of the mass of living matter remains stationary, while, if the re- constructive process is the more rapid, the living body growM, But the increase of size which constitutes growth is the result of a process of molecular intus-susception, and there- fore differs altogether from the process of growth by accre- tion, which may be observed in crystals and is effected purely by the external addition of new matter — so that, in the well-known aphorism of LinnsBUs,* the word " grow," as applied to stones, signifies a totally different process from what is called " growth " in plants and animals.
* ** Lapidu erescnnt: vegettiriUa ereseont et vivunt: onimaHa eres- eanti vivoni et Mnttunt."
CHASACTBB8 OV LIVINO MATTER. 3
3. Its tendency to undergo eyeUcal changes.
In the ordinary course of nature, all living matter pro- ceeds from pre-existing living matter, a portion of the latter being detached and acquiring an independent existence. The new form takes on the characters of that from which it arose; exhibits the same power of propagating itself by means of an offshoot ; and, sooner or later, like its pre- decessor, ceases to Uve, and is resolved into more highly oxidated compounds of its elements.
Thus an individual living body is not only constantly changing its substance, but its size and form are under- going continual modifications, the end of which is the death and decay of that individual ; the continuation of the kind being secured by the detachment of portions which tend to run through the same cycle of forms as the parent. No forms of matter which are either not living, or have not been derived from living matter, exhibit these three pro- perties, nor any approach to the remarkable phenomena defined under the second and third heads. But in addi- tion to these distinctive characters, living matter has some other peculiarities, the chief of which are the dependence of all its activities upon moisture and upon heat, within a limited range of temperature, together with the fact that it usually possesses a certain structure, or organization.
Ajb has been said, a large prox>ortion of water enters into the composition of all living matter ; a certain amount of drying arrests vital activity, and the complete abstraction of this water is absolutely incompatible with either actual or potential life. But many of the simpler forms of life may undergo desiccation to such an extent as to arrest their vital manifestations and convert them into the semblance of not-living matter, and yet remain potentially alive ; that is to say, on being duly moistened they return to life again. And this revivification may take place after months, or even years, of arrested life.
The propertieB of liriDg matter are intimately related to tonpeimtiire. Not only does exposure to heat sufi&cient to deeompote proton matter destroy life, by demolishing the
B 2
4 THE AKATOMT OF IITVEBTBBBATBD AKIMAL8.
molecular stmotnre apon which life depends ; bat all vital activitj, all phenomena of nutritive growth, movement, and reproduction are possible only between certain limits of temperature. As the temperature approaches these limits the manifestations of life vanish, though thej maj be teoovered bj return to the normal conditions; but if it pass far beyond these limits, death takes place.
This much is clear ; but it is not easy to say exactly what the limits of temperature are, as they appear to vary in part with the kind of living matter, and in part with the conditions of moisture which obtain along with the temperature. The conditions of life are so complex in the higher organisms, that the experimental investigation of this question can be satisfactorily attempted only in the lowest and simplest forms. It appears that, in the dry state, these are able to bear far greater ex- tremes both of heat and cold than in the moist condition. Thus Pasteur found that the spores of fungi, when dry, could be exposed without destruction to a temperature of 120°-125° 0. (248°-257° Fahr.), whHe the same spores, when moist, were all killed by exposure to 100^ G. (212^ Fahr.) On the other hand, Cagniard de la Tour found that dry yeast might be exposed to the extremely low temperature of solid carbonic acid (—60** C. or —76° Fahr.) without being killed. In the moist state he found that it might be frozen and cooled to — 5° G. (23° Fahr.), but that it was killed by lower temperatures. However, it is very desirable that these experiments should be repeated, for Gohn's careful observations on Bcusteria show that, though they fall into a state of torpidity, and, like yeast, lose all their powers of exciting fermentation at, or near, the freezing-point of water, they are not killed by exposure for five hours to a temperature below —10° G. (14° Fahr.) and, for some time, sinking to —18° 0. (— 0°*4 Fahr.) Speci- mens of Spirillum vohUans, which had been cooled to this extent, began to move about some little time after the ice containing them thawed. But Gohn remarks that EuglefUB, which were frozen along with them, were all killed and
BXSISTAKCX TO HXAT AND COLD. 5
disorgasiBed, and that the same fate had befallen the higher Infiuoria and Botifera, with the exception of some encysted VoHieelloR, in which the rhythmical movements of the contractile vesicle showed that life was preserved.
Thns it would appear that the resistance of living matter to cold depends greatly on the special form of that matter, and that the limit of the Euglena, simple organism as it is, is mnch higher than that of the Bacterium.
Considerations of this kind throw some light upon the apparently anomalous conditions under which many of the lower plants, such as Prataeoccus and the DiatomacecB, and some of the lower animals, such as the Badiolariaf are observed to flourish. Protaeoccus has been found, not only on the snows of great heights in temperate latitudes, but covering extensive areas of ice and snow in the Arctic T^ons, where it must be exposed to extremely low tem- peratures,— in the latter case for many months together; while the Arctic and Antarctic seas swarm with DiatomacecB and Badiolaria. It is on the DiatoTitacecB, as Hooker has well shown, that all surface life in these regpions ultimately depends ; and their enormous multitudes prove that their rate of multiplication is adequate to meet the demands made upon them, and is not seriously impeded by the low temperature of the waters, never much above the freezing- point, in which they habitually live.
The maximum limit of heat which living matter can resist is no less variable than its minimum limit. Kiihne found that marine Amcebm were killed when the tempera- ture reached 35° 0. (95° Fahr.), while this was not the case with fresh-water Amc^m, which survived a heat of 5°, or evea 10°, C. higher. Aetinophrya EichJiomii was not killed until the temperature rose to 44° or 45° G. Didymium aerjnda is killed at 35° 0.; while another Myxom/ycetef JEthO' Hum sepUcwn, succumbs only at 40° G.
Cohn (" XJntersuchungen iiber Bacterien," Beiirdge «ir Biologie der Pflanaen, Heft 2, 1872) has given the results of a series of experiments conducted with the view of Ascer- taining the temperature at which Bacteria are destroyed
6 TBI AKATOKT OV IimBTZBBA.TXD A1TIMAI.S.
Then liring m a fluid of definite chemical compocition, Etnd free from all snoh compUcationa as most arise from the inequslitieB of phjaical condition when solid particles other than the Baeteria co-exiet with them. The fiuid employed oontAined 0*1 gramme potasBiom phosphate, O'l gr. 0170- tallised magneainm Balpfaat«, O'l gr. tribasic calcium phosphate, and 0'2 gr. ammoniom torttate, dissolved in 20 cnbic centimeteee of distilled water. If to a certain qoantitj of this "normal fluid" a email proportion of water containing Baeteria was added, the moltiplication of the Baetwia went on with rapidity, whether the mouth of the containing flask was open or hermeticallj closed. Hermetically-sealed flasks, containing portions tA the normal fluid infected with Baeieria, were submerged in water heated to various temperatures, the flask being care- fully shaken, without being raised out of the water, during its submergence.
The result was, that in those flasks which were thus aubjected. for an hour, to a. heat of 60°-62° 0. (140°-143= Fahr.), the Baeteria noderwent no development, and the fluid remained perfectly clear. On the other band, in r eiperimenta in which the flasks were heated only
BB8IBTAV0S TO HBAT AND GOLD. 7
they haye been heated aboTe the boiling-point, require renewed inTeetigation.
Both in Kiihne's and in Cohn's experiments, which last have lately been confirmed and extended by Dr. Boberts of Manchester, it was noted that long exposure to a lower temperature than that which brings about immediate destruction of life, produces the same effect as short ex- posure to the latter temperature. Thus, though all the Baderia were killed, with certainty, in the normal fluid, by short exposure to temperatures at or above 60° 0. (140° Efthr.), Cohn observed that, when a flask containing in- fected normal fluid was heated to 50°-52° G. (122°-125° Fahr.) for only an hour, the consequent multiplication of the Baderia was manifested much earlier, than in one which had been exposed for two hours to the same tempe- rature.
It appears to be very generally h^d that the simpler vegetable organisms are deprived of life at temperatures as high as 60° 0. (140° Fahr,) ; but, it ia affirmed by competent observers, that AlgcR have been found living in hot springs at much higher temperatures, namely, from 168° to 208° Fahr., for which latter surprising fact we have the high authority of Descloiseaux. It is no explanation of these phenomena, but only another mode of stating them, to say that these organisms have become " accustomed " to such temperatures. If this degree of heat were absolutely incompatible with the activity of living matter, the plants could no more resist it than they could become "accus- tomed *' to being made red hot. Habit may modify sub- sidiary, but cannot affect fundamental, conditions.
Recent investigations point to the conclusion that the immediate cause of the arrest of vitality, in the first place, and of its destruction, in the second, is the coagulation of certain substances in the protoplasm, and that the latter €emkKBM various ooagukble matters, which solidify at dif - ferent temperatures. And it remains to be seen, how far the death of any form of living matter, at a given temperature, depends on the destaruction of its fundamental substance
8
THB AVATOMT OP nrVX&TBBBATSD AVIMALS.
at that heat, and how far death is hronght about by the coagalation of merely accessory compounds.
It may be safely said of all those living things which are large enough to enable us to trust the eyidenoe of microscopes,* that they are heterogeneous optically, and that their different parts, and especially the surface layer, as contrasted with the interior, differ physically and chemi- cally ; while, in most living things, mere heterogeneity is exchanged for a definite structure, whereby the body is distinguished into visibly diverse parts, which possess dif- ferent powers or functions. Living things which present this visible structure are said to be organiaed; and so widely does organisation obtain among living beings, that organised and living are not unfrequently used as if they were terms of co-extensive applicability. This, however, is not exactly accurate, if it be thereby implied that all living things have a visible organisation, as Uiere are numerous forms of living matter of which it cannot properly be said that they possess either a definite visible structure or per- manently specialised organs : though doubtless, the simplest particle of living matter must possess a highly complex molecular structure, which is far beyond the reach of vision.
The broad distinctions which, as a matter of fact, exist between every known form of living substance and every other component of the material world, justify the sepa- ration of the biological sciences from all others. But it must not be supposed that the differences between living and not-living matter are such as to bear out the assump- tion that the forces at work in the one are different from
* In considering the question of the complication of molecalar Btructore wliich even the smallest and simplest of living beings may possess, it is well to recollect that an organic particle rgjon of an inch in diameter, in which our best microscopes may be incom- petent to reveal the slightest
differentiation of parts, may be made up of 1,000,000 particles Tgoiiwo of an inch in diameter, wnile the molecules of matter are probably much less than |(jooo<kj of an inch in diameter. Hence in such a body there is ample scope for any amount of complexity of molecular structure.
MOBPHOLOGT. 9
those which are to be met with in the other. Considered apart from the phenomena of conscioasnees, the phenomena of life are all dependent upon the working of the same physical and chemical forces as those which are active in the rest of the world. It maj be convenient to use the terms " vitalitj " and " vital force " to denote the causes of certain great groups of natural operations, as we employ the names of '* electricity " and " electrical force " to denote others ; but it ceases to be proper to do so, if such a name implies the absurd assumption that either " electricity " or ''vitality" are entities playing the part of efficient causes of electrical or vital phenomena. A mass of living proto- plasm is simply a molecular machine of great complexity, the total results of the working of which, or its vital pheno- mena, depend, on the one hand, upon its construction, and, on the other, upon the energy supplied to it ; and to speak of " vitality " as anything but the name of a series of opera- tions is as if one should talk of the " horologity " of a clock.
Living matter, or protoplasm and the products of its metamorphosis, may be regarded under four aspects : —
(1.) It has a certain external and internal form, the latter being more usually called structure ;
(2.) It occupies a certain position in space and in time ;
(3.) It is the subject of the operation of certain forces, in virtue of which it undergoes internal changes, modifies external objects, and is modified by them ; and
(4.) Its form, place, and powers are the effects of certain causes.
In correspondence with these four aspects of its subject. Biology is divisible into four chief subdivisions — I. Mor- phology; n. DiSTsiBTjnoN ; III. Physiolooy; rv.
JSjT10JA>QY.
I. MOKPHOLOOY.
So far as living beings have a form and structure, they faJH within the province of AncUimiy and Histology, the latter being merely a name for that ultimate optical analysis
10 THE ANATOMY OF IKYBBTBBBATBD AKIMAL8.
of living Btmctnre which can be carried out only by the aid of the microscope.
And, in so far as the form and stractnre of anj living being are not constant during the whole of its existence, but undergo a series of changes from the commencement of that existence to its end, living beings have a Develop' ment. The history of development is an account of the anatomy of a living being at the successive periods of its existence, and of the maimer in which one anatomical stage passes into the next.
FmaUy. the syBtematic statement and generalisation of the facts of Morphology, in such a maimer as to arrange living beings in groups, according to their degrees of like* ness, is Taaonomy,
The study of Anatomy and Development has brought to light certain generalisations of wide applicability and g^reat importance.
1. It has been said that the great majority of living beings present a very definite structure. Unassisted vision and ordinary dissection suffice to separate the body of any of the higher animals, or plants, into fabrics of different sorts, which always present the same general arrangement in the same organism, but are combined in different ways in different organisms. The discnmination of these com- paratively few fabrics, or tissues, of which organisms are composed, was the first step towards that ultimate analysis of visible structure which has become possible only by the recent perfection of microscopes and of methods of preparation.
Histology, which embodies the results of this analysis, shows that every tissue of a plant is composed of more or less modified structural elements, each of which is termed a ceU; which cell, in its simplest condition, is merely a spheroidal mass of protoplasm, surrounded by a coat or sac — ^the ceU-waU — ^which contains cellulose. In the various tissues, these cells may undergo innumerable modificationa of form — the protoplasm may become differentiated into a nucleus with its nucleolus, a primordial utricle, and a
HI8TOLOOT. 11
cafitj filled witli a waterj fluid, and the cell- wall may be Tarionaly altered in composition or in stmoture, or may coaleace with others. But, howerer extensive these change may be, the fact that the tissaes are made np of morpholo- gically distinct nnits — the ceUs — remains patent. And, if any dbonbt could exist on the subject, it would be removed by the stady of development, which proves that every plant commences its existence as a simple cell, identical in its fondaznental characters with the less modified of those odls of whidi the whole body is composed.
Bui it is not necessary to the morphological unit of the plant that it should be always provided with a cell-walL Certain plants, sueh as Frotococcue, spend longer or shorter periods of their existence in the condition of a mere spheroid of protoplasm, devoid of any cellulose wall, while, at other times, the protoplasmic body becomes enclosed within a cell-wall, fabricated by its superficial layer.
Therefore, just as the nucleus, the primordial utricle, and the central fluid are no essential constituents of the morphological unit of the plant, but represent results of its metamorphosis, so the cell-wall is equally unessential ; and either the term " cell " must acquire a merely technical significance as the equivalent of morphological unit, or some new term must be invented to describe the latter. On the whole, it is probably least inconveuient to modify the sense of the word " cell."
The histological analysis of animal tissues has led to snnilar results and to difficulties of terminology of precisely the same character. In the higher animals, however, the modifications which the cells undergo are so extensive, thai the fact that the tissues are, as in plants, resolvable into an aggregation of morphological units, could never hafie been established without the aid of the study of de- velopment, which proves that the animal, no less than the phuit, commences its existence as a simple cell, fundament- ally identical with the less modified cells which are found m tbe tissaes of the adult.
Thofogh the nnoleas is very constant among animal cells,
12 THE ANATOMY OF IKYBBTEBBATED AKIMALS.
it is not uniyersall J present ; and among the lowest forms of animal life, tlie protoplasmic mass which represents the morphological imit maj be, as in the lowest plants, devoid of a nucleus. In the animal, the cell-wall never has the chai*acter of a shut sac containing cellulose ; and it is not a little difficult, in manj cases, to saj how much of the so-called "cell-wall'* of the animal cell answers to the "primordial utricle "and how much to the proper " cellulose cell-wall " of the vegetable cell. But it is certain that in the animal, as in the plant, neither ceU-wall nor nucleus are essential constituents of the cell, inasmuch as bodies which are unquestionablj the equivalents of cells — true morphological units — ^maj be mere masses of protoplasm, devoid alike of cell- wall and nucleus.
For the whole living world, then, it results: — ^that the morphological unit — the primary and fundamental form of life — is merely an individual mass of protoplasm, in which no fui*ther structure is discernible ; that independent living forms may present but little advance on this structure; and that slII the higher forms of life are aggp*egates of such morphologpLcal units or cells, variously modified.
Moreover, all that is at present known tends to the con- clusion, that, in the complex aggregates of such imits of which all the higher animals and plants consist, no cell has arisen otherwise than by becoming separated from the pro- toplasm of a pre-existing cell ; whence the aphorism " Omnit eellula e cellvld."
It may further be added, as a general truth applicable to nucleated cells, that the nucleus rarely undergoes any considerable modification, the structures characteristic of the tissues being formed at the expense of the more super- ficial protoplasm of the cells; and that, when nucleated cells divide, the division of the nucleus, as a rule, precedes that of the whole celL
2. In the course of its development every cell proceeds from a condition in which it closely resembles every other cell, through a series of stages of gradually increasing divergence, until it reaches that condition in which it
DBTSLOPMBITT. 13
presents the characteristic features of the elements of a special tissae. The development of the cell is therefore a gradual progress from the general to the special state.
The like holds good of the development of the body as a whole. However complicated one of the higher animals or plants may be, it begins its separate existence under the form, of a nucleated cell. This, by division, becomes con- verted into an aggregate of nucleated cells: the parts of tills aggregate, following different laws of growth and multiplication, give rise to the rudiments of the organs ; and the parts of these rudiments again take on those modes of growth multiplication and metamorphosis which are needful to convert the rudiment into the perfect structure.
The development of the organism as a whole, therefore, repeats in principle the development of the celL It is a progress from a general to a special form, resulting from the gradual differentiation of the primitively similar mor- phological units of which the body is composed.
Moreover, when the stages of development of two Mtiimala are compared, the number of these stages which are similar to one another is, as a general rule, proportional to the doseness of the resemblance of the adult forms; whence it f oUows that the more closely any two animals are allied in adult structure, the later are their embryonic conditions distinguishable. And this general rule holds for plants no less than for animals.
The broad principle, that the form in which the more complex living things commence their development is always the same, was first expressed by Harvey in his famous aphorism " Onvne vivwm ex ovo" which was intended simply as a morphological generalisation, and in no wise implied the rejection of spontaneous generation, as it is commonly supposed to do. Moreover, Harve/s study of the development of the chick led him to promulgate that theory of " epigenesis,'* in which the doctrine that develop- ment 18 a progress from the general to the special is inq^ieitly contained.
Caspar F. Wolff furnished further, and indeed condu-
14 THE ANATOMY OF nTYBBTSBBATED ANIMALS.
give, proof of the troth of the theory of epigeneais : but, unfortunatelj, the anthoritj of Haller and the speculatioiiB of Bonnet led science astray, and it was reserved for Yon Baer to put the nature of the process of deyelopment in its true light, and to formulate it in his famous law.
8. Deyelopment, then, is a process of differentiation by which the primitiyely similar parts of the living body become more and more unlike one another.
This process of differentiation may be effected in seyeral ways.
(1.) The protoplasm of the germ may not undergo divi- sion and conversion into a cell aggregate ; but various parts of its outer and inner substance may be metamorphosed directly into those physically and chemically different materials which constitute the body of the adult. This occurs in such animals as the Infusoria, and in such plants as the unicellular Algm and Ftrngi.
(2.) The germ may undergo division, and be converted into an aggregate of dtvmon masses, or hlastomeres, which become cells and give rise to the tissues by undergoing a metamorphosis of the same kind as that to which the whole body is subjected in the preceding case.
The body, formed in either of these ways, may, as a whole, undergo metamorphosis by differentiation of its parts ; and this differentiation may take place without reference to any axis of symmetry, or it may have reference to such an axis. In the latter case, the parts of the body which become dis- tinguishable may correspond on the two sides of the axis (bilateral symmetry), or may correspond along several lines parallel with the axis (radial symmetry).
The bilateral or radial symmetry of the body may be further complicated by its segmentation, or separation by divisions transverse to the axis, into parts, each of which corresponds with its predecessor or successor in the series.
In the segmented body, the segments may or may not give rise to symmetrically or asymmetrically disposed pro- cesses, which are appendages, using that word in its most general sense.
DirFBSMHTIATZON OF BTBUOTVBS. 15
And the biggest degree of complication of stractnre, in both aTiiTnalH and plants, it attained by the body when it becomes divided into segments proyided with appendages ; when the segments not only become veiy different from one another, bnt some coalesce and lose their primitiTe distinct- ness; and when the appendages and the segments into which thej are sabdivided similarly become differentiated and coalesce.
It is in Tirtne of snch processes that the flowers of plants, and the heads and limbs of the Atihropoda and of the VerUibraia, among animals, attain their extraordinary diyerBity and complication of stracture. A flower-bad is a segmented body or axis, with a certain number of whorls of appendages ; and the perfect flower is the result of the gradnal differentiation and confluence of these primitiTcly similar segments and their appendages. The head of an insect or of a crustacean is, in like manner, composed of a namber of segments, each with its pair of appendages, which by differentiation and confluence are converted into the feelers and variously modified oral appendages of the adult.
In some complex organisms, the process of differentiation, by which they pass from the condition of aggregated embryo cells to the adult, can be traced back to the laws of growth of the two or more cells into which the embryo cell is divided, each of these cells giving rise to a particular portion of the adult organism. Thus the fertilised embryo cell in the arcbegonium of a fern divides into four cells, one of which gives rise to the rhizome of the young fern, another to its first 'rootlet, while the other two are converted into a placenta-like mass which remains imbedded in the pro- thallus.
The stracture of the stem of Chora depends upon the different properties of the cells, which are successively derived by transverse division from the apical cell. An hUer-nodal cell, which elongates greatly, and does not divide, is succe^ed by a nodal cell, which elongates but Htde, and becomes greatly subdivided; this by another
16 THB ANATOMY OF INYEBTBBBATBD ANIMALS.
inter-nodal cell, and bo on in r^pilar alternation. In the same way the stracture of the stem, in all the higher plants, depends upon the laws which govern the manner of diyiflioa and of metamorphosis of the apical cells, and of their oon- tinnation in the camMwn layer.
In all animals which consist of cell-aggregatee, the cells of which the embryo is at first composed arrange them- selves by the splitting, or by a process of invagination* ol the blastoderm into two layers, the epibloui and the hffp(h hlastf between which a third intermediate layer, the meto* blast, appears ; and each layer gives rise to a definite group of organs in the adult. Thus, in the Veriebraia, the epiblast gives rise to the cerebro-spinal axis, and to the epidennk and its derivatives ; the hypoblast, to the epithelium of the alimentary canal and its derivatives; and the mesobUsi^ to intermediate structures. The tendency of recent in- quiry is to prove that the several layers of the germ evolve analogous organs in invertebrate animals, and to indicate the possibility of tracing the several germ layers back to the blastomeres of the yelk, from the subdivision of which they proceed.
It is conceivable that all the forms of life should have presented about the same differentiation of structure, and should have differed from one another by superficial charac- ters, each form passing by insensible gradations into those most like it. In this case Taxonomy, or the classification of morphological facts, would have had to confine itaelf to the formation of a serial arrangement, representing the serial gradation of these forms in nature.
It is conceivable, again, that living beings should have differed as widely in structure as they actually do, but that the interval between any two extreme forms should have been filled up by an unbroken series of gradations; in which case, again, classification could only effect the forma- tion of series — the strict definition of groups would be as impossible as in the former case.
As a matter of fact» living beings differ enormously, not
\.
MOBPHOLOOICAL aBOUPS. 17
only in differentiation of stmctnre, bat in the modes in
; wbicli that differentiation is brought abont; and the in- tervals between extreme forms are not filled up, in the
I existing world, by complete series of gradations. Hence it arises that living beings are, to a great extent, sosceptible of classification into groups, the members of each group resembling one another, and differing from all the rest, by
. certain definite peculiarities.
' U No two liring beings are exactly alike, but it is a matter of observation that, among the endless diyei*sitie8 of living things, some constantly resemble one another so closely that it is impossible to draw any line of demarcation between them, while they differ only in such characters as are associated with sex. Such as thus <4oeely resemble one another constitute a morphological species; while different morphological species are defined by constant characters which are not merely sexuaL
The comparison of these lowest groups, or morphological species, with one another, shows that more or fewer of them possess some character or characters in common — some feature in which they resemble one another and differ from all other species — and the group or higher order thus formed is a genus. The generic groups thus constituted are susceptible of being arranged in a similar manner into groups of successively higher order, which are known as families, orders, classes, and the like.
The method pursued in the classification of living forms is. in fact, exactly the same as that followed by the maker of an index in working out the heads indexed. In an alphabetical arrangement, the classification may be truly termed a morphological one, the object being to put into close relation all those leading words which resemble one another in the arrangement of theii* letters, that is, in their form, and to keep apart those which differ in structure. Headings which begin with the same word, but differ other- wise, might be compared to genera with their species ; the groups of words with the same first two syllables to families; those with identical first syllables to orders ; and
18 THE ANATOMY OF IlfYEBTBBSATED ANIMALS.
those with the same initial letter to classes. But there is this difference between the index and the Taxonomic arrangement of living forms, that in the former there is nothing but an arbitrary relation between the various classes, while, in the latter, the classes are similarly capable of co-ordination into larger and larger groups, until all are comprehended under the common definition of living beings.
The differences between " artificial " and " natural " clas- sifications are differences in degree, and not in kind. In each case the classification depends upon likeness ; but in an artificial classification some prominent and easily observed feature is taken as the mark of resemblance or dissemblance ; while, in a natural classification, the things classified are arranged according to the totality of their morphological resemblances, and the features which are taken as the marks of groups are those which have been ascertained by ob- servation to be the indications of many likenesses or un- likenesses. And thus a natural classification is a great deal more than a mere index. It is a statement of the marks of similarity of organisation ; of the kinds of struc- ture which, as a matter of experience, are found universally associated together; and, as such, it furnishes the whole foundation for those indications by which conclusions as to the nature of the whole of an animal are drawn from a knowledge of some part of it.
When a palaeontologist argues from the characters of a bone or of a shell to the nature of the animal to which that bone or shell belonged, he is guided by the empirical morphological laws established by wide observation, that such a kind of bone or shell is associated with such and such structural features in the rest of the body, and no others. And it is these empirical laws which are embodied and expressed in a natural classification.
V
THX PHENOMSNA OF DISTBTBTTTION. 19
n. DiSTBIBUTION.
Liying bemgs occnpj certain portions of the surface of the earth, inhabiting either the dry land, or the fresh or salt waters ; or being competent to maintain their existence in either. In anj given locality, it is found that these different media are inhabited bj different kinds of living beings; and that the same medium, at different heights in the air and at different depths in the water, has dif- ferent living inhabitants.
Moreover, the living populations of localities which differ considerablj in latitude, and hence in climate, alwajs pre- sent considerable differences. But the converse proposition is not true; that is to saj, localities which differ in longi- tude, even if they resemble one another in climate, often have veiy dissimilar Faurue and Florce,
It has been discovered bj careful comparison of local Faunae and FlorsD, that certain areas of the earth's surface are inhabited bj groups of animals and plants which are not found elsewhere, and which thus characterize each of these areas. Such areas are termed Provineea of Distrtbu- Hon, There is no parity between these provinces in extent, nor in the physical configuration of their boundaries ; and, in reference to existing conditions, nothing can appear to be more arbitrary and capricious than the distribution of living beings.
The study of distribution is not confined to the present order of nature; but, by the help of geology, the naturalist is enabled to obtain clear, though too fragmentary, evidence of the characters of the faun® and florae of antecedent epochs. The remains of organisms which are contained in the stratified rocks prove that, in any given part of the earth's surface, the living popidation of earlier epochs was different from that which now exists in the locality ; and that, on the whole, the difference becomes greater tha farther we go back in time. The organic remains which are found in the later Oainozoic deposits of any district
c 2
20 THB ANATOMY OF INYEBTBBBATED ANIMALS.
are alwajs closely allied to those now found in the province of distribution in which that locality is included ; while in the older Cainozoic the resemblance is less; and in the Mesozoic, and the PalsBozoic strata, the fossils maj be simi- lar to creatures at present liying in some other province, or may be altogether unlike any which now exist.
In any given locality, the succession of living forms may appear to be interrupted by numerous breaks — ^the associated species in each fossiliferous bed being quite distinct from those above and those below them. But the tendency of all palseontological investigation is to show that these breaks are only apparent, and arise from the incompleteness of the series of remains which happens to have been preserved in any given locality. As the area over which accurate geological investigations have been carried on extends, and as the fossiliferous rocks found in one locality fill up the gaps left in another, so do the abrupt demarcations between the faunsB and florsB of successive epochs disappear — a certain proportion of the genera and even of the species of every period, great or small, being found to be continued for a longer or shorter time into the next succeeding period. It is evident, in fact, that the changes in the living population of the globe which have taken place during its history, have been effected, not by the sudden replacement of one set of living beings by another, but by a process of slow and gradual introduc- tion of new species, accompanied by the extinction of the older forms.
It is a remarkable circumstance, that, in all parts of the globe in which fossiliferous rocks have yet been examined, the successive terms of the series of living forms which have thus succeeded one another are analogous. The life of the Mesozoic epoch is everywhere characterised by the abundance of some groups of species of which no trace is to be found in either earlier or later formations ; and the like is true of the Palsoozoic epoch. Hence it follows, not only that there has been a succession of species, but that the general nature of thai succession has been the same all
DI8TBIBUTION IN TIME. 21
oyer the globe; and it is on this ground that fossils are 80 important to the geologist as marks of the relative age of rocks.
The determination of the morphological relations of the species which have thus succeeded one another is a problem of profound importance and difficulty, the solution of which, howerer, is already dearly indicated. For, in several cases, it is possible to show that, in the same geographical area, a form A, which existed during a certain geological ex)och, has been replaced by another form B, at a later period ; and that this form B has been replaced, stiU later, by a third form C. When these forms, A, B, and C, are com- pared together they are found to be organized upon the same plan, and to be very similar even in most of the de- tails of their structure ; but B differs from A by a slight modification of some of its parts, which modification is carried to a still greater extent in C.
In other words, A, B, and 0 differ from one another in the same fashion as the earlier and later stages of the embryo of the same animals differ ; and, in successive epochs, we have the group presenting that progressive specialisa- tion which characterises the development of the individual. Clear evidence that this progressive specialisation of structure has actually occurred has as yet been obtained in only a few cases {e.g., EquidcR, Crocodilia), and these are confined to the highest and most complicated forms of life; while it is demonstrable that, even as reckoned by geological time, the process must have been exceedingly slow.
Among the lower and less complicated forms, the evi- dence of progressive modification, furnished by compari- son of the oldest with the latest forms, is slight, or absent ; and some of these have certainly persisted, with very little change, from extremely ancient times to the present day. It is as important to recognise the fact that certain forms of life have thus i>ersisted, as it is to admit that others have undergone progressive modification.
It luiB been said that the successive terms in the series
22 THE ANATOm OF INYEBTEBRATED ANOCALS.
of living forms are analogous in all parts of the globe. But the species which constitute the corresponding or hcmotcuBie terms in the series, in different localities, are not identical. And, though the imperfection of our knowledge at present precludes positive assertion, there is every reason to believe that geographical provinces have existed through- out the period during which organic remains furnish us with evidence of the existence of life. The wide distribu- tion of certain Palaeozoic forms does not militate against this view ; for the recent investigations into the nature of the deep-sea fauna have shown that numerous Orudacea, Echinodermata, and other invertebrate animals, have as wide a distribution now as their analogues possessed in the Silurian epoch.
m. Physiolooy.
Thus far, living beings have been regarded merely as definite forms of matter, and Biology has presented no considerations of a different order from those which meet the student of Mineralogy. But living things are not only natural bodies, having a definite form and mode of struc- ture, growth, and development. They are machines in action ; and, under this aspect, the phenomena which they present have no parallel in the mineral world.
The actions of living matter are termed its funeUons ; and these functions, varied as they are, may be reduced to three categories. They are either — (1), functions which affect the material composition of the body, and determine its mass, which is the balance of the processes of waste on the one hand and those of assimilation on the other. Or (2), they are functions which subserve the process of re- production, which is essentially the detachment of a part endowed with the power of developing into an independent whole. Or (3), they are functions in virtue of which one part of the body is able to exert a direct influence on another, and the body, by its parts or as a whole, ^becomes A ionrce of molar motion. The first may be termed austen'
riTNCTIONS AND OBOAITS. 23
ioHve, the second generative, and the third eorreloHve functionB.
Of these three dasses of functions the first two onlj can be said to be inyariably present in living beings, all of which are nonrish'ed, grow, and ninltiplj. But there are some forms of life, snch as manj Fwngi, which are not known to possess any powers of changing their form ; in which the protoplasm exhibits no moyements, and reacts npon no stimnlos ; and in which anj influence which the different parts of the body exert upon one^ another must be transmitted indirectly from molecule to molecule of the common mass. In most of the lowest plants, however, and in all animals yet known, the body either constantly or temporarily changes its form, either with or without the application of a special stimulus, and thereby modifies the relations of its parts to one another, and of the whole to surrounding bodies ; while, in aU the higher animals, the different parts of the body are able to affect, and be affected by, one another, by means of a special.tissue, termed nerve. Molar motion is effected on a large scale by means of another special tissue, muscle; and the organism is brought into relation with surrounding bodies by means of a third kind of special tissue — that of the sensory organs — by means of which the forces exerted by surrounding bodies are transmuted into affections of nerve.
In the lowest forms of life, the functions which have been enumerated are seen in their simplest forms, and they are exerted indifferently, or nearly so, by all parts of the protoplasmic body ; and the like is true of the functions of the body of even the highest organisms, so long as they are in the condition of the nucleated ceU, which constitutes the starting-point of their development. But the first pro- cess in that development is the division of the germ into a number of morphological units or blastomeres, which, eventually, give rise to cells; and as each of these pos- sesses the same physiological functions as the germ itself, it follows that each morphological iinit is also a physio- logical nnit, and the multicellular mass is strictly a com-
24 THE ANATOMY OW INYEBTEBBATED ANIMALS.
pound organiBm, made np of a multitude of ph jsiologicallj independent cells. The physiological actiyities manifested bj the complex whole represent the sum, or rather the resultant, of the separate and independent physiological actiyities resident in e&ch of the simpler constituents of that whole.
The morphological changes which the cells undergo in the course of the further development of the organism, do not affect their individuality; and, notwithstanding the modification and confluence of its constituent cells, the
«
adult organism, however complex, is still an aggregate of morphological units. Nor is it less an aggregate of physio- logical units, each of which retains its fundamental inde- pendence, though that independence becomes restricted in various ways.
Each cell, or that element of a tissue which proceeds from the modification of a cell, must needs retain its sustentative functions so long as it grows or niaint4unH a condition of equilibrium ; but the most completely meta- morphosed cells show no trace of the generative function, and many exhibit no correlative functions. Contrariwise those cells of the adult organism which are the unmeta- morphosed derivatives of the germ, exhibit aU the primary functions, not only nourishing themselves and growing, but multiplying, and frequently showing more or less ' marked movements.
Organs are parts of the body which perform particular functions. In strictness, perhaps, it is not quite right to speak of organs of sustentation or generation, each of these functions being necessarily performed by the morphological unit which is nourished or reproduced. What are called the organs of these functions are the apparatuses by which certain operations, subsidiary to sustentation and genera- tion, are carried on.
Thus, in the case of the sustentative functions, all those organs may be said to contribute to these functions which are concerned in bringing nutriment within the reach of the ultimate ceUs, or in removing waste matter from them ;
MTTBCIil AVD NBBTI. 25
while in the case of the generative function, all those organs contribute to the function which produce the cells froxD. which germs are giren off; or help in the eracuation, or fertilisation, or deyelopment of these germs.
On the other hand, the correlative functions, so long as thej are exerted bj a simple undifferentiated morphological unit or cell, are of the simplest character, consisting of those modifications of position which can be effected bj mere changes in the form or arrangement of the parts of the protoplasm, or of those prolongations of the proto- plasm which are called pseudopodia or cilia. But, in the higher animals and plants, the movements of the organism and of its parts are brought about by the change of the form of certain tissues, the property of which is to shorten in one direction when exposed to certain stimuli. Such tissues are termed eovUractile; and, in their most fully developed condition, muscular. The stimulus by which this contraction is naturally brought about is a molecular change, either in the substance of the contractile tissue itself, or in some other part of the body ; in which latter case, the motion which is set up in that part of the body .uust be propagated to the contractile tissue through the ii termediate substance of the body. In plants, there seems to be no question that parts which retain a hardly modified ceL ^ar structure may serve as channels for the transmis- sion of this molecular motion ; whether the same is true of animals is not certain. But, in all the more complex animals, a peculiar fibrous tissue — nerve — serves as the agent by which contractile tissue is affected by changes occurring elsewhere, and by which contractions thus ini- tiated are co-ordinated and brought into harmonious com- bination. While the sustentative functions in the higher forms of life are still, as in the lower, fundamentally de- pendent upon the powers inherent in all the physiological units which make up the body, the correlative functions are, in the former, deputed to two sets of specially modified units, which constitute the muscular and the nervous tissues.
When the different forms of life are compared together
26 THB ANATOMY OF IKYXBTEBRATED ANXMALS.
as physiological machines, thej are found to differ as machines of human constmction do. In the lower forms, the mechanism, though perfectlj well adapted to do the work for which it is required, is rough, simple, and weak ; while, in the higher, it is finished, complicated, and powerfuL Considered as machines, there is the same sort of difference between a polype and a horse as there is between a distaff and a spinning-jenny. In the progress from the lower to the higher organism, there is a gradual differentiation of organs and of functions. Each function is separated into many parts, which are severally entrusted to distinct organs. To use the striking phrase of Milne-Edwards, in passing from low to high organisms, there is a division of physio- logical labour. And exactly the same process is observable in the development of any of the higher organisms; so that, physiologically as well as morphologicaUy, develop- ment is a progress from the general to the special
Thus far, the physiological activities of living matter have been considered in themselves, and without reference to anything that may affect them in the world outside the living body. But living matter acts on, and is powerfully affected by, the bodies which surround it ; and the study of the influence of the ** conditions of existence " thus deter- mined constitutes a most important part of Physiology.
The sustentative functions, for example, can only be exerted under certain conditions of temperature, pressure, and light, in certain media, and with supplies of particular kinds of nutritive matter; the sufficiency of which supplies, again, is greatly influenced by the competition of other organisms, which, striving to satisfy the same needs, give rise to the passive " struggle for existence." The exercise of the correlative functions is influenced by similar condi- tions, and by the direct conflict with other organisms, which constitutes the active struggle for existence. And, finally, the generative functions are subject to extensive modifi- cations, dependent partly upon what are commonly called external conditions, and partly upon wholly unknown agencies.
AOAMOOSirBSIS. 27
In the lowest forms of life, the only mode of generation at present known is the division of the body into two or more parts, each of which then grows to the size and assumes the form of its parent, and repeats the process of mnltiplication. This method of mnltiplioation bj fission is properly called generation, becanse the parts which are separated are seyerally competent to give rise to individoal oi^anisms of the same nature as that from which they
In many of the lowest organisms the process is modified ■0 far that, instead of the parent dividing into two eqiud parts, only a small portion of its substance is detached, as a bod, which developes into the likeness of its parent. This is generation by gemmation. Generation by fission and by gemmation are not confined to the simplest forms of life, however. On the contrary, both modes of multipli- cation are common not only among plants, but among animals of considerable complexity.
The multiplication of flowering plants by bulbs, that of annelids by fission, and that of polypes by budding, are weU-known examples of these modes of reproduction. In all these cases, the bud or the segment consists of a multi- tude of more less metamorphosed cells. But, in other instances, a single cell detached from a mass of such un- differentiated cells contained in the parental organism is the foundation of the new organism, and it is hard to say whether such a detached cell may be more fitly called a bud or a segment — whether the process is more akin to fission or to gemmation.
In all these cases the development of the new being from the detached germ takes place without the influence of other living mattsr» Common as the process is in plants and in the lower animals, it becomes rare among the higher animals. In these, the reproduction of the whole organism from a part, in the way indicated above, ceases. At most we find that the cells at the end of an amputated portion of the organism are capable of reproducing the lost part; in the very highest animals, even this power
28 THE AKATOMY OF INYBBTEBBATED ANIMALS.
vanishes in the adult; and, in most parts of the body, thongh the undifferentiated cells are capable of mnltipli* cation, their progeny grow, not into whole organisms like that of which they form a part, but into elements of the tissues.
Throughout almost the whole series of living beings, however, we find concurrently with the process of agixmiih genesis, or asexual generation, another method of genera- tion, in which the development of the germ into an organism resembling the parent depends on an influence exerted by living matter different from the germ. This is gamogenetit or sexual generation. Looking at the facts broadly, and without reference to many exceptions in detail, it may be said that there is an inverse relation between agamogenedc and gamogenetic reproduction. In the lowest organisms gamogenesis has not yet been observed, while in the highest agamogenesis is absent. In many of the lower forms of life agamogenesis is the common and predominant mode of reproduction, while gamogenesis is exceptional ; on the contrary, in many of the higher, while gamogenesis is the rule, agamogenesis takes place exceptionally.
In its simplest condition, which is termed *' conjngaiwn,^ sexual generation consists in the coalescence of two wTniliiy masses of protoplasmic matter, derived from different parts of the same organism, or from two organisms of the same species, and the single mass which results from the fusion develops into a new organism.
In the majority of cases, however, there is a marked morphological, difference between the two factors in the process, and then one is called the nude, and the other the female element. The female element is relatively large, and undergoes but little change of form. In all the higher plants and animals it is a nucleated cell, to which a gpreatei or less amount of nutritive material, constituting a food- yelk, may be added.
The male element, on the other hand, is relatively smalL It may be conveyed to the female element by an out- growth of the wall of its cell, which is short in many
OAMOGBITESIS. 29
[lg(B and Fungi, but becomes an immensely elongated ibnlar filament, in the case of the pollen cell of flowering lants. But, more commonlj, the protoplasm of the nude dl becomes conyerted into rods or filaments, which gnally are in active yibratile moyement, and sometimes re propelled bj numerous cilia. Occasionallj, however, B in many Nematoidea and Artkropoda, they are devoid of lobility.
The manner in which the contents of the pollen tube ifect the embryo cell in flowering plants is unknown, as
0 perforation through which the contents of the pollen abe may jmiss, so as actually to mix with the substance of be embryo cell, have been discovered; and there is the une difficulty with respect to the conjugative processes f some of the Cryptogamia, But in the great nugority f plants, and in all animals, there can be no doubt that be substance of the male element actually mixes with that f the female, so that, in all these cases, the sexual process emains one of conjugation; and impregnation is the hysical admixture of protoplasmic matter derived from wo sources, which may be either different parts of the same rganism, or different organisms.
The effiect of impregnation appears in all cases to be bat the impregnated protoplasm tends to divide into ortions {blagtomeres), which may remain united as a single ^-aggregate, or some or all of which may become separate rganisms. A longer or shorter period of rest, in many ues, intervenes between the act of impregnation and the ommencement of the process of division.
As a general rule, the female cell which directly receives he influence of the male is that which undergoes division nd eventual development into independent germs; but here are some plants, such as the FloridecB, in which this
1 not the case. In these, the protoplasmic body of the richogyne, which unites with the spermatozooids, does not mdergo division itself, but transmits some influence to djaoent cells, in virtue of which they become subdivided nto independent germs or spores.
30 THE ANATOMY OF INYBBTEBBATBD AKIMAIiS.
There is still much obscuritj respecting the reprodnctaTe processes of the Infusoria ; but, in the VoHiedlidcR^ it would appear that conjugation merely determines a condition of the whole organism, which gives rise to the division of the endoplast or so-caUed nucleus, by which germs are thrown off; and if this be the case, the process would have some analogy to what takes place in the FloridecB.
On the other hand, the process of conjugation by which two distinct DiporpcB combine into that extraordinary double organism, the Diplosoon paradoxwn, does not directly give rise to germs, but determines the development of the sexual organs in each of the conjugated individuals ; and the same process takes place in a large number of the Infusoria, if what are supposed to be male sexual elements in them are really such.
The process of impregnation in the Floridea is remark- ably interesting, from its bearing upon the changes which fecundation is known to produce upon parts of the parental organism other than the ovum, even in thohighest i^wimnla and plants.
The nature of the influence exerted by the male ^ment upon the female is wholly unknown. No morphological distinction can be drawn between those cells which are capable of reproducing the whole organism without im* pregnation, and those which need it, as is obvious from what happens in insects, where eggs which ordinarily re- quire impregnation, exceptionally, as in many moths, or regularly, as in the case of the drones among bees, develop without impregnation. Even in the higher ft-niTrialu^ such as the fowl, the earlier stages of division of the germ may take place without impregnation.
In fact, generation may be regarded as a particular case of cell multiplication, and impregnation simply as one of the many conditions which may determine or affect that process. In the lowest organisms, the simple protoplasmic mass divides, and each part retains all the physiological pro- perties of the whole, and consequently constitutes a germ whence the whole body can be reproduced, Ii^ more ad-
I
Tm TBIOBT or nOWDATIOIT, 31a
need organimu, each ci the mnltitade of cells into which
eembiTOcell wconTcrted at fint, probably retaiiu all, or
■rij all, the phjaiolo^cal capabQitieH of the whole, and is
pable of ■erring at a reprodnctiTe geim ; but at diTision
ea on, and many of the cells which re«alt from divirion
jnire special morphological and phjeiological properties,
■eema not improbable that thej, in proportion, loae their
)re general characteia. In proportion, for example, aa the
lieacj of a given cell to become a mnacle cell or a car-
Kge cell is more nurlced and definite, it is readily con-
TaUe that ita primitiTe capacity to reproduce the whole
janiam should be reduced, though it might not be alto-
ther abolished. If this view is well based, the power of
pvodncing the whole organism would be limit«d to those
!la which had acquired no special tendencies, and conse-
ently had retained all the powers of the primiliye cell
which the organism commenced its existence. The more
enaively diSnsed such cells were, the more generally
■ht multiplication by budding or fission take place; the
e localised, the more limited would be the parta of the
nism in which such a process would take place. And
where such cells occurred, their development or non-
opment might be connected with conditions of nntri-
It depends on the nutriment supplied to the female
of abee whether it shall become a neuter or a sexually
-t female ; and the sexual perfection of a large pro-
n of the internal parasites is similarly dependent
heir food, and perhaps on other conditions, such as
iperature of the medium in which Uiey live. Thus the
Idisappearanceofagamogenesis in tbehigher animals
be related with that increasing specialisation of
t which is their essential charact«riatic ; and when
to occur altogether, it may be supposed that no cells
which retain nnmodified the powers of the primi-
i^ cell. The organism is like a society in which
I ia so engrossed by bis special business that he
a time nor inclination to marry.
be female elements in the highest organisms.
402 THE ANATOMY OF INTEBTEBBATED ANIMALS.
little as thej differ to all appearance from undifferentiated ceUs, and though thej are directlj derived from epithelial cells which have undergone yerj little modification from the condition of blastomeres, are incapable of full develop- ment unless they are subjected to the influence of the male element, which may, as Caspar Wolff suggested, be compared to a kind of nutriment. But it is a living nutri- ment, in some respects comparable to that which would be supplied to an animal kept alive bj transfusion, and its molecules transfer to the impregnated embryo cdl all the special characters of the organism to which it belonged.
The tendency of the germ to reproduce the characters of its immediate parents, combined, in the case of sexual generation, with the tendency to reproduce the characters of the male, is the source of the singular phenomena of hereditary transmission. No structural modification is so slight, and no functional peculiarity is so insignificant in either parent that it may not make its appearance in the offspring. But the transmission of parental peculiarities depends greatly upon the manner in which they have been acquired. Such as have arisen naturally, and have been hereditary through many antecedent generations, tend to appear in the progeny with great force; while artificial modifications, such for example, as result from mutilation, are rarely if ever, transmitted. Circumcision through in- numerable ancestral generations does not appear to have reduced that rite to a mere formality, as it should have done, if the abbreviated prepuce had become hereditary in the descendants of Abraham ; while modem lambs are bom with long tails, notwithstanding the long-continued prac- tice of cutting those of every generation short. And it remains to be seen whether the supposed hereditary trans- mission of the habit of retrieving among dogs is really what it seems at first sight to be; on the other side, Brown- Sequard*s case of the transmission of aitificially induced epilepsy in guinea-pigs is undoubtedly very weighty.
Although the germ always tends to reproduce, directly or indirectly, the organism from which it is derived, the
THB AI/TBBNATION OF OBNEBATIONS. 801
lesnlt of its derelopment differs somewhat from the parent, UsaaUy the amount of variation is insignificant; but it may be considerable, as in the so-called "sports;" and sach Tariations, whether useful or useless, may be trans- mitted with great tenacity to the offspring of the subjects of them.
In many plants and animals which multiply both asexu- ally and sexually, there is no definite relation between the agamogenetic and the gamogenetic phenomena. The or- ganism may multiply asexually before, or after, or con- currently with, the occurrence of sexual generation.
But in a great many of the lower organisms, both animal tnd Tegetable, the organism (A) which results from the im- pregnated germ produces offspring only agamogenetically. It thus gives rise to a series of independent organisms, (B, B, B, . . .), which are more or less different from A, and which sooner or later acquire generative organs. From their impregnated germs A is reproduced. The process thus described is what has been termed the " alternation of generations " under its simplest form, — for example, as it is exhibited by the SalpcB, In more complicated cases, the independent organisms which correspond with B may pre rise agamogenetically to others (Bj), and these to others Bj), and so on (e.g. Aphis). But, however long the series, i final term appears which developes sexual organs, and reproduces A. The " alternation of generations " is, there- :are, in strictness, an alternation of asexual with sexual generation, in which the products of the one process differ ^m those of the other.
The Hydrozoa offer a complete series of gradations be- tween those cases in which the term B is represented by a Tee, self -nourishing organism (e.g., CyancBo), through those n which it is free but unable to feed itself (Calycophoridce), o ihoee in which the sexual elements are developed in xidies which resemble free zooids, but are never detached, ind are mere generative organs of the body on which they ire developed {Cord/ylophora).
In the last case, the " individual " is the total product of
D
34 THE ANATOMY OV IVTBBTSBBATBD AITIMALS.
the development of the impregnated embryo, all the parts of which remain in material continuity with one another. The multiplication of mouths and stomachs in a Cordy- lophora no more makes it an aggregation of different individuals than the multiplication of segments and legs in a centipede converts that Arthropod into a compound animal. The Cordylophora is a differentiation of a whole into many parts, and the use of any terminology which im- plies that it results from the coalescence of many parts into a whole is to be deprecated.
In Cordylophora the generative organs are incapable of maintaining a separate existence; but in nearly allied Hyd/rozoa the unquestionable homologues of these organs become free zooids, in many cases capable of feeding and growing, and developing the sexual elements only after they have undergone considerable changes of form. Mor- phologically, the swarm of MedMsoi thus set free fi'om a Hydrozoon are as much organs of the latter, as the multi- tudinous pinnules of a Comahda, with their genital glands, are organs of the Echinoderm. Morphologically, therefore, the equivalent of the individual Comatula is the Hydrozoic stock plus all the MeduscB which proceed from it.
No doubt it sounds paradoxical to speak of a million of Aphides, for example, as parts of one morphological indivi- dual ; but beyond the momentary shock of the paradox no harm is done. On the other hand, if the asexual Aphides are held to be individuals, it follows, as a logical conse- quence, not only that all the polyx>es on a Cordylophora tree are "feeding individuals," and all the genital sacs ''generative individuals," while the stem must be a " stump individual," but that the eyes and legs of a lobster are "ocular" and "locomotive individuals." And this conception is not only somewhat more paradoxical than the other, but suggests a conception of the origin of the com- plexity of animal structure which is wholly inconsiBtent with fact.
•i ■
CAUBXS OF THS PHSNOXBNA OV IJFB. 85
IV. -Etiology.
Morphology, Distribation, and Fhjsiologj inrestigate and determine the facts of Biology. Etiology has for its object the ascertainment of the cansee of these facts, and the explanation of biological phenomena, by showing that they constitute particular cases of general physical laws. It is hardly needful to say that setiology, as thus conceived, is in its infancy, and that the seething controversies, to which the attempt to found this branch of science made in the Origin of Species has given rise, cannot be dealt with in this place. At most, the general nature of the problems to be solved, and the course of inquiry needful for their solutioii, may be indicated.
In any investigation into the causes of the phenomena of life, the first question which arises is, whether we have any knowledge, and if so, what knowledge, of the origin of living matter?
In the case of all conspicuous and easily-studied organ- isms, it has been obvious, since the study of nature began, that living beings arise by generation from living beings of a like kind; but before the latter part of the 17th cen- tury, learned and unlearned alike shared the conviction that this rule was not of universal application, and that multitudes of the smaller and more obscure organisms were produced by the fermentation of not-living, and especially of putrefying dead matter, by what was then termed gene- ratio cequivoca or eponianea, and is now called abiogenesis, Redi showed that the general belief was erroneous in a multitude of instances; SpaUanzani added largely to the list ; while the investigations of the scientific helmintholo- gists of the present century have eliminated a further category of cases in which it was possible to doubt the appUcability of the rule " omnfie tntn*m e vivo " to the more complex organisms which constitute the present fauna and flora of the earth. Even the most extravagant supporters of abiogenesis at the present day do not pretend that
D 2
28 THE ANATOMY OF INYEBTBBRATED ANIMALS.
vanishes in the adult; and, in most parts of the body, though the undifferentiated cells are capable of mnltipli* cation, their progeny grow, not into whole organimiB like that of which they form a part, but into elements of the tissues.
Throughout almost the whole series of living beings, however, we find concurrently with the process of agamo* genesis, or asexual generation, auother method of genera- tion, in which the development of the germ into an organism resembling the parent depends on an influence exerted by Hving matter different from the germ. This is gwmogenemM or sexual generation. Looking at the facts broadly, and without reference to many exceptions in detail, it may be said that there is an inverse relation between agamogenetic and gamogenetic reproduction. In the lowest organisms gamogenesis has not yet been observed, while in the highest agamogenesis is absent. In many of the lower forms of life agamogenesis is the common and predominant moda of reproduction, while gamogenesis is exceptional ; on the contrary, in many of the higher, while gamogenesis is the rule, agamogenesis takes place exceptionally.
In its simplest condition, which is termed " eonjugaiion,^ sexual generation consists in the coalescence of two similar masses of protoplasmic matter, derived from different parts of the same organism, or from two organisms of the same species, and the single mass which results from the fusion develops into a new organism.
In the majority of cases, however, there is a marked morphological difference between the two factors in the process, and then one is called the male, and the other the female element. The female element is relatively large, and undergoes but little change of form. In all the higher plants and animals it is a nucleated cell, to which a greater or less amount of nutritive material, constituting a food' yelkf may be added.
The male element, on the other hand, is relatively small. It may be conveyed to the female element by an out- growth of the wall of its ceU, which is short in many
OAMOaBNSSIS. 29
AlgcB and Fungi, but becomes an immenselj elongated tubular filament, in the case of the pollen ceU of flowering plants. Bnt, more commonly, the protoplasm of the male oeU becomes converted into rods or filaments, which Qsoallj are in active vibratile movement, and sometimes are propelled bj numerous cilia. Occasionally, however, as in many Nematoidea and Arthropoda, they are devoid of mobility.
The manner in which the contents of the pollen tube affect the embryo cell in flowering plants is unknown, as no perforation through which the contents of the pollen tube may pass, so as actually to mix with the substance of the embrjo cell, have been discovered; and there is the nine difficulty with respect to the conjugative processes of some of the Cryptogamia, But in the great msgority of plants, and in all animals, there can be no doubt that the substance of the male element actually mixes with that of the female, bo that, in all these cases, the sexual process remains one of conjugation; and impregnation is the physical admixture of protoplasmic matter derived from two sources, which may be either different parts of the same organism, or different organisms.
The effect of impregnation appears in all cases to be that the impregnated protoplasm tends to divide into portions {bl4uio7n€re8\ which may remain united as a single <^-Aggregatey or some or all of which may become separate organisms. A longer or shorter period of rest, in many eases, intervenes between the act of impregnation and the commencement of the process of division.
As a general rule, the female cell which directly receives the influence of the male is that which undergoes division and eventual development into independent germs; but there are some plants, such as the Floridece, in which this is not the case. In these, the protoplasmic body of the trichogyne, which unites with the spermatozooids, does not undergo division itself, but transmits some influence to adjacent cells, in virtue of which they become subdivided into independent germs or spores.
r-i
38 THB ANATOICY OF INYBBTSBBATBD ANIMALS.
that the more careful the investigator, and the more com- plete his mastery over the endless practical difficulties which surronnd experimentation on this subject, the more certain are his experiments to give a negative result ; while positive results are no less sure to crown the efforts of the clumsy and the careless.
It is argued that a belief in abiogenesis is a necessary corollary from the doctrine of Evolution. This may be true of the occurrence of abiogenesis at some time ; but if the present day, or any recorded epoch of geological time, be in question, the exact contrary holds good. If all living beings have been evolved from pre-existing forms of life, it is enough that a single particle of living protoplasm should once have appeared on the globe, as the result of no matter what agency. In the eyes of a consistent evolutionist, any further independent formation of protoplasm would be sheer waste.
The production of living matter since the time of its first appearance, only by way of biogenesis, implies that the specific forms of the lower kinds of life have undergone but little change in the course of geological time, and this is said to be inconsistent with the doctrine of evolution. But, in the first place, the fact is not inconsistent with the doctrine of evolution properly understood, that doctrine being perfectly consistent with either the progression, the retrogression, or the stationary condition of any particular species for indefinite periods of time ; and secondly, if it were, it would be so much the woi^se for the doctrine of evolution, inasmuch as it is unquestionably true, that certain, even highly organised, forms of life have persisted without any sensible change for very long periods. The Terebrcdtila psiUcuxa of the present day, for example, is not distinguishable from that of the Cretaceous epoch, while the highly organised Teleostean fish, BeryXf of the Chalk differed only in minute specific characters from that which now lives. Is it seriously suggested that the existing Tere- hratuUB and Beryoes are not the lineal descendants of their Cretaceous ancestors, but that their modem represen-
ORIGIN OF 8PBCIB8. 39
tatiyes have been independentlj dereloped from primordial germs in the interval ? But if this is too fantastic a sug- gestion for grave oonsideration, why are we to beUeve that the GlobigerincB of the present day are not lineally descended from the Cretaceous forms P And if their unchanged gene- rations have succeeded one another for all the enormous time represented by the deposition of the Chalk and that of the Tertiary and Quaternary deposits, what difficulty is there in supposing that they may not have persisted unchanged for a greatly longer period P
The fact is, that at the present moment there is not a shadow of trustworthy direct evidence that abiogenesis does take place, or has taken place, within the period during which the existence of life on the globe is recorded. But it need hardly be pointed out, that the fact does not in the slightest degree interfere with any conclusion that may be arrived at, deductively, from other considerations that, at some time or other, abiogenesis must have taken place.
If the hypothesis of evolution is true, living matter must have arisen from not-living matter ; for by the hypothesis, the condition of the globe was at one time such that living matter could not have existed in it,* life being entirely incompatible with the gaseous state. But living matter once originated, there is no necessity for another origina- tion, since the hypothesis postulates the unlimited, though perhaps not indefinite, modifiability of such matter.
Of the causes which have led to the origination of living matter, then, it may be said that we know absolutely nothing. But postulating the existence of living matter endowed with that power of hereditary transmission, and with that tendency to vary which is found in all such matter, Mr. Darwin has shown good reasons for believing that the interaction between living matter and surround-
* It nmkefl no difference if we reason for sapposing that all
adopt Sir W. Thomson's hypothe- stellar and planetary components
sis, and sappose that the germs of the universe are or have been
uf liTing things have been trans- gaseous, as that the earth has
ported to oar globe from some passed through this stage, other, teeixig that there is as much
40 THB ANATOHT OV IVTEBTBBBATED ANIMAIidB.
ing conditions, which results in the snrviyal of the fittest, is sufficient to account for the gradual evolution of f^snts and animals from their simplest to their most complicated forms, and for the known phenomena of Morphology, Physiology, and Distribution.
Mr. Darwin has further endeavoured to give a physical explanation of hereditary transmission by his hjrpothesis of Pangenesis; while he seeks for the principal, if not the only cause of variation in the influence of changing conditions.
It is on this point that the chief divergence exists among those who accept the doctrine of Evolution in its general outlines. Three views may be taken of the causes of variation : —
a. In virtue of its molecular structure, the organism may tend to vary. This variability may either be indefinite, or may be limited to certain directions by intrinsic condi- tions. In the former case, the result of the struggle for existence would be the survival of the fittest among an indefinite number of varieties ; in the latter case, it would be the survival of the fittest among a certain set of varieties, the nature and number of which would be predetermined by the molecular structure of the organism. ^
b. The organism may have no intrinsic tendency to vary, but variation may be brought about by the influence of conditions external to it. And in this case also, the varia- bility induced may be either indefinite or defined by in- trinsic limitation.
c. The two former cases may be combined, and variation may to some extent depend upon intrinsic, and to some extent upon extrinsic, conditions.
At present it can hardly be said that such evidence as would justify the positive adoption of any one of these views exists.
If all living beings have come into existence by the gradual modification, through a long series of generations, of a primordial living matter, the phenomena of embryonic development ought to be explicable as particidar cases of
PHYLOaSNT. 41
the general law of hereditaiy transmission. On this yiew, a tadpole is first a fish, and then a tailed amphibian, pro- Tided with both gills and Itmgs, before it becomes a fro^, because the frog waa the last term in a series of modifica- tions whereby some ancient fish became an orodele amphi- bian; and the nrodele amphibian became an anurous amphibian. In fact, the development of the embryo is a recapitulation of the ancestral history of the species.
If this be so, it follows that the derelopment of any organism should famish the key to its ancestral history ; and the attempt to decipher the full pedigree of organisms from so much of the family history as is recorded in their derelopment has given rise to a special branch of biological speculation, termed phylogeny.
In practice, however, the reconstruction of the pedigree of a group from the developmental history of its existing members is fraught with difficulties. It is highly probable that the series of developmental stages of the individual organism never presents more than an abbreviated and condensed summary of ancestral conditions; while this summary is often strangely modified by variation and adaptation to conditions; and it must be confessed that, in inost cases, we can do little better than guess what is genuine recapitulation of ancestral forms, and what is the effect of comparatively late adaptation.
The only perfectly safe foundation for the doctrine of Evolution lies in the historical, or rather archsBological, evidence that particular organisms have arisen by the gradual modification of their predecessors, which is fur- nished by fossil remains. That evidence is daily increasing in amount and in weight ; and it is to be hoped that the comparison of the actual pedigree of these organisms with the phenomena of their development may furnish some criterion by which the validity of phylogenetic conclusions, dednced from the facts of embryology alone, may be satis- factorily tested.
II.
42 THB ANATOMT GF nrYSBTBBSATBD AJXJMAIA,
CHAPTER I.
I.— THE DISTINCTIYE CHABACTBB8 OF ANIMALS.
Tab more complicated forma of the living things, the general characters of which have now been discussed, appear to be readily distinguishable into widely separated groups. Animals and Plants. The latter have no power of locomo- tion and only rarely exhibit any distinct movement of their parts when these are irritated, mechanically or otherwise. They are devoid of any digestive cavity ; and the matters which serve as their nutriment are absorbed in the gaseous and fluid state. Ordinary animals, on the contrary, not only possess conspicuous locomotive activity, but their parts readily alter their form or position when irritated. Their nutriment, consisting of other animals or of plants, is taken in the solid form into a digestive cavity.
But even without descending to the very lowest forms of animals and plants, we meet with facts which weaken the force of these apparently broad distinctions. Among animals, a coral or an oyster is as incapable of locomotion as an oak ; and a tape- worm feeds by imbibition and not by the ingestion of solid matter. On the other hand, the Sensitive Plant and the Sundew exhibit movements on irritation, and the recent observations of Mr. Darwin and others leave little doubt that the so-called "insectivorous plants *' really digest and assimilate the nutritive matters contained in the living animals which they catch and de- stroy. All the higher animals are dependent for the protein compounds which they contain upon other animals or upon plants. They are unable to manufacture protein out of simpler substances ; and although positive proof is wanting that this incapacity extends to all animals, it may safely be
THS DiarnrcTiYB cha&actxks ov utimals. 43
lined to exist in all those forms of ft.TiiTna.1 life which i in solid nutriment, or which live parasiticall j on other nals or plants, in situations in which thej are provided 1 abundant supplies of protein in a dissolved state. he great nugoritj of the higher plants, on the contrary, able to manufacture protein when supplied with car- ic acid, ammoniacal salts, water, and sundry mineral sphates and sulphates : obtaining the carbon which they lire by the decomposition of the carbonic acid, the gen of which is disengaged. One essential factor in the formance of this remarkable chemical process is the Kzophyll which these plants contain, and another is the 8 light.
extain animals (It^usoria, Ccelenteraia, Tu/rbellaria) less chlorophyll, but there is no evidence to show what i it plays in tlieir economy. Some of the higher plants n parasitic, and a great group of the lower plants, the igi (which may be parasitic or not), are, however, devoid chlorophyll, and are consequently totally unable to ve the carbon which they need from carbonic acid, ertheless they are sharply disting^uished from animals, much as they are still, for the most part, manufac- ra of protein. Thus such a Fungus as PenicUlium is
to fabricate all the constituents of its body out of aonium tartrate, sulphate and phosphate, dissolved in *r (see su^pra, p. 6, note) ; and the yeast-plant flourishes multiplies with exceeding rapidity in water containing IT, ammonitim tartrate, potassium phosphate, calcium iphate, and magnesium sulphate, evertheless, the experiments of Mayer have shown
when peptones are substituted for the ammonium rate, the nutrition of the yeast-plant is favoured ead of being impeded. So that it would seem that the (t-plaut is able to take in protein compounds and milate them, as if it were an animal ; and there can be reasonable doubt that many parasitic Fungi, such as Botrytis Bassiana of the silk-worm caterpillar, the TUBa of the house-fly, and, very probably, the Peronospora
44 THE UTATOMY OF IKYEBTEBRATED AKHCALS.
of the potato-plant, directl j assiinilate the protein substances contained in the bodies of the plants and animals which they infest; nor is it clear that these Fungi are able to maintain themselyes upon less fully elaborated nutriment.
Cellulose, amyloid and saccharine compounds were formerly supposed to be characteristically Tegetable pro- ducts ; but cellulose is found in the tests of Ascidians ; and amyloid and saccharine matters are of Tery wide, if not universal, occurrence in animals.
And on taking a comprehensive survey of the whole animal and vegetable worlds, the test of locomotion breaks down as completely as does that of nutrition. For it is the rule rather than the exception among the lowest plants, that at one stage or other of their existence they should be actively locomotive, their motor organs being usually eUia, altogeUier similar in character and function to the motor organs of the lowest animals. Moreover, the protoplasmic substance of the body in many of these plants exhibits rhythmically pulsating spaces or e(ynira4;Hle wicuoUs of the same nature as those characteristic of so many animals.
No better illustration of the impossibility of drawing any sharply defined distinction between animals and plants can be found, than that which is supplied by the history of what are commonly termed ' Monads.'
The name of ' Monad ' * has been commonly applied to minute free or fixed, rounded or oval bodies, provided with one or more long cilia {flagella), and usually provided with a nucleus and a contractile vacuole. Of sufth bodies, all of which would properly come under the old group of Monadidce, the history of a few has been completely worked out ; and the result is that, while some {e.g. ChtamydonKmas, zoo- spores of Peronogpora and Coleochcete) are locomotive condi- tions of indubitable plants; others (Badiolaria, NodUuca) are embryonic conditions of as indubitable animals. Yet others (zoospores of Myxomycetes) are embryonic forms of organisms which appear to be as much animals as plants ;
* O. F. MUIler, ' Historia oontpicuus, simplicitsimus, pcl- Yexmium/ 1773. " Vermis in- lucidos, punctifonnis."
THS DI8TJJI0TIVJS CHABACTEB8 OV AITIMALB. 45
inasmucli as in one condition thej take in solid nntriment, and in another have the special morphological, if not physiological peculiaritiee of plants; while, lastly, in the case of such monads as those recently so carefully studied by Messrs. Dallinger and Drysdale, the morphological characters of which are on the whole animal, while their mode of nutrition is xmknown, it is impossible to say whether they should be regarded as animals or as plants.
Thus, traced down to their lowest terms, the series of plant forms gpradually lose more and more of their dis- tinctiye vegetable features, while the series of wTiimal forms part with more and more of their distinctive animal characters, and the two series converge to a common term. The most characteristic morphological peculiarity of the plant is the investment of each of its component cells by a sac, the walls of which contain cellulose, or some closely analogous compound ; and the most characteristic physio- logical peculiarity of the plant is its power of manufacturing protein from chemical compounds of a less complex nature.
The most characteristic morphological peculiarity of the ffTiimstl is the absence of any such cellulose investment.* The most chaxacteristic physiological peculiarity of the ^nimal is its Want of powcr to manufacture protein out of simpler compounds.
The great majority of living things are at once referable to one of the two categories thus defined ; but there are some in which the presence of one or other characteristic mark cannot be ascertained, and others which appear at different periods of their existence to belong to different categories.
* No analvsis of the tubstanee with cellulose when heated with composing the cyets in which so the double hyposulphite of cop- many of the Protozoa inclose them- per and ammonia, it is possible, seWes temporarily, haa yet been therefore, that the difference be- made. But it is not improbable tween the chitinous investment of that it may be analogous to diitin ; an animal and the cellulose invest - and if so, it is worthy of remark ment of a plant may depend upon that though ehitin ic a nitroge- the proportion of nitrogenous nous body, it readily vields a matter which is present in each substance appareBtly Identical case in addition to the ehitin.
46 THB AVATOMY OV nrYBBTBBSATBD AHIMALS.
II. — THB MORPHOLOGICAL DIFFEBBNTIATIOV OF
ANIMALS.
The simplest form of animal life imaginable would be a protoplasmic body, devoid of motility, maintaining itself bj the ingestion of such proteinaceons, fattj, amjloid and mineral matters as might be brought into contact with it by external agencies ; and increasing by simple extension of its mass. But no animal of this degree of simplicity is known to exist. The very humblest a,Tiimalfl with which we are acquainted exhibit contractility, and not only in- crease in size, but, as they grow, divide, and thus undergo multiplication. In the simplest known animals — the Protozoa — ^the protoplasmic substance of the body does not become differentiated into discrete nucleated masses or cells, which by their metamorphosis give rise to the different tissues of which the adult body is composed. And, in the lowest of the ProtoBoa, the body has neither a constant form nor any further distinction of parts than a greater density of the peripheral, as compared with the central, part of the proto- plasm. The first steps in complication are the appearance of one or more rhythmically contractile vacuoles, such as are found in some of the lower plants ; and the segregation of part of the interior protoplasm as a rounded mass, the " endoplast " or " nucleus." Other Prototoa advance further and acquire permanent locomotive organs. These may be developed only on one part of the surface of the body, which may be modified into a special organ for their support. In some, a pedicle of attachment is formed, and the body may acquire a dense envelope (Injuaoria), or secrete an internal skeleton of calcareous or silicious matter (Forammifera, Badhlaria), or fabricate such a skele- ton by gluing together extraneous particles {Foraminifera),
A mouth and gullet, with an anal aperture, may be formed, and the permeable soft central portion of the protoplasm may be so limited as to give rise to a virtual alimentary tract between these two apertures. The contractile vacuole
\
MOBPHOLOOIOAL DIFFBRBHTIATIOIT. 47
may be derreloped into a complicated sjstem of canals [Paramaemm), and the endoplast may take on more and more definitely the characters of a reproductive organ, that is, may be the focus of origin of germs capable of repro- ducing the indiridual {VoriieeUa). In hct, rudiments of all the chief system of organs of the higher animals, with the exception, more or less doubtful, of the nervous, are thus sketched out in the Proionoa, just as the organs of the higher plants are sketched out in Caulerpa,
In the Meicuoa, which constitute the rest of the animal kingdom, the a^iiwiftJ, in its earliest condition, is a pro- toplasmic mass with a nucleus — ^is, in short, a Protozoon. But it never acquires the morphological complexity of its adult state by the direct metamorphosis of the protoplasmic matter of this nucleated body — ^the ovum — into the different tissues. On the contrary, the first step in the development of all the Mekuoa is the conversion of the single nucleated body into an aggregation of such bodies of smaller size — ^the Morula — ^by a process of division, which usually takes place with great regularity, the ovum dividing first into two segments, which then subdivide, giving rise to four, eight, sixteen, Ac., portions, which are the so-called division ma9$e$ or hUuiomerea,
A similar process takes place in sundry Protouoa and gives rise to a protozoic aggregate, which is strictly comparable to the Morula, But the members of the protozoic aggregate become separate, or at any rate independent existences. What distinguishes the metazoic aggregate is that, though its component blastomeres also retain a certain degree of physiological independence, they remain united into one morphological whole, and their several metamorphoses are so ordered and related to one another, that they constitute members of a mutually dependent commonalty.
The MdoMoa are the only animals which fall under common observation, and have therefore been known from ihb earliest times. All the higher lang^uages possess general naimw aqpiivalent to our beasts bird, reptile, fish.
48 THE AKATOmr OF IHrYEBTEBSATBD AlfTlTATrft.
insect and worm ; and this shows the very early perception of the fact, that not¥dth8tanding the wonderfol divendtj of i»-TiiTwn.1 forms, thej are modelled upon oomparatiyelj few great types.
In the middle of the last century the founder of modem Taxonomy, Linnaeus, distinguished animals into Mammalia^ Aves, Amphibia, Pisces, Insecta and Vermes, that is to say, he converted common sense into science by defining and giving precision to the rough distinctions arrived at by ordinaiy observation.
At the end of the century, Lamarck made a most impor- tant advance in general morphology, by pointing out that mammals, birds, reptiles, and fishes, are formed upon one type or common plan, the essential character of which is the possession of a spinal column, interposed between a cere- bro-spinal and a visceral cavity; and that in no other ajiimals is the same plan of construction to be discerned. Hence he drew a broad distinction between the former and the latter, as the Yebtebbata and the Inyebtebbata. But the advance of knowledge respecting the structure of inverte- brated animals, due chiefly to Swammerdam, Trembley, Reaumur, Peyssonel, Goeze, Boesel, Ellis, Fabricius, O. F. Miiller, Lyonet, Pallas, and Cuvier, speedily proved that the Invertebrata are not framed upon one fundamental plan, but upon several; and, in 1795, Cuvier* showed that, at fewest, three morphological types, as distinct from one another as they are from that of the vertebrated animals, are distinguishable among the Invertebrata, These he named -pi. Mollusques ; II. Insectes et Vers ; III. Zoophytes. In ^ tne ' Begne animal ' (1816), these terms are Latinised, Ani- "m malia MoUusca, Artietdata, and Badiaia. Thus, says Cuvier : "It will be found that there exist four principal forms, four general plans, if it may thus be expressed, on which all animals appear to have been modelled; and the ulterior divisions of which, under whatever title naturalists may have designated them, are merely slight modifications, founded on the development or addition of certain parts. * Tableau ^lementaire de llilttoire det animaux. An vi.
COMMON PLANS. 49
These four common plans are those of the Vertebrata, the MoUusea, the Ariictdaia, and the Radiata."
For extent, variety, and exactness of knowledge, Cuvier was, beyond all comparison, the greatest anatomist who has ever lived ; but the absence of two conditions rendered it impossible that his survey of the animal kingdom should be exhaustive, grand and comprehensive as it was.
Up to the time of Cuvier's death in 1832, microscopic inves-" tigation was in its infancy, and hence the great majority of the lowest forma were either imknown or little understood ; and it was only in the third decade of the present century that Rathke, Dollinger and Yon Baer commenced that won- derful series of exact researches into embryology, which Yon Baer org^anised into a special branch of morphology, developing all its most important consequences and raising it to its proper position, as the criterion of morphological theories.
Upon embryological grounds Yon Baer arrived at the same conclusion as Cuvier, that there are four common plans of animal structure.
In the course of the last half -century the activity of anatomists and embryologists has been prodigious, and it may be reasonably doubted whether any form of animal life remains to be discovered which will not be found to accord with one or other of the common plans now known. But at the same time this increase of knowledge has abolished the broad lines of demarcation which formerly appeared to separate one common plan from another.
Even the hiatus between the Vertehrata and the IrvceV' td^aia, is partly, if not wholly, bridged over ; and though among the Invertebrata, there is no difficulty in distinguish- ing the more completely differentiated representatives of such types or common plans as those of the Arthropoday the AjMtelida, the MoUuaca, the Tunicaia, the Echinodermaiay the CcBUfnieralay and the Porifera, yet every year brings forth fresh evidence to the effect, that just as the plan of the plant is not absolutely distinct from that of the animal, so that of the Yertebra^ has its points of commimity with
50 THE ANATOMY OF INYEBTEBBATED ANIMALS.
that of certain of the Invertebrates; that the Arthropod, the Mollusk, and the Echinoderm plans are united by that of the lower worms, and that the plan of the latter is separated by no very great differences, from that of the Coelenterate and that of the Sponge.
Whatever speculative views may be held or rejected as to the origin of the diversities of animal form, the facts of anatomy and development compel the morphologist to regard the whole of the Metcaoa as modifications of one actual or ideal primitive type, which is a sac with a double cellular wall, enclosing a central cavity and open at one end. This is what Haeckel terms a Oagtrcea, The inner wall of the sac is the hypoblast {endoderm of the adult), the outer the epiblast [ectoderm). Between the two, in all but the very lowest Metazoa, a third layer, the mesoblast {mesoderm of the adult), makes its appearance.
In the Porifera, the terminal aperture of the gastrsea becomes the egestive opening of the adult animal, and the ingestive apertures are numerous secondary pore-like apertures formed by the separation of adjacent cells of the ectoderm and endoderm. The body may become variously branched, a fibrous or spicular endoskeleton is usually deve- loped in the ectoderm, and no perivisceral cavity is developed. There are no appendages for locomotion or prehension ; no nervous system nor sensory organs are known to exist ; nor are there any circulatory, respiratory, renal, or generative organs.
In the Coelenterataf the terminal aperture of the gastraea becomes the mouth, and if pores perforate the body walls, they do not subserve the ingestion of food. There is no separate perivisceral cavity, but, in many, an enteroccele or system of cavities, continuous with, but more or less separate from the digestive cavity, extends through the body. Prehensile appendages, tentaeula, are developed in great variety. A chitinous exoskeleton appears in some, a calcareous or chitinous endoskeleton in others. There are no circulatory, respiratory, or renal organs (though it is possible that certain cells in the PorpiUB, e,g, may have an
AKNULOSB DI7FBBBNTIATION. 51
nropoietic f xmction) ; but special genital organs make their appearance, as do a definitelj-arranged nervous system and organs of sense.
The lowest TurheUaria are on nearly the same grade of organisation as the lower CcdentercUa, but the thick mesoderm is traversed by canals which constitute a water- vascular tydem. In the adult state these canals open, on the one side, into the interstices of the mesodermal tissues, and, on the other, communicate with the exterior. Their analogy to the contractile vacuoles of the Infusoria on the one hand, and to the segmental organs of the Annelids on the other, lead me to think that they are formed by a splitting of the mesoblast, and that they thus represent that form of perivisceral cavity which I have termed a fdiiaaecde. A nervous system consisting of a single or doable ganglion with two principal longitudinal nerve cords, is found in many ; and there may be eyes and auditory sacs.
Upon this foundation a gradual complication of form is based, brou^t about by —
1. The elongation of the bilaterally symmetrical body and the formation of a chitinous exoskeleton.
2. The development of a secondary aperture near the anterior end of the body, which becomes the permanent mouth.
3. The division of the mesoblast into successive segments (gomites),
4. The development of two nervous ganglia in each somite.
5. The outgrowth of a pair of appendages from each somite, and their segmentation.
6. The gradual specialisation of the somites into cephalic, thoracic and abdominal groups; and that of their appen- dages into sense organs, jaws, locomotive limbs, and respi- ratory organs.
7. The conversion of the schizocoele into a spacious perivisceral cavity oontaining blood ; the reduction of the water- vascular system and the appearance of pseudo-hsemal vessels ; and the replacement of these, in the higher forms, by a heart, arteries, and veins, which contain blood.
£ 2
52 THE ANATOMY OF INYBBTEBBATED ANIMALS.
8. The converaion of the simple inner sac of the gastrsea into a highly complex alimentary canal, with special glan- dular appendages, representing the liver and the kidneys.
9. A similar differentiation of the genital apparatus.
10. A gradual complication of the eye, which, in its most perfect form, presents a series of crystal-clear conical rods, disposed perpendicularly to fhe transparent corneal region of the chitinous ezoskeleton, and connected by their inner ends with the optic nerves of the prse-oesophageal ganglia.
By such modifications as these the plan of the simple Turbellarian gradually passes into that of the highest Arthropod.
Starting from the same point, if the mesoblast does not become distinctly segmented; if few, probably not more than three, pairs of ganglia are formed; if there are no segmented appendages, but the chief locomotive organ is a muscular foot developed in the neural aspect of the body ; if, in the place of the chitinous exoskeleton, a shell is secreted by a specially modified part of the hsBmal wall termed the mantle ; if the schizocoele is converted into a blood cavity, which communicates with the exterior by an organ of Bojanus, which appears to represent the water-vascular system and the segmental organs ; and if, along with these changes, the alimentary, circulatory, respiratory, genital, and sensory organs take on special characters, we arrive at the complete Molluscan plan.
From the Turbellarian to the Tunicate, or Ascidian, the passage is indicated, if not effected, by Balanoglasstu, which, in its larval state, is comparable to an Appendievlaria with- out its caudal appendage. On the other hand, the large pharynx of the Tunicaia and the circle of tentacula around the oral aperture, with the single ganglion, approximate them to the Polyzoa, In the perforation of the pharynx by lateral apertures, which communicate with the exterior, either directly or by the intermediation of an atrial cavity, the Tunio(dareBeiah\eordjBaJanoglo88U8 and the VerMnrata. The axial skeleton of the caudal appendage has no parallel except in the yertebrate notochord. In the structure of
THB PLAN OF THE ECHINODEBMS. 53
it and the regular reversal of the direction of its tions, the Tunieata stand alone. The general »e of a test solidified by cellulose is a marked rity, but in estimating its apparent singularity the ce of cellulose as a constituent of chitin must be leered. Finally, the tadpole-like larvsB of many Asci- are comparable only to the CerearioB of Trematodcs, one hand, and to vertebrate larval forms on the
another apparently very distinct type is met with in ensive group of the Echinodermata, 1 the other Metazoa, except the Porifera and Ccelen' the plan of the body is, obviously, bilaterally sym- il, the halves of the body on each side of a median L plane being similar. Any disturbance of this sym- such as is found in some Arthropoda and in many Ml, arises from the predominant development of one But, in a Sea-urchin or Star-fish, five or more similar parts are disposed around a longitudinal axis, which i mouth at one end and the anus at the other ; there lial symmetry, as in a sea anemone or a Ctenopho- N^evertheless, close observation shows that, as is also le in the Actinia or Ctenophoran, this radial sym- is never perfect, and that the body is really bila- symmetrical in relation to a median plane which les the centre of length of one of the radiating eres.
ther marked peculiarity of the Echinoderm type, general, if not universal, presence of a system of jocral vessels "' consisting of a circular canal around >uth, whence canals usually arise and follow the line of each of the ambulacral metameres. And in deal Echinoderm, these canals give off prolongations enter certain diverticula of the body wall, the pediceh cers.
Schinoderms have a calcareous endoskeleton. le chapter allottei to these animids, it will be shown
54 THE AKATOMY OF nTYBBTSBBATBD ANIMALS.
that they are modificatioiis of the Tnrbellaria.n type, brought about by a singular series of changes undergone by the endoderm and mesoderm of the larva or Echinopoedium.
III. — THE PHYSIOLOGICAL DIFFERENTIATION OF ANIMALS, AND THE MOBPHOLOaiCAL DIFFERENTIATION OF THEIB OBOANS.
Regarded as machines for doing certain kinds of work, animals differ from one another in the extent to which this work is subdivided. Each subordinate group of actions or functions is allotted to a particular portion of the body, which thus becomes the organ of those functions ; and the extent to which this division of physiological labour is carried differs in degree within the limits of each common plan, and is the chief cause of the diversity in the working out of the common plan of a group exhibited by its members. Moreover, there are certain types which never attain the same degree of physiological differentiation as others do.
Thus, some of the Protozoa attain a grade of physiologi- cal complexity as high as that which is reached by Uie lower Metazoa. And notwithstanding the multiplicity of its parts, no Echinoderm is so highly differentiated a physio- logical machine as is a snail.
A mill with ten pairs of millstones need not be a more complicated machine than a mill with one pair; but if a mill have two pairs of millstones, one for coarse and one for fine grinding, eo axranged that the substance ground passes from one to the other, then it is a more complicated machine — a machine of higher order — ^than that with ten pairs of similar grindstones. In other words, it is not mere multiplication of organs which constitutes physiological differentiation; but the multiplication of organs for dif- ferent functions in the first place, and the degree in which they are co-ordinated, so as to work to a common end, in the second place. Thus, a lobster is a higher animal, from a physiological point of view, than a Cyclops, not
THE TEOUMBNTABY SYSTEM. 55
because it has more distingoishable organs, bnt because these organs are so modified as to perform a much greater variety of functions, while they are all co-ordinated towards the maintenance of the animal, by its well-developed nervous system and sense organs. But it is impossible to say that e.g, the Arthropoda, as a whole, are physiologically higher than the MoUusca, inasmuch as the simplest embodi- ments of the common plan of the Arthropoda are less differentiated physiologically than the great majority of Mollusks.
I may now rapidly indicate the mode in which physio- logical differentiation is effected in the different groups of organs of the body among the Metazoa.
Iniegumentary Organs, — In the lowest Metazoa, the inte- gument and the ectoderm are identical, but so soon as a mesoderm is developed, the layer of the mesoderm which is in contact with the ectoderm becomes virtually part of the integument, and in all the higher animals is distin- g^uished as the dermis {enderon), while the ectodermal cells constitute the epidermis {ecderon). The connective tissue and muscles of the integument are exclusively developed in the enderon ; while, from the epidermis, all cuticular and cellular ezoskeletal parts, and all the integumentary glands are developed. The latter are always involutions of the epidermis. The hard protective skeletons in all invertebrate Metaaoa, except the Porifera, the Actiiiozoa, the Echinoder- maioj and the Tunicata, are cuticular structures, which may be variously impregnated with calcareous salts formed on the outer surface of the epidermic cells.
In the Porifera^ the calcareous or silicious deposit takes place within the ectoderm itself, and, probably, the same process occurs, to a greater or less extent, in the Actinozoa. In those Tunicaia which possess a test, it appears to be a structure sui generis, consisting of a gelatinous basis excreted by the ectoderm, in which cells detached from the ectoderm divide, multiply, and give rise to a deposit of cellulose. The test may take on the structure of cartilage or even of
56 THE ANATOMY OF INYEBTEBKATED ANIMALS.
oonnective tissue. In the Vertebrata alone do we find hard exoskeletal parts formed by the comification and cohesion of epidermic cells.
In the Actinozoa and the Echinodermata, the hard skeleton is, in the main, though perhaps not wholly, the result of calcification of elements of the mesoderm. In some Mol- lusks, portions of the mesoderm are converted into true cartilage, while the enderon of the integument often becomes the seat of calcareous deposit. The endoskeleton and the dermal ezoskeleton of the Vertebrata are cellular (cartilage, notochord) or fibrouH (connective tissue) modifi- cation of the mesoderm, which may become calcified (bone, dentine). Recent investigations tend to show that the enamel of the teeth is derived from the ectoderm.
The Alimentary Apparatus. — Prom the simple sac of the Hydra or aproctous Twrbellarian, we pass to the tubular alimentary tract of the proctuchous Turbellaria, In the Botifera and Polyzoa^ there is a marked distinction into buccal cavity, pharynx, oesophagus, stomach and intestines ; while distinct salivary, hepatic, and renal glands, are found in the majority of the higher invertebrates, and, not un- frequently, ghmds secreting an odorous or coloured fluid, appear in the region of the termination of the alimentary canal.
The oral and gastric regions are armed with cuticular teeth in many Ini>ert€brata ; but teeth formed by the calcifi- cation of papillary elevations of the enderon of the lining of the mouth, are confined to the Vertebrata ; unless, as seems probable, the teeth of the Echinidea have a similar origin.
The lining membrane of the oral cavity is capable of being everted, as a proboscis, in many Invertebrata. The margins of the mouth may be raised into folds, armed with c'uticular plates. In the Vertebrata, the jaws are such folds, supported by endoskeletal cartilages, belonging to the system of the visceral arches, or by bones developed in and around them; but, in the Arthropoda, what are usually termed jaws are modified limbs.
THE BLOOD 8T8TBM. 57
2%« Blood cmd CircuUdory Apparatus, — In the Codenierata, the aomatio cayitj, or enteroccele, is in free communication with the digestive cavity, and not nnf requentl j communicates with the exterior hj other apertures. The fluid which it contains represents hlood ; it is moved by the contractions of the body, and, generally, by cilia developed on the endo- dermal lining of the enteroccele. In the Twrhellaria, Trema" toda and Cestoidea, the lacimee of the mesoderm and the interstitial fluid of its tissues are the only representatives of a blood- vascular system. It is probable that these communi- cate directly with the terminal ramifications of the water- vas- cular system. In the Botifera, a spacious perivisceral cavity separates the mesoderm into two layers, the gplanehiwplevTe, which forois the enderon of the alimentary canal, and the somaioplefure, which constitutes the enderon of the integu- ment. The terminations of the water vessels open into this cavity. In Annelids, there is a similar perivisceral cavity communicating in the same way with the segmental organs ; but, in most, there is, in addition, a system of canals with contractile walls, which, in some, communicate freely with the perivisceral cavity; but, in the majority, are shut off from it. These canals are filled by a clear, usually non-corpuscu- lated fluid, which may be red or green, and constitute the pseitd-hcemal system. The fluid which occupies the peri- visceral cavity contains nucleated corpuscles, and has the characters of ordinary blood. It seems probable that the fluid of the pseud-hfiemal vessels, as it contains a substance resem- bling haemoglobin, represents a sort of respiratory blood.
In the Arthropodat no segmental organs or pseud-hsemal vessels are known. In the lowest forms, the perivisceral cavity and the interstices of the tissues represent the whole blood system, and colourless blood cells float in their fluid contents. In the higher forms, a valvular heart, with arteries and capillaries, appears, but the venous system re- mains more or less lacunar. In the MoUusca, the same gradual differentiation of the blood- vascular system is ob- servable. In very many, if not all, the blood cavities communicate directly with the exterior by the ' organs of
^t.1
58 THB ANATOMY OF IKYBHTEBBATBD ANIMALS.
Bojanns' — ^whicH resemble very simple segmental organs, and appear to be always associated with the renal apparatus. In the Vertebrataf AmphiooBus has a system of blood vessels with contractile walls, and no distinct heart. In aU the other Yertebrates there is a heart with at fewest three chambers {Hnua venosuSf cUriwm, ventricle)^ arteries, capillaries, and veins, and a system of lymphatic vessels connected with the veins. The lymphatic fluid consists of a colourless plasma, with equally colourless nucleated corpuscles ; the blood plasma contains, in addition, red corpuscles, which are nucleated in Ichthyopsida and Sauropsida, but have no nucleus in the Mcmtmalia, The lymphatic vessels always communicate with the interstitial lacunsB of the tissues, and, in the lower Vertebrates are themselves, to a great extent, irregular sinuses. The venous system presents many large sinuses in the lower vertebrates; while, in the higher forms, these sinuses are for the most part replaced by definite vessels with muscular walls. But the " serous cavities " remain as vast lymphatic lactmsB. Yalves make their apx>earance in the lymphatics and in the veins, and the heart becomes sub- divided in such a manner as to bring about a more and more complete separation of the systemic circulatory appa- ratus from that which supplies the respiratory organs.
The BespircUory System. — In the lower Metiizoa respira- tion is effected by the general surface of the body. In the Annelids, processes of the integument, which are sometimes branched and usually are abundantly ciliated and supplied with pseud-hffimal vessels, give rise to hranchi4B, Bi'anchiffi, abundantly supplied with blood-vessels, but never ciliated, attain a great development in the Crustacea, The access of fresh water to them is secured by their attachment to some of the limbs ; and, in the higher Crustaceans, one of the appendages, the second maxilla, serves as an accessory organ of respiration. Although especially adapted for aquatic respiration, they are converted into air-breathing organs in the land crabs, being protected and kept moist in a large chamber formed by the carapace. ^
THE BESPIRATOliY SYSTEM. 59
In some molliiBks {e,g. Fieropoda), the delicate lining nemhrane of the pallial cavity seires as the respiratory aigan ; but, in most, branched or laminated processes of the body give rise to distinct branchiffi. The mantle becomes in aocesaory organ of respiration, being so modified as to direct, or to canse, the flow of currents of water over the bnmcbicB contained in its cavity. In many adult urodele Aw^^kibia {Perennibranehiata), and in the embryonic con- dition of all Amphibia and of many fishes, branchifls of a Bmilar character, abundantly supplied with blood vessels, are attached to more or fewer of the visceral arches.
In all these cases the branchisB are external, and are developed from the integument. In Crustaceans and Mol- Insks the blood with which they are supplied is return- ing to the heart ; while, in the Vertebrata mentioned, it is flowing from the heart; and it will be observed that the gradual perfectioning of the respiratory machinery consists, first, in the outgrowth of parts of the integument specially adapted to subserve the interchange between the gases contained in the blood and those in the surrounding medium; secondly, in the increase of the surface of the branchise, so as to enable them to do their work more rapidly ; thirdly, in the development of accessory organs, by which the flow of water over the branchias is rendered definite and constant, and may be increased or diminished in accordance with the needs of the (Bconomy.
It is probable that the water- vascular system and the segmental organs of Tnrbellarians and Annelids ; the cloacal tubes of the Gephyrea and of some Holothuridea ; the ambii- lacral vesicles of the Eohinoderms, and the large pharyngeal cavity of the Polyzoa ; to a greater or less extent, subserve respiration, and constitute internal respiratory organs.
In Myriapoda and Insecta, the tracJieoe — tubes which open on the surface of the body and contain air, and are curiously Kiftiilar in their distribution to the water vessels of the worms— constitute a very complete internal aerial re8pii*a- tory apparatus.
In Arachnida, trachee may exist alone, or be accompanied
60 THE ANATOMY OF INYEBTEBBATED ANIMALS.
bj folded pulmonary sacs, or the latter may exist alone, as in the Scorpion. In this case, these lungs are supplied bj blood which is returning from the heart.
In these animals, the flow of air into and out of the air cavities is governed by the contractions of muscles of the body, disposed so as to alter its vertical and longitudinal dimensions. In the higher forms, the entrance and exit of air is regulated by valves, placed at the external openings (stigmata) of the trachess, and provided with muscles, by which they can be shut.
In the Enteropneuata and the Tunicata a new form of internal aquatic respiratory apparatus appears. The large pharynx is perforated by lateral apertures, which place its cavity in communication with the exterior ; and water, taken in by the mouth, is driven through these branchial clefts and aerates the blood, which circulates in their interspaces.
The respiratory apparatus of Amphioxus, of aU adult fishes, and of the tadpoles of the higher anurous Amphibia^ in a certain stage of their existence, is of an essentially similar character. The accessory respiratory apparatus for the maintenance and the regulation of the currents of water over the gills is furnished by the visceral arches and their muscles; and the respii*atory blood flows from the heart.
In MoUusks which live on land (PtUmogaderopoda), the lining wall of the mantle cavity becomes folded and highly vascular, and subserves the aeration of the venous blood, which flows through it on its way to the heart. The lung is here a modification of the integument, and might be termed an external lung. The lungs of the air- breathing Vertebrata, on the contrary, are diverticula of the alimentary canal, posterior to the hindermost of the visceral arches. They receive their blood from the hinder- most aortic arch. It therefore flows from the heart. The gradual improvement of these lungs as respiratory machines is effected, first, by the increase of the surface over which the venous blood brought to the lungs is distributed; secondly, by changes in the waUs of the cavity in which
THE UBOPOIBTIC SYSTEM. 61
the limgs are contained, bj which that cavitj gradually becomes shut off from the peritoneal chamber, and divided from it by a mnscular partition. Concurrently with these modifications, a series of alterations takes place in the accesaory apparatus of respiration, wheref>y the machinery of inspiration, which, in the lower Vertebrata, is a buccal force-pump, which drives air into the lungs, in the same way aa water is driven through the branchiss, is replaced by a thoracic suction-pump, which draws air into the lungs by dilatation of the walls of the closed cavity in which they are contained. Along with these changes, modifications of the heart take place, in virtue of which one half of its total mechanical power becomes more and more exclusively appropriated to the task of driving the blood through the lungs. The term ' double circulation ' applied to the course of the blood in the highest VertebrcUa is, however, a mis- nomer. In the highest, as in the lowest, of these animals, the blood completes but one circle, and the respiratory organ is in the course of the outward current.
Many animals are truly amphibious, combining aquatic and aerial respiratory organs.
Thus, among MoUusks, AmptdLaria and Onchidum combine branchisD with pulmonary organs ; many Teleostean fishes have the lining membrane of the enlarged branchial chamber vascular and competent to subserve aerial respiration. And in the Ganoids and Teleostei, the presence of an air bladder, which is both functionally and morphologically of the same nature as a lung, is very common. But, in the majority of the Teleoatei, the air bladder is turned aside from its pulmonary function to subserve mechanical pur- poses, in affecting the specific gravity of the body. On the other hand, in the Ganoids and Dipnoh the whole series of modifications by which the air bladder passes into the lung are patent. In such lower Amphibia as Proteus and Metw- branchns, branchial respiration is predominant, and the lungs are subsidiary ; but in the higher, the lungs acquire greater importance, while the branchi® diminish, and eventually dinppear.
62 THE ANATOMY OF nTYEBTEBRATED ANIMALS.
The UropaieUe System. — ^Uropoietic organs, distinct from tHe alimentajy canal, are probably represented bj the water-vascular system and segmental organs of the worms. The 'organs of Bojanus' of Mollusks are sacs or tubes opening, on the one side, on the exterior of the bodj, and, on the other, into some part of the blood-vascular system. So far, as Gregenbaur has shown, they resemble the segmental organs of Annelids. In the majority of the MoUusca, some part of the wall of the organ of Bojanus is in close relation with the venous system near the heart, and the nitrogenous waste of the body is here eliminated from the venous blood. In the Vertebraia, the renal apparatus is constructed on the same principle. If for simplicity's sake we reduce a mammalian kidney to an ureter with a single uriniferous tubule, it corresponds with an organ of Bojanus, so far as it contains a cavity communicating with the exterior at one end, and having a vascular plexus — ^the MaJpighian body — in intimate contact with the opposite end. In the adult mammal there is no direct communication between the urinary duct and the blood-vascular system. But, inasmuch as recent researches have proved that the ureter is formed by subdivision of the Wolfl5an duct, and that the WolflBan duct is primitively a diverticulum of the perito- neal cavity, and remains for a longer or shorter time (per- manently, in some of the lower Yertebrata, as Myxine) in communication therewith; and since it has further been shown that the peritoneal cavity communicates directly with the lymphatics, and therefore indirectly with the veins ; it follows that the vertebrate kidney is an extreme modifica- tion of an organ, the primitive type of which is to be found in the organ of Bojanus of the Mollusk, and in the seg- mental organ of the Annelid ; and, to go still lower, in the water- vascular system of the Turbellarian. And this, in its lowest form, is so similar to the more complex conditions of the contractile vacuole of a Protozoon, that it is hardly straining analogy too far to regard the latter as the primary form of uropoietic as well as of internal respiratory ap- paratus.
THE NBBYOnS SYSTEM. 63
The Nervous System, — In its essential nature, a neire ii a definite tract of living substance, through which the molecular changes which occur in anj one part of the arganism are conveyed to and affect some other part. Thofl, if, in the simple protoplasmic bodj of a Profcozoon, a sdmulos applied to one part of the body, were more readily transmitted to some other part, along a particular tract of the protoplasm, that tract would be a virtual nerve, althoagh it might have no optical or chemical characters which should enable us to distinguish it from the rest of the protoplasm.
It is important to have this definition of nerve clearly before us in considering the question whether the lowest Miimiila possess nerves or not. Assuredly nothing of the kind is discernible, by such means of investigation as we it present possess, in Ptvtozoa or Porifera ; but any one who has attentively watched the ways of a Colpoda, or still more of a Vorticella, will probably hesitate to deny that they possess some apparatus, by which external agencies give rise to localised and co-ordinated movements. And when we reflect that the essential elements of the highest nervous system — the fibrils into which the axis fibres break up — are filaments of the extremest tenuity, devoid of any definite structural or other characters, and that the nervous system of animals only becomes conspicuous by the gather- ing together of these filaments into nerve fibres and nerves, it will be obvious that there are as sti*ong morphological, as there are physiological, grounds for suspecting that a nervous system may exist very low down in the animal scale, and possibly even in plants.
The researches of Kleinenberg, which may be readily verified, have shown that, in the common Hydra, the inner ends of the cells of the ectoderm are prolonged into deli- cate processes, which are eventually continued into very fine longitudinal filaments, forming a layer between the ectoderm and the endoderm.
Kleinenberg terms these neuro-muscular elements, and thinlrg that they represent both nerve and muscle in their
64 THE ANATOMY OF INYEBTEBHATBD ANIMALS.
nndifferentiated state. But it appears to me tHat while the assumed contractility of these fibres might account for the shortening of the body of the Polype, they can have nothing to do with its lengthening. As the latter move- ments are at least as vigorous as the former, we are there- fore obliged to assume sufficient contractility in the general constituents of the body to account for them. And if so, what ground is there for supposing that this contractility can be exerted by only one tissue when the body shortens ? To my mind, it is more probable that " Kleinenberg's fibres" are solely intemuncial in function, and therefore the primary form of nerve. The prolongations of the ecto- dermal cells have indeed a strangely close resemblance to those of the cells of the olfactory and other sense organs in the Vertebrata; and it seems probable that they are the channels by which impulses affecting any of the ceUs of the ectoderm are conveyed to other «cells and excite their contraction.
The researches of Eimer* upon the nervous system of the Cienophora are in perfect accordance with this view. The mesoderm is traversed in all directions by very fine fibrils, varying in diameter from j^j^j^ to ttoiht ^^ <^ inch. These fibrils present numerous minute varicosities, and, at intervals, larger swellings which contain nuclei, each with a large and strongly refracting nucleolus. These fibrils take a straight course, branch dichotomously, and end in still finer filaments, which also divide, but become no smaller. They terminate partly in ganglionic cells, partly in muscular fibres, partly in the cells of the ectoderm and endoderm. Many of the nerve fibrils take a longitudinal course beneath the centre of each series of paddles, and these are accompanied by ganglionic cells, which become particu- larly abundant towards the aboral end of each series. The eight bands meet in a central tract at the aboral pole of the body ; but Eimer doubts the nervous nature of the cellular mass which lies beneath the lithocyst and supports the eye- spots.
* ' Zoologiflche Studien anf Capri.' Leipzig, 1873.
8BH80BT OBOAKS. 65
The neirous sjstem of the Ctenophoran is, therefore, jost such as would arise in Hydra, if the deyelopment of a thick mesoderm gave rise to the separation and elongation of Kleinenberg's fibres ; and if special bands of snch fibres, developed in relation with the chief organs of locomotion, united in a central tract directly connected with the higher sensory organs. We haye here, in short, virtual, though incompletely differentiated, brain and nerves.
All recent investigation tends more and more completely to establish the following conclusions: firstly, that the central ganglia of the nervous system in all animals are derived from the ectoderm ; secondly, that all the nerves of the sensory organs terminate in cells of the ectoderm; thirdly, that all motor nerves end in the substance of the mnscnlar fibres to which they are distributed. So that, in the highest animals, the nervous system is essentially oimila.T' to that of the lowest ; the difference consisting, in part, in the proportional size of the nerve centres, and, in part, in the gathering together of the intemuncial filaments into bundles, having a definite arrangement, which are the nerveBf in the ordinary anatomical sense of the term.
And as respects the ectodermal ceUs which constitute the fondamental part of the organs of the special senses, it is becoming clear that the more perfect the sensory apparatus, the more completely do these sensigenous cells take on the form of delicate rods or filaments. Whether we consider the organs of the lateral line in Fishes and Amphibia ; the gustatory bulbs ; the olfactory cells ; the auditory cells ; or the elements of the retina, this rulu holds good,
Every one of the organs of the higher senses makes its appearance in the animal series as a part of the ectoderm, the oeHs of which have undergone a slight modification. In the case of the eye, accessory structures, consisting of variaoflly-coloured masses of pigment, which surround the risiial cells, and of a transparent refracting cuticular or f^Xhnl^r stnicture which lies superficially to them — a rudi- ■wntary choroid and cornea — are next added. The highest form of oomponnd Arthropod eye differs from this only in
^ THE ANATOMY OF nrTBBTBBBATBD ANUCALS.
the differentiation of the lajer of sensigenoas ceUs into the crystalline cones and their appendages, and it has not been clearly made ont that the simple eyes of most other Inver- UbraJka have undergone any farther change.
Bnt in NaviiluB the nerre cells and choroid line the walls of a deep cnp open externally ; which, thongh its develop- ment has not been traced, may be safely assumed to result from the involution of the retinal ectoderm. It may be compared to an arthropod compound eye become concave instead of convex.
In the higher Cephabpadat the margins of the ocular pouch unite and give rise to a true cornea, which, how- ever, frequently remains perforated ; and a crystalline lens is developed. In the higher Vertebraia the retina is still a modified portion of the ectoderm^ For inasmuch as the anterior cerebral vesicle is formed by involution of the epiblast, and the optic vesicle is a diverticulum of the anterior cerebral vesicle — it neoessafily follows that the outer wall of the optic vesicle 4s I'eally i>art of the ecto- dtfm, its inner face being, morphologically, a portion of the surface of the body^ The rods and cones of the verte- brate eye, therefore, exactly correspond with the crystalline cones, &c., of the Arthropod eye ; and the reversal of the ends which are turned towairds the light in the Vertebraia, is a necessary result of the extraordinary change of position which the retinal surface undergoes in them%
In the part of the ectoderm which takes on the auditory function, two kinds of accessory organs, solid particles suspended in a fluid and fine hair-like filaments, are deve- loped in close relation with the neirve endings. In the Crudacea both are combined^ and an involution of the sensory region takes place, which usually remains open throughout life, and represents the niost rudimentary form of auditory labyrinth. The Orustaoean ear is the parallel of the Nautilua eye^ In the VerMfraicL, the membranous laby- rinth is similarly an involution of the integument, which remains open throughout life in many Fishes, but becomes ■hut off and surrounded by thick mesoblaatie stractures in
KSPRODUcnrs obgavb. 67
all the higlier VerUbrata, The Uffnpaimtm aad the ossicula audUus are additional aoceesorj stmctares, formed at the expense of the hjomandibnlar cleft and its boundary walls.
Hie Bqirod/iidwe System, — The relation of the reprodne- tire elements to the primitive layers of the germ, is as jet uncertain. E. van Beneden has brought forward verj strong evidence to the effect that in Hydractinia, the spermatozoa are modified cells of the ectoderm, and the ova of those of the endoderm; but whether it can be safely condnded that this rale holds good for animals generally, is a question that can only be settled by much and difficult investigation. The fact that, in the Vertebrata, the ova and spermatozoa are products of the epithelial lining of the peritoneal cavity, and therefore proceed from the mesoblast, appears at first sight directly to negative any such generalisation. But it must be remembered that the origin of the mesoblast itself is yet uncertain, and that it is quite possible that one portion of that layer may originate in the ectoderm and another in the endoderm.
Theare is some reason to suspect that hermaphrodism was the primitive condition of the sexual apparatus, and that unisexuality is the result of the abortion of the organs of the other sex, in males and females respectively.
Yery low down in the animal series, among the TwrbeUaria, the accessory organs of generation acquire a great com- plexity* In the lower Turbellaria the excretory duct is a ave short wide passage. But, in the higher Turbellaria and Timmatoda, the female apparatus presents a germarium, in which the ova are developed ; viteUarian glands, which give rise to a supplemental or food yelk ; an oviduct ; a uterus ad TAgina; and a spermatheca, in which the semen is tered up. The male apparatus presents a testis, a vas and a penis. The function of the viteUarian may be taken on by cells of the ovary, or oviduct; yelk substance may be formed within the pri- ■ikiire omxn itself, in the ^ri^ropoda and in most MoUueea ; hit the vefj^roduotive organs in all these animals are redn- to the Turbellarian type.
t2
68 THE ANATOMY OF IKYEBTSBBATBD ANIMALS.
In the Annelids {OligochoBta and Polychaia), the oyaria and tastes often have no special dnots, and their products make their waj out of the body bj canals which appear to be modified segmental organs.
In the Cephalopoda^ again, the ovaria and testes part with their contents hj dehiscence into chambers connected with the water cavities, which are prolongations of the organs of Bqjanns. And they are conveyed away from these chambers by ducts, the oviducts or vasa def erentia, which commence by open mouths in them.
In the Vertebrata, the reprpductive organs either dehisce and pour their contents into the peritoneal cavity, whence they are conveyed outwards by abdominal por^ (Marsipo- branehii, many Teleottei), or they are continued into ducts which open behind the anus separately from the renal opening in the females, but in common with it in the males (most Teleosteans) ; or their ducts are derived from portions of the prin^iitive renal apparatus which, as we have seen, is a structure of the same order as the organs of Bojanus and the segmental organs. The testes is usually convprtcd into a mass of tubuli, which eventually open directly into the ducts (epididymis, vc^s deferens) derived from the renal org^ans. The ovary, on the other hand, becomes an aggre- gation of sacs — the Graafian f oUideq — and th^ oviducts open into the peritoneal cavity,
Development, — The embryo either passes through all stages from the morula to a condition differing from the adult only in size, proportions, and sexual characters, or it leaves the egg in a condition more or less remote from the adult state, and sometimes exceedingly different from it. In the latter case, the animal is said to undergo a meiamor- phosis. Each of these modes of development occurs in mem- bers of the same group, and often in closely allied forms : as, for example, the former in the crayfish (Astaeus), and the latter in the lobster {Homarus).
When metamorphosis occurs, the larva may live under oonditiona totally different from those under which the ftdnlt paaaee its eodatence, and its stractnre maybe variously
DSYSLOPMSNT.
69
wM>i^^fiw«1 in relation to these conditions. Thus the larva of an *«^w»«1 which is fixed in the adult state maj be provided with largely developed locomotive organs ; while that of an adult which f eedis bj suction maj be provided with power- ful afiparataB for the seiznre and manducation of vegetable and animal prey.
Tha larva of a free adult maj be parasitic, ot ih&t of a parasitic adtilt free and activelj locomotive. Moii-eover, the whole course of development may take place outside the body of the parent, or more or less extensivel j within it ; whence the distinction of oviparous, owwiviparous, and wf^^arous* animals.
Finally, when development takes place within the body of the parent, the fcetus may receive nourishment from the latter by means of an apparatus termed a placenta, by which an exchange between the parental and f oBtal blood is readily effected. Examples of placente are found not only in the higher mammals, but in some Plagiostome fishes and among the Tunieata.
In many Insects and in the higher Vertebrates, the embryo acquires a special protective envelope, the amnion, which is thrown off at birth ; while, in many Y ertebrates, another foetal appendage, the allantoie, subserves the respiration and nutrition of the fodtus.
The strange phenomena included under the head of the Alternation of GknerationSj and which result from the division, by budding or otherwise, of the embryo which leaves the egg, into a succession of independent zooids, only the last of which acquires sexual organs, have already been generally discussedi
* As eggi capable of develop- ment are alive, ttiis terminology is ety Biologically bad; and otwriri- panmt is particularly objection, able, as ail animals bring forth live eggB or that which proceeds from them. But as understood to apply to animals which lay
eggs ; to those in which the eggs are hatched within the interior of the body, without any special foetal nutritive apparatus; and to those in which the young are provided with such an apparatus, they have a certain convenience.
70 THB AN ATOMT OF IHYSBTBBBATBD ANIMALS.
IV.— THB DISTBIBUTIOK OP ANIMALS.
The distribution of aTiinmlH has to be considered under two points of view ; first, in respect of the present condition of natare; and secondly, in respect of past conditions. The hret is commonly termed Cfeographieal, the second Qtologieal, or PakBowMogieal, DistrOmtion, A little con- sideration, howerer, will show that this classification of the facts of distribution is sssentially faulty, inasmuch as many of the phenomena included under the second head are of the same order as those comprehended under the first. Zoologi- cal Distribution comprehends all the facts which relate to the occurrence of animals upon the earth's surface, throughout the time during which animal life has existed on the globe. Therefore it embraces*-
First, Zoologieai Chronology, or the duration and order of succession of living forms in time ; and —
Secondly, Zoological Geography, or the distribution of life on the earth's surface at any given epoch.
What is commonly termed Gleographical Distribution, is simply that distribution which obtains at the present epoch; but it is obvious, that at any given moment in their past history, animals must have had some sort of geographical distribution; and considerable acquaintance with the nature of that distribution has now been obtained for all the epochs, the nature of the livmg population of which has been revealed by fossil remains. I do not propose to deal at length with either branch of .distribution in this place, but a few broad truths which have been established may be mentioned,
Oeogrc^hical Digtrtbtdion ai the present epoch, — ^The Fauna of the deep sea (below 500 fathoms) has been shown by the investigations of Wy ville Thomson and his associates of the ' Ohallenger,' to present a striking general uniformity in all parts of the world hitherto explored, in correspondence with the general uniformity of conditions at such depths.
MABIini DISTBIBUTIOK. 71
Witli reepeot to the surface of the eea, the ohservationB of the same naturalists tend to establish a like uniformity of the great tjpes of Foraminiferal life throughout the tropical and temperate zones — ^with a diminution in the fthandance of that life towards the arctic and antarctic regioma, where it appears to be replaced by Radiolaria and Diatomaceoos plants.
With regard to higher organisms, the oceanic Hydrosoa and the Cienophara are undoubtedly yery widely spread. it is probable that they attain their maximum develop- flwnt in warm seas, though the known facts are insuf&cient for the definite conclusion. SagiUa and Appendiculaaria, with many genei>a of Copepoda, Crtuiacea, and Pteropoda, are of world-wide distribution ; and it is at present doubtful whether any well-marked pfoyinces of the ocean can be defined by the occuirence of purely pelagic animals. On the other hand, shallow-water marine animals fall into aaaemblages characteristic of definite areas or provinces of diMtribuiion — that is to say, though many species have a world-wide distribution, others occur only in particular localities, and certain geographical areas are marked by the existence in them of a number of such peculiar species. The basins of the Pacific, the Indiau Ocean, the Atlantic, the Mediterranean, and the Arctic seas, are thus especially characterised; and even limited areas of these great geographical diyisions, such as the Celtic, the Lusitanian, and the Australian, have their peculiar features.
But though the shallow- water marine Faun® thus follow the broad features of physical geography, and though, with- in each great province of distribution thus marked out, tem- perature and other physical conditions have an obvious influence in determining the range of species; yet, on comparing any two great areas together, differences in cli- matal conditions are at once seen to be inadequate to account for the differences between the Faunae of the two areas. Climate in no viray enables us to understand why the Trigonia, the pearly NawtUus, the Cetiracion^ the eared seals, and the penguins are found in the Pacific,
72
THE ANATOMY OF nfYBHTSBKATSD ANIMAI/S.
and not in the Atlantic area ; * nor why the Cetacea of the arctic and antarctic regions should he as different as they are. When we torn to the distrihntion of land aninlals, the boundaries of the provinces of distribu- tion correspond neithei* with physical features nor with climatic conditions; MatnnialB, birds, reptiles, and am- phibians are so distributed at the present day as to mark out f oui^ great areas or provinces of distribution of very unequal extent, in each of which a number of chlu-acteristic types, not found elsewhere, occur. These are, 1. The Aretogoeal, including North America, Europe, Africa^ and Asia as far as Wallace's line, or the boundary between the Indian and the Papuan divisions of the Indian Archipeki.go. 2. The AuitrOcolwnhian, comprising all the American con- tinent south of Mexico. 3. The Aushxtlian, from Wallace's line to Tasinania. 4. The Novotelanian, including the islands of N^w Zealand.f
There is n6w no doubt that provinces of distribution, closely corresponding with these, existed at the time of the Quatemaiy and later Tertiary rocks. In Eurox>e, North America, and Asia, the Arctogsal province was as distinctly characterised in the Miocene, and probably in the Eocene epoch, as it is at present What may have been the case in Austrocolumbia, Australasia, iuid Novozelania we have no means of being certain, in the absence of sufficient knowledge of the Miocene and Eocene deposits of those regions.
Our present knowledge of the geographical distribution which obtained in the older periods, does not enable us to speak with any confidence as to the limits of the provinces of distribution in the past. But this much is certain, that as far back as the epoch of the Trias — at the dawn of the
* Penguins are found at the Cape of Good Hope and at the Falkland Islands, bnt not in the northern parts of the We»t Coast of Africa, nor of the East Coast of South America. In the Pacific they stretch nortli to the Papuan and Peruvian ooasts.
t On the clasrification and dit-
tribntion of the Aledommorpha and Heteromorp/uB : Proceedings of the Zoological Society, 186S. Plater on the 'Geographical Distribution of Birds.' Ibid, vol. ii. Pucheran, *Revne et Magasin de Zoologie,' 18*^5. Blumy,* The Geographical Dis- tribution of Maminals.'
BXTINCT TBANSmOKAL FOB1C8. 73
Seooadttrj period — ^the Beptilia and Amphibia of Europe, laduk sad South Africa, and probablj North America, pwentod the same kind of resemblance as the mammals ind birdB of the corresponding Arctogseal Fauna do now. But then there is no information respecting the Reptiles and Amphibians of the corresponding epoch in Austro- eohimbia and Australia, so that it is impossible to say whether, in Triassic times, the Arctogseal province was Umited aa it is now.
Outside the limits of the Arotogseal province, the materials for forming a judgment of the distribution of aniroalu are altogether insufficient to enable us to draw any conclnBion as to the existence, and still less as to the boun- daries} of definite provinces of distribution in Paleozoic timesi No remains of land animals have yet been dis- coveredi The fresh-Water fauna consists of Amphibians and Fishes, and we know nothing, or next to nothing of these in any part of the world except the Arctogseal pro- vince.
A good deal is known of the older Silurian fauna outside the boundaries of the present Arctogseal province, and within those of both the Austrocolumbian and Australasian provinces. With a generally similar fades the faunsB of these regions present clear differences. And, considering that the groups of animals which are represented are chiefly deep sea and pelagic forms, it is not wonderful that this similarity of facies should exist; The investigations of the ' Challenger ' expedition show that such forms present a like similarity of facies, at the present day.
One of the most important facts which have been esta- blished under the head of Zoological Chronology, is, that in all parts of the world, the faima of the later part of the Tertiary period, in any province of distribution, was made up of forms either identical with, or very similar to those now living in that area.
For example, the elephants, tigers, bears, bisons, and hippopotamuses of the later tertiary deposits of England are all closely allied to members of the existing ArctogSBal
74 THB AHATOMT OF IKTIItTSBSATED AKIMALS.
fauna ; the great armadilloe, anteaters, and platjrrhine apes of the cavea of South America are a^ cloeelj related to the existing AnstroooloiBbian fauna; and the fossil kang^arooe, wombats and phalangers of the Australian tertiaries to those which now live in the Australasian province.
No remains of elephants occur in Australia, nor kan- garoos in Austrocolumbia ; nor anteaters and armadilloB in Europe in tertiary deposits.
But as we go back in time from the Tertiary to the Secondary, this law no longer holds good. Most of the few terrestrial mammals of secondary age which have been discovered belong to Australasian and not to Arctogseal types, and the marine fauna resembles that of the existing Pacific more than it does that of the Atlantic area, but differs from both in the presence of numerous wholly extinct groups. It looks as if , in the latter part of the Cretaceous epoch, a great change in the limits of the then existing distributional area had taken place, and the types now characteristic of the Arctogseal province had invaded re- gions from which they had before been shut out. And the assumption of a process of a similar character appears to me to be the only rational explanation of the rapid advent of types absent in the palaeozoic deposits known to us, in the earlier Secondary rocks.
Yet other results of first-rate importance have come out of the study of the chronological relations of fossil remains. Cuvier's investigations proved that the hiatuses between existing groups of ungulate mammals tend to be filled up by extinct forms. Later investigations have not only con- firmed this conclusion, but have shown that, in several cases, an existing much modified form can be shown to have been preceded in time, in the same distributional area, by exactly such forms as it is necessary should have existed, if the much modified existing animal had proceeded by way of evolution from a simpler form.
For certain groups of animals, then, there is as much and SB good evidence of their having been evolved by suc- cessive modification of a primitive form as the nature of
TBB OLDB8T SHOWN FAUKA.
75
the ease permitB ub to expect. But the groupe in which Uwfe is eridenoe of such modifications daring geologically raooided tuooie, all belong to the most differentiated members of their clamien Lower forms, co-extensive in duration, ex- hibit no sign of haying undergone any notable modification. While the former are mutable, the latter are penident types m relation to geological time.
Leaving the debateable question of the nature of Eoaoon made, the oldest f ossilif erous rocks are the Cambrian. The Msntj fauna therein preserved consists of forms which are aeitlMr Protoaoa nor Pori/era, nor even appertain to the knrestgrovips of their respective classes. There is no reason to bdiere that it gives a just notion of the contemporaneous fnna* nor is there any vaHd reason for the supposition that it r^ffesents the forms of animal life which were the list to make their appearance on our planet
76 THE ANATOHT OF INYBBTBBllATBD AKIMALS.
CHAITER II.
THB t»BOTO^OA.
In its fd^bleet manifestations, the contraotilitj of animaJB results in niere changes of the form of the body, as in the adult QregarifUB ; but, from the sluggish shortenings and lengthenings of the different diameters of the body which these creatures exhibit^ all gradations are traceable, through those o-Tiimnln which push out and retraci^ broad lobular processes, to those in which the contractile prolongatiouB take the form of long and slender filaments. YiThether thick or filamentous, such contractile processes are called *' pseudopodia," when their movements are slow, irreg^ar. and indefinite ; '* cilia " or " flagellar" when they are rapid and occur rhythmically in a defiinite direction ; but the two kinds of organs are essentially of the same nature. It will be convenient to distinguish those Protozoa which possess pseudopodia, as myxopods, and those which ai^ provideil with cilia or flagella, as nuistigopods.
The Protozoa are divisible into a lower and a higher group. In the former — the Monbba — ^no defiinite structure is discernible in the protoplasm of the body ; in the latter — ^the Endoplastica — a certain portion of this substance (the so-called nucleus) is distinguishable from the rest ; * and, Yety commonly, one or more " contractile vacuoles " ai*e present. The name of oontraetile va€W)le8 is given to spaces in the protoplasm, which slowly become filled with a clear watery fluid, and, when they have attained a certain
* I adopt this distinction as a whether it will stand the test of matter of tmnporarjr convenience, further inv estigatiou. thoogh I entertain great doubt
1
I
THB PROTOZOA. 77
oxe, are suddenly obliterated by the coming together, on all sides, of the protoplasm in which they lie. This systolic and diastolic movement nsnally occurs at a fixed point in the protoplasm, at regolar intervals, or rhythmically. But the TBcnole has no proper wall, nor, in most cases, is any trace of it discernible at the end of the systole. Occasionally, the vacuole certainly communicates with the exterior, and there is some reason to think that such a communication may always exist. The function of these organs is entirely onknown, though it is an obvious conjecture that it may be respiratovy or excretoiy.
The ** nucleus " is a structure which is often wonderfully similar to the nucleus of a histological cell, but, as its identitj with this is not fully made out, it may better be tennad "endoplast." It is, usually, a rounded or oval bod^ embedded in the protoplasm, and but slightly dif- ferent therefrom in either its optical or chemical characters* Generally it becomes more deeply stained by such colouring matters as hBematoxylin or carmine, and resists the action of acetic acid better than the surrounding protoplasm.
In a few Protozoa there are many endoplasts in the sub- gtajice of the body, and the protoplasm shows some tendency to become partially differentiated into cells. But where, ■8 in the higher Infusoria, the body presents a definite organisation, with permanently differentiated constituents, which may be properly termed tissues, these tissues do not result from the metamorphosis of cells, but originate from the protoplasm directly by changes of its physical and <^iemical characters.
Conjugation* followed by the development of germs, which are set free and assume the form of the parent, has been observed in several groups of the Protozoa, but it is not yet quite certain how far sexual distinctions are esta- blished among these animals.
78 THE ANAT(1MY OF INVERTKBRATED ANIMALS.
I. — THB ICONEBA.
In these lowest forms of aniinalB the entire liying body consists of a particle of gelatinous protoplasm, in which no nucleus, contractile vacuole, or other definite structure is visible ; and which, at most, presents a separation into an outer, more clear, and denser layer— the eeto9are; and an inner, more granular and fluid matter — the endo9arc. The outer layer is the seat of active changes of form, whereby it is produced into pseudopodia» which attain a certain length and are then retracted, or are effaced by the development of others from adjacent parts of the body. These pseudopodia are sometimes broad short lobes, at others, elongated filaments. When lobate, the pseudopodia remain distinct from one another, their margins are clear and transparent, and the granules which they may contain plainly flow into their interior from the more fluid central part of the body. But, when they are filiform, they are very apt to run into one another, and give rise to networks, the constituent filaments of which, however, readily separate and regain their previous form ; and, whether they do this or not, the surfaces of these pseudopodia are often beset by minute granules, which are in incessant motion — ^like those which are observable on the reticulations of the jxrotoplasm of the cells in a TradescanHa hair.
The myxopod thus described moves about by means of its contractile pseudopodia, and takes the solid matters which serve as its food into all parts of its body by their aid; while the undigested exuvia of the food are r^ected from all parts of the body in the same indiscriminate way. It is an organism which is devoid of any visible organs except pseudopodia; and, so far as is known at present^ it multiplies by simple division.
The Protamcoba (with lobate pseudopodia) and Protogenes (with filamentous pseudopodia) of Haeckel are Monera of this estremely simple character. In Mymodiatywn (Haeckel) the pieodopodia of a number of such Monera run together,
THB MONBSA. 79
and give rise to a complex network, or common plas- modium.
It IB open to doubt, however, whether either Froiamoeba, Ptotogenet, or Mymodieiywm is anything but one stage of a cjcle of forms, which are more completely, though perhaps not jet wholly, represented by some other very int^esting MomerOt also described by HaeckeL
ThiUy the genus VamvpyreUa is a myxopod with filamen- tous pseudopodia, a species of which infests one of the stalked Diatomacesa, Chmphonema, feeding upon the soft parts of the frustules of its host, by inserting some of its pseudopodia through the raphe of the frustide, which it enrelopee, and absorbing the contained protoplasm* Having thus provided itself with abundant noiuishment, by creep- ing fromfrustule to frustule of ih<^fihmphonemaf it thrusts aside the last evacuated frustule from its peduncle, and, taking its place, draws in its pseudopodia, becomes sphe- rical, and surrounds itself with a structureless cyst, enclosed in which it remains, perched upon the peduncle of the Qnfmphcmema, Soon, its protoplasm undergoes division into four equal masses, and each of these becoming converted into a young VanupyreUOf escapes from the cyst, and re- oommences the predatory life of its parent. In this case, Uie myxopod becomes encysted, and then undergoes fission into bodies, each of which passes directly into the form of the parent.
In another genus (JfffHXiiinim) an additional complication is introduced ; the myxopod becomes encysted, and then divides into many portions ; each of these elongates, and mrrounds itself with a delicate, fusiform, silicious case. Thus enclosed, the germs are set free by the bursting of the cyst; and, after a while, the contents of the silicious cases emerge, and pass at once into the myxopod state.
In other genera, not only does the myxopod become encysted, before it undergoes fissive multiplication, but the forms thus produced differ from the myxopod in being fr^ee-swimming org^anisms, propelled by a long vibratile fnifcTpjmt or flagellum, like those flagellate If^vMoria which
TBI ABATOHT Or HTVXBTEBKATKD A
I. THB MONERA.
In Uieae lowest forms of ftnimala the entu conaista of a particle of gelatinous protoplaai nncl^is, contractile vacnole, or other definil -risible; and which, at moat, presents a sepft outer, more clear, and denser lajer— the Ml inner, more granular and fluid matt«r — The outer lajer is the seat of active cImi wherebj it is produced into pseudopodia, w certain leoigdi and are then retracted, or an derelopment of otliers from a^acent parte These psendopodia »re sometimes broad d others, elongated filaments. When lobate, tti remain .WhuuA Fn>iu .^n^- imolh^-r. iWir lunq and transpureut, anil tlio ^ujiule^ which thl{ plainly flow into their int^arior from the t part of the body. But, when they are filifof! apt to nm into one another, and give rise it conatituent filamente i.>f which, kowever, t
rHE ARA^TOmr OT UmSTEBBATCD AiriMAI.8.
Tig. 1.
„ _ . , , , Bckel),— o, the ilill condition ior-
Toandad b* k itmcturelsn syit ; l>, encntcd ferm, tb« pRiIoplum of which U diTidiDg; c, ths otR biintinf[ ud giving exit to tin bodin Into whioli the prolopltim bntkt ap. There m at fint * moittdl,* d, neh being pravlded with > flsgcllilbna dUuin, b; mean*
THI FOBAMINIF£BA« 81
of which it propels itself (d). After a time each monad retracts its cilium and resumes an Amoeba-like form ; (e) many of these coalesce and form a single Plasmodium, which grows and feeds under the form /. The specimen figured contains a Ffridinium (above), three IHetyocysUe (below), and two IsthmuB (Diatomaceous plants) in the centre. (Haeckel, * Studien iiber Moneien,' 1870.)
are termed " monads." After swimming about for a while, these mastigopods draw in their flagella, and become creep- ing myxopods. This cycle of forms is exhibited bj the genus Protomonaa of Haeckel. Lastly, in Proiomyxa (Fig. 1) (Haeckel), there is an alternation of a mastigopod {d) with a myxopod form (e), as in Protomonas. But each myxopod does not usually become encydted alone. On the contrary, a certain number of the myxopods unite together, and become fused into an active Plasmodium (/), which exhibits no trace of their primitive separation. The plasmodium becoming qxdescent and spheroidal, surrounds itself with a structureless cyst (a), divides into numerous portions (5), which are converted into flagellate mastigopods, and these finally return to the myxopod condition (c, d, e). The cycle of life is here singularly sinulai' to that presented by the Myxomycetes, which have hitherto been usually regarded as plants.
There is no means of knowing whether the cycle of forms presented by Protomonas and Protomyxa is complete, or whether some term of the series is still wanting ; and con- sidering how low down among plants the sexual process occurs, it seems quite possible that some corresponding sexual process yet waits to be discovered among the Monera. It is possible that the fusion of separate Myxodictya and Protamyza into a plasmodium may be a process of sexual conjugation. On the other hand, it may well be that these extremely simple organisms have not yet reached the stage of sexual differentiation.
The FoBAMiNiPEBA. — ^Doubtless many Monera remain to
be discovered, but they will probably be minute and inconspi-
caooB organisms like the majority of those already described.
The Fcraminifera, on the other hand, are Monera of the
Protof^enea type, which, nevertheless, play and have played
o
Ha TBI AlTXTOn or mnsTIBBATBD anhialb.
on important part in the liistory of th« ^lobe, by reason of their power of fabricating Bkeletons or ahelle, whicb maj be compoeed of homj (chitinoaa Pj matter, or of carbonate of lime, secreted from the water in which they live, or may be fabricated by sticking together extraneotia matters, sncb aa porticlea of aand.
The first step from such an organism as Prologeaet to the Foramin^era is seen iath6LUi>erkiihnia of Claparede, where the peeudopodia are given off from only a small part of the Borface of the body, the rest remaining naked and flexible.
In Oromia there is a similar retrtnction of the area from
T]g a.
which psendopodia proceed, bat the rest of the body is in- vested by a case of a membranous eubstance. Let this case become hardened by the attachment of foreign bodies — as particles of sand, or fragments of shelly matter, as in the so-called arenaoeons Foratnitt\fera, — or let a deposit of calcareons salts take place in it, and the Gromia would be converted into a Foraminifer.
The infinitely diversified characters of the skeleton of the fbraminiftra depend — firstly, upon the structure of the skeletal enbatonce itself ; and, secondly, upon the form of the protoplasmic body, which last, again, is largely depen-
TBI voKixnrirsaA. S3
dent upon the numner in whick *ncc«saiTe bods of proto< plasm are developed from the parent mau, which, to begin wiUi, a alwaje rample in form and oommoiilf globular.
The enbstance of the calcareoas skeleton itaelf, whatever be its form, is dtlier perforated or imperforate. In the 7m- perforata (OnHntdcE, LUuitida, IfUiolida) the psendopodia are protruded from on!; one end of the body, the rest of which is cnt off from the ecterior by the skeleton. In Ute Pmforata, the rabstance of the Bhell is traversed bj more or less delicate canals, filled with the protoplasm,
Fig. 3
'#-^
fig- S.— DUgimna of Fi>ramiiiifera.~~A, monothftUinlaii ; B, C, poly- thkUmJu; D, horiiontal; E ukd F, vvrllcal woliaiii of hel]a>id fbtrn. la E, the otiMnben of e«oh torn of Che iplnl oTerlap their pndxetmon md conceal them, W in the gentu NnmmHlita.
which thus reaches the sorface and gives off psendopodia all over the bodj. Hence, while the hard parts of the Imparforata form a sort of exo-skeleton, those of the Perforata have rather the nature of an endo-skeleton.
The simplest skeletons are spherical or flaaV-sbaped, and single-chambered. Bnt complication ariees bj the addition of new chambers, which maj form a linear series, or becoiled npou one another in various ways, or be irregnlarly aggre- gat«d. Horeover, the new chambers maj overlap those a 2
84 THE ANATOKT OF IKTBSTBBBATED ANIMALS.
already formed in different degrees, and the interspaces between the walls of the chambers maj be Tariouslj filled up by secondary deposition, until such large and apparently complicated bodies as the Nummulites are built up.
The Foraminifera are almost all marine animals, living in the sea, from the surface to great depths, sometimes free» and sometimes attached to other bodies.
The investigations of Major Owen, confirmed and extended by the recent work of H.M.S. * Challenger,* have proved that such forms as Ghbigerina, PvlvinulcMria, and Orbulina constantly occur at the surface of all temperate and tropical seas, and, together with the Radiolaria and the Diatomaceous plants which accompany them, form an important ingre- dient in he food of pelagic animals, such as the Sdlpce.
It is no less certain that at all depths down to 2400 fathoms or thereabouts, Ghbigermce in all stages of growth and containing more or less protoplasmic matter are found at the bottom, mixed with the cases of the surface Diatoms and the skeletons of Badiolaria, The proportion of Olohi- geriruB, OrbvlincB, and PtUvintdarice in the deep-sea mud increases with the depth, until, at depths beyond 1000 fathoms, the sea- bottom is composed of a fine chalky ooze made up of little more than the remains of these Foramini- fera and their associated Diatoms and Baddolaria,
It may be regarded as certain, therefore, that some of the chalky ooze arises from the precipitation to the bottom of the skeletons of dead GhligerincB, PtUvinularicB and OrhulincBy and it may be that the whole has this origin. On the other hand, it may be that a greater or smaller pro- portion of these Foraminifera really live at the bottom, as their congeners are known to do at less depths.
It has been said that the condition of the surface waters and sea-bottom which has just been described, obtains in all temperate and hot seas ; or, speaking roughly, for 55^ on either side of the equator. Towards the northern and southern limits of this zone the Foraminifera diminish, while BadioUma remain and IHaUmuLeem increase in proportion, 80 that, in the circnmpolar areas north and aouUi of 60° in
PROTOZOA AS BOCK-BniLl>£B8. 85
each liemispliere, the siirf ace organisms are chiefly such as have silicious skeletons. In accordance with this condition of the Borface life, the ooze coyering the sea-bottom in these r^ons is no longer calcareous but silicious, being composed of the cases of Diatoms and the skeletons of Badiolaria often largely mixed with ice, drifted mud, stones, gravel, and boidders.
If we suppose the globe to be uniformly covered with an ocean 1000 fathoms deep, the solid land forming its bottom would be out of reach of rain, waves, and other agents of degradation and no sedimentary deposits woidd be formed. But if Foraminifera and Diatoms, following the same laws of distribution as at present obtain, wert) introduced into this ocean, the fine rain of their silicious and calcareous hard parts woidd commence ; and a circumpolar cap of silicious deposit would begin to make its appearance in the north and in the south ; while the intermediate zone would be covered with Ghhigerina ooze, containing a comparatively small proportion of silicious matter. The thickness of the calcareo-silicious and silicious beds thus formed would be limited only by time and the depth of the ocean. These strata, once accumulated, would be liable to all those influences of percolating moisture and subterranean heat, which are known to suffice to convei-t silicious matters into opal, or quartzite, and calcareous matters into the various forms of limestone and marble. And such metamorphic agencies might more or less completely obliterate the traces of their primitive structure.
But yet other changes might be effected. At the present day, in the Gulf of Mexico, off the Agulhas bank, and else- where, at no great depths (100 to 300 fathoms) the Fora- minif eral mud is undergoing a metamorphosis of another character. The chambers of the Foraminifera become filled by a green silicate of iron and alumina, which penetrates into even their finest tubuli, and takes ex- quisite and almost indestructible casts of their interior. The calcareous matter is then dissolved away, and the casts are left, constituting a ^q dark sand, which, when
86 THE ANATOMY OF nTYBBTEBBATED ANIMALS.
crashed, leayes a greenisli mark, and is known as * Green- sand.'
Moreover, the researches of the ' Challenger ' have shown, that in great areas of the Atlantic and Pacific oceans over which the sea has a depth exceeding 2400 fathoms — areas in some cases of many thousand square miles in superficies — ^the bottom is covered not by Glohigerina ooze, but by a fine red clay, which is also a silicate of iron and alumina. In this clay no remains of Gtohigerina or other calcareous organisms are found; but where these great depths graduidly pass into shallower water, they make their appearance in a fragmentary condition — graduaUy be- coming more and more perfect as the depth diminishes to 2400 fathoms or thereabouts.
Nevertheless the OlobigerinoB and other Foraminifera abound at the surface over these areas as they do elsewhere, and their remains must be rained down upon it. Why they disappear, and what relation the red clay mud has to them, is a problem not yet satisfactorily solved. It has been suggested that they are dissolved and that the red clay is merely the insoluble residue, left after the calcareous portion of their skeletons has disappeared. In this case the red clay, like the Ohbigerina ooze, the silicious mud, and the greensand, will be an indirect product of living action.
Metamorphic processes operating upon clay, however, may convert it into slate ; and thus, all the fundamental minerals of which rock masses are composed may have formed part of living organisms, though no trace of their origin may be discernible in them in their final state.
FalsBontology lends much support to the view that what is here suggested as a theoretically possible origin of much of the superficial crust of the globe, may have been its actual origin.
The nummulitic limestones of the Eocene epoch cover an enormous area of central and southern Europe, North Africa, West Asia and India. And their chief mass is made up of the more or less metamorphosed remains of Foramiwifera.
PROTOZOA JL8 BOCK-BUILDEBB. 87
The beds of chalk which underlie the nummnlitic lime- stones, and occnpj a still greater area, are essentially identical with the Olohigerina ooze, the species of Glohi' gerina found in it being indistinguishable from those now living. The remains of Foraminifera have been detected in the limestones r>f all epochs as far as the Silurian, and Ehrenberg discovered that an old Silurian greensand, near St. Petersburg, is composed of casts of Foraminifera just such as are now being formed in the Gulf of Mexico. And if the Eozoon canademe be, as it appears to be, nothing but an encrusting form of Foraminifer, the existence of these organisms is carried back to an epoch far beyond that at which any other evidence of life has yet been found. So that it is possible that, as Wyville Thomson has suggested, the enormously thick "azoic" slaty and other rocks, which constitute the Laurentian and Cambrian formations, may be to a great extent the meta- morphosed products of Foraminif eral Hf e.
Hence the words of Linnseus may be literally true—
** Petrefacta non a calce, sed calx a petrefactis. Sic lapides ab animalibus, nee vice versa. Sic rupes saxei non primsevi, sed temporis filias."
And there may be no part of the common rocks, which enter into the earth's crust, which has not passed through a living organism at one time or another.
n. — THE ENDOPLASTICA.
1. The Radiolabia. — Most species of the genus Actino- phry8 or " sun-animalcule," which is common in ponds, are simply free swimming myxopods with stiffish pseudopodia, which radiate from all sides of the globular body. The sab- stance of the latter presents one or more " contractile spaces " or " vacuoles," which, rhythmically, become distended vrith water, and are then obliterated by the contraction of the surrounding protoplasm. But in the Actinophrys (or more properly AeHnaaphcBrivmi) Eiehhomii (Fig. 4), the central part of the protoplasm is distinguished from the rest by
THB XHATOlfT OF
ooDtaiuing a number of endoplasta. It thos leads to the BadiotiOria [FolyaMitia of Ehienbei^), the eimplest forms of
Fig. 4.—ActiMoipliariirm EiMonui (»HtT Bertwig knd Lener, ' lleber
Bhliopaden,' SehnlK's Aiohi*, 1676). I.— The endre anlinkl ; r, c, eoutnctlle vacnala. TI._Put of the petiphcrf maoh nugnlfled -, a, a, a, paeudopodu wilh
■UffuUI nibituice; ■, nuclei or endoplMti. in. — A va7 young Adno^iliiirmai, wiUi onljr tiro micld Uid tiro
pModopMls, much nxgnlfied.
wludi ooiuUt Msentiallj of a myxopod, prorided with filamentoufl, radiating, and often anastomoHiiig, peendo-
TBI kU>IOLUtUL.
podia, ntecflntreof the body iaooonpiedbj a capsule filled
wiih protoplaemj this Bometdmen containa onlj an oil-
fig i.
— A, a nun oftha n: tiro of tbe oral ecntral ma wlUi tha onlonrad vetlo
Flc.6.
1
-^
"»:"«'^
-^Si^'i^""^
1)
Flf . 6.~(!pAiEn»owK
(mfter Hteckel), magnified.
globule, at others cella or nnclei, and ci^BtaUine bodies. In the layer of protoplaam from which the paeudopodia proceed,
90 THE AKATOMT OF ZNYBSTEBBATED ANIMALS.
cellffif orm bodies of a briglit jellow colonr, whicli liave been found to contain starcli, are usaally developed,* and this layer also gives rise to a skeleton of a homy, or, more osuallj, silicions cbaracter, which may have the form of detached spicula, or of coarticulated rods, or of networks, or of plates of silicions matter, often of the most exquisite delicacy and beauty. Most of the Eadiolaria are simple, solitary, and microscopical in size ; but some, such as CoUoaphcBra and 8ph€Bro%oum (Figs. 5 and 6), are formed of aggregates of such simple forms, and float, as visible gelatinous masses, at the surface of the sea, which is the habitation of the great majority of the Badiolaria,
The manner of multiplication and the development of the Rcfdiolaria have not yet been thoroughly worked out. Gienkowsky, however, has observed, in CoUosphasra, that the protoplasm contained in the central capsule breaks up into numerous rounded masses. The several capsules which are associated together in the compound Badiolarian then become isolated, by the dissolution of the protoplasm which invested and connected them, and finally burst, giving exit to the rounded bodies; which, while yet within the capsules, were observed to be in active motion. The germs (for such they appear to be) thus set free are 0'008 mm. long, ovate, and carry two flagellif orm cilia at their nar- row ends; so that they are 'monads.' Each has in its interior a crystalline rod and a few minute oil-globules. The further development of these mastigopods has not yet been traced; but if , as is probable, they pass into young Eadiolaria (which, according to Haeckel, possess no capsule, but resemble ActinospIuBria), the Badiolaria, as members of the Endoplcuiica, would typify ProtoTnonas among the Monera, Neither conjugation nor fission has been observed among the ordinary Eadiolaria, but both these processes take place in AcHnosphoBriwen, ; and, consider- ing the resemblance of the young Badiolaria to ActinO'
* Even after the death of the multiply, and the possibility that Badiolaiian, these jellow oellf are they may be parasites most be said by Gifliikowiky to thrive and borne in mind.
THB PBOTOPLA0TA. 9l
iphcerium, it seems probable that coigugation and fission will yet be discoyered among them.
AetinosphcBrium has been observed to undergo multiplica- tion, hj division of its central substance into a certain number of spheroids, and every spheroid becomes enclosed in a silicious case. After a period of rest, a young Actmo- wphiBrivmih emerges from each of these cysts.
The marine Badiolaria all inhabit the superficial stratum of the sea, and must fabricate their skeletons at the expense of the infinitesimally small proportion of silex which is dissolved in sea-water ; but when they die, these skeletons sink to the bottom, and there accumulate, together with the Fcraminifera^ in warm and temperate regions; and with the cases of the diatomaceous plants, which abound at the surface, along with the B^Mdiolaria, all over the globe (see p. 85). The late investigations of Archer and others have demonstrated the existence of a considerable number of fresh- water Badiolaria,
Extensive masses of tertiary rock, such as that which is found at Gran, and that which occurs at Bissex Hill, in Barbadoes, are very largely made up of exquisitely preserved skeletons of Badiolaria. But, though there can be little doubt that Badiolaria abounded in the cretaceous sea, none are found in the chalk, their silicious skeletons having probably been dissolved and redeposited as flint.
2. The Peotoplasta. — The proper Avuj^hb have broad and ovate pseudopodia, and resemble Frotamaba (p. 78) very closely ; but they present an advance upon its structure by exhibiting a distinct endoplast (nucleus) and a contractile vacuole. In ArceUa, there are many such nuclei. They thus stand in somewhat the same relation to FrotamoBba as Actvnaphrya does to Frotogenes.
Moreover, there are AmcebcB in which the power of throw- ing out pseudopodia is confined to one region of the body ; and others, as Arcella, in which a shell is formed over the rest of the body. In other Amcebce, as A. radiosa, the pseu- dopodia are few, narrow, and but little mobile. But the AmodbcB present no such diversity of skeletal develop-
92 THE ANATOMY OF ZNYBBTBBBATBD ANIMALS.
ment as the Foraminifera do. They multiply by division, and in some cases — e.g., A. sphcaroeoecua of Haeckel — become encysted before they divide.
AtruBlHB (the " proteos animalcules '* of the older writers) are not uncommon, and sometimes are very abundant, in fresh waters ; they also occur in damp earth and in the sea, but there is much doubt whether many of them are to be regarded as independent organisms, or whether they are not rather stages in the development of other animals or even of plants, such as Myxomycetee. Leaving out the contractile vacuole, the resemblance of an Amoeba in its structure, manner of moving, and even of feeding, to a colourless corpuscle of the blood of one of the higher animals is par- ticularly noteworthy.*
3. The Gbegabinidje are very closely allied to the AmcdHB, but, in the cycle of forms through which they pass, they curiously resemble Myxastrum, In form, they are spheroidal or elongated oval bodies, sometimes divided by constrictions into segments. Occasionally, one end of the body is produced into a sort of rostrum, which may be armed with recurved homy spines.
In the ordinary GregarincB, the body presents a denser cortical layer (ectosarc) and a more fluid inner substance (endosarc), in which last the endoplast (nucleus) is imbedded. The presence of contractility is manifested merely by slow changes of form, and nutrition appears to be effected by the imbibition of the fluid nutriment, prepared by the organs of the wTiiTn5i.1a in which the QregarinoB are parasitic. There is no contractile vacuole.
The GregariruB have a peculiar mode of multiplication, sometimes preceded by a process which resembles conjuga- tion. A single Gregarina (or two which have become applied together) surrounds itself with a structureless cyst. The nucleus disappears, and the protoplasm breaks up (in a manner very similar to that in which the protoplasm of a sporangium of Mucor divides into spores) into small
* Gontraetile vaenoles have been observed in the colourlete blood MTpnsclet of Awnpkiinu under certain oonditiont.
TBI aBKaAAurisx. 93
bodiea, each of which acqnirefl a Bpindle-ehaped case, and ia known as a pasudo-nameella. On the bnrsting of the cjnt theoe bodies are net free, and, when placed in favour* able circnmatancea, the contained protoplasm eacapea aa a amaU active body like a Prolam<eba. M. E. van B^ieden has recently diacorered a vei; large Greganva [Q. gigantea), which inhabits the intestiiie of the lobster, ojid hia careful
Fig. 7.
paoida-iuTieellK ; G, H, fr(« uniebiform
iiiTestigatioti of its Htmctnre and derelopment has yielded very interesting reaolts.
Qregarina giganUa attains a leng;th of two-thirds of an inch. It is long and slender, and tapers at one extremity, while the other ia obtnse, roonded, and separated by a slight constarictum fnnn the rest of the body, which is i^lindroidaL
94 THE ANATOlfT OF IKYEBTEBBATED AKIHALS.
The outer inyeBtment of the body is a thin stractureless onticle ; beneath this lies a thick cortical lajer (ectosarc), dis- tinguished bj its clearness and firmness from the semifluid central substance (endosaro), which contains many strongly refracting granules. In the centre of the body, the ellipsoid " nucleus," with its " nucleolus/' fills up the whole cavity of the cortical layer, and thus divides the medullary substance into two portions. The body of this Qregarina may present longitudinal striations, arising from elevations of the inner surface of the cortical layer, which fit into depressions of the medullary substance; but these are inconstant. On the other hand, there are transverse striations which are constant, and which arise from a layer of what are ap- parently muscular fibrillee, developed in a peripheral part of the cortical layer, immediately below the cuticle. The fibrills themselves are formed of elongated corpuscles joined end to end. A transverse partition separates the cephalic enlargement from the body, and the layer of muscular fibres only extends into the posterior part of the enlargement.
The embryos of Gregarina gigantea, when they leave their pseudo-navicellse, are minute masses of protoplasm similar to Protamoebce, and like them devoid of nucleus and contractile vacuole. They soon cease to show any change of form, and acquire a globular shape, the peri- pheral region of the body at the same time becoming clear. Kext, two long processes bud out from this body; one is actively mobile, the other stilL The former, detaching itself, assumes the appearance and exhibits the motions of a minute thread- worm, whence M. van Beneden terms it a pseudo'filafia. The enlargement at one end becomes apparent, the pseudo-filaria passes into a quiescent state, and the " nucleolus " makes its appearance in its interior. Around this a clear layer is differentiated, giving rise to the ** nucleus," and the pseudo-filaria passes into the con- dition of the adult Qregarina gigantea.
4. The Oatallacta of Haeokel, represented by the genus Magottphcera^ are, in one Btage, myxopods with long pseudo-
TBI HfTUBOBU. 95
>du, whicli, broad and lobe-like at the haae, break up into
le filaments at their ends, and tnaj therefore be said
be intenuediatfl between tho«e of Protogenea and those
Prolammba. Tike myiopod is provided with a dUtinct
idoplaet and a well-marked contractile apace. When
ilj fed, it eecretea a cjet and diridea into a number of
aasea, each of which it converted into a conical bodj,
ith its baoe tnrned ontwarda and ita apex inwards. These
inical bodiea are imbedded in a gelaiinoas matter, and
iHs cohere into a ball, from the centre of which tbej
diat«. Each derelopea cilia around ita baae, and contain a
1 endoplast and a contractile vacnole. After the complex
obe thus formed has bnrst ita envelope, it awima about
r a while, like a Folttw. The several ciliated animalcnlei
ed bj taking in solid particlea thronj^h the disk. Tbej
m aeparote. and, finallj, retracting their cilia, become
lopoda ench aa thoae with which the aeriea Htart«d.
goaphcera ia thus very nearly an endoplaatic repetition
the moneran Frotomonaa — the maatigopod being pro-
d with many small cilia, instead of with a couple of
1 fla^ella. On the other hand, the Calallacta are closely
1 to the next group, and, I am diapoaed to think,
t well be included in it.
rhe IirrusoBU. — Excluding from the miscellaneous
blage of heterogeneous forms, which have passed
this name, the Deamidia, Dialomaeeas, VolvociTteiB, and
nda, which are true plants, on the one hand ; and
uparatirely highly organised Rotifera, on the other;
tmain three assemblages of minute organisms, which
conveniently comprehended under the general title
iori&. These are — (o) the ao-called " Monads," or
t jbxgdUita ; {b) the AotneUE, or lafaaoria tetttaca-
jd (c) the Infmaria ciliata.
e Plaoxllata. — Theae are characterised by pos- nly one or two long whip-like cilia, sometimes -e than one are present] situated at the same end r, eometimea far apart. The body very generally endoplaat and a contractile vacuole. There is
96 THE ANATOMY OF IHnTEBTEBBATED AKIMALS.
no permanently open oral aperture, but there is an oral region, into which the food is forced, and, passing into the endosarc, remains for some time surrounded by a globule of contemporaneously ingested water — a so-called "food racuole." Professor H. James Clark, who has recently carefully studied the FlageUatay points out that, in Bicosceea and Codonoeea, a fixed monadif orm body is enclosed within a structureless and transparent calyx. In Codosiga a similar transparent substance rises up round the base of the flagel- lum, like a collar. In Salpingceca the coUar around the base of the flagellum is combined with a calycine inrest- ment for the whole animal In Anthophyaa, there are two motor organs — ^the one a stout and comparatively stiff flagellum, which moves by occasional jerks, and the other a very delicate cilium, which is in constant vibratoiy motion.
The discrepancy between the two kinds of locomotive organs attains its maximum in Anisonema, which presents interesting points of resemblance to Noctiluca,
Multiplication by longitudinal fission was observed in Codosiga and Ardhophysa, and probably occurs in the other genera. In Codosiga the flagellum is retracted before fission takes place, but the body does not become encysted; in Anthophysa the body assumes a spheroidal form, and is surrounded by a structureless cyst, before division occurs.
Conjugation has not been directly observed among most of the Infusoria flageUata, nor do any of them exhibit a structure analogous to the endoplastule of the CUiata.
Messrs. Dallinger and Drysdale have recently worked out the life history of several flagellate " Monads," which occur in putref3ring infusions of fish. They show that these FlageUata not only present various modes of agamic multi- plication by fission, preceded or not by encystment, but that they conjugate, and that the compound body which results (the equivalent of the zygospore in plants) becomes encysted. Sooner or later, the contents of the cyst become divided either into comparatively larg^ or excessively minnte bodies, which enlarge and gradually take on the form of the parent.
THE FLAOELLATA. 97
The careful inyestigatioiiB of these authors lead them to oiclnde that, while the adult forms are destroyed at from L**-8(P C, the excessively minute sporules which have been mentioned, and which may have a diameter of less than \^^^(i of an inch, may be heated to 148^ G. without the astruction of their vitality.
In Euglena viridis (which, however, may be a plant) tein * has observed a division of the " nucleus " to take lace, whereby it becomes converted into separate masses, >me of which acquire an ovate or fusiform shape, sur- ^unding themselves with a dense coat, while others become un-waUed sacs, full of minute granules, each of which is rovided with a single cilium. The ultimate fate of these )die8 has not been traced.
A careful study of the singular genus Noctiluca, led me. 1 1855, to assign it a place among the Infusoria^ and recent vestigations have conclusively proved that it is one of the ageUata.
The spheroidal body of Noctihtca miliaris (Fig. 8) is about
^eightieth of an inch in diameter, and, like a peaob, pre-
ts a meridional groove, at one end of which the mouth is
ated. A long and slender, transversely striated ten-
2 overhangs the mouth, on one side of which a hard-
^ed ridge projects. Close to one end of this is a vibratHe
m. A funnel-shaped depression leads into a central
of protoplasm, connected by fine radiating bands with
er of the same substance which lines the cuticular
>pe of the body. There is no contractile vacuole, but
il endoplast lies in tlie central protoplasm. Bodies
are ingested are lodge<l in vacuoles of the latter until
■e digested.
rding to the observations of Cienkowsky.t if a
w be injured, the body bursts and coUapses, but tbo
ismic and other contents, together with the tentacle,
irregular mass, the periphery of which eventually
nismas der Infusions- (Schiilze's *Archiv fur Mikro M. skop. Anutomie,' 1872.)
tr Koctiluca miliaria.**
98 THE AHATOHY Of IITTBItTEBIUTSD A1IJ11A1£.
becomes vacuolated, enloi^ea. and secretes a new inveat- ment. But even a small portion of the pixitoplasm of a mutilated NoetUvca is able to become a, perfect animal. Under some conditions, the tentacle of a Noctiluea maj be retracted into the bod;, and, at all times of the jear. spheroidal NodUuca, devoid of flageUum, tooth, or meridio- nal groove, hut otherwise normal, are t« be found. These last are probabl; to be regarded as encjBt«d form«. Multi- plication ma; take place in at least two wajs. Fission majr
Fig. 8. — ffncfiltica «i7iari(.— «, Guirin vacuule; jr, radiating Elunents; ' ""■[ aterture. (f)
occur in the spbcroidsl forma, aa well as in those poBsessed of a tentacle ; it is inaugurated bj the enlargement, con- striction, and division of the endoplajst. This process takes plac« more especially in the latter part of the year.
Another mode of aaemal multiplication, which has a sin- gular resemblance to the process of partial jelk division, occun onlf in the spheroidal NbctUveee. The endoplaat Aisftppesra^ and th^ proto^asmt sconmnlating on the inner
THX FLAOELLATA.
i of one region of the cnticle, divides first into t Q four, eight, sixteen, thirty-two, or more masses : t !sion of the protoplasm being accompanied by the ele\ I of the cuticle into protaberances, which, at first, con nd in number and dimensions with these division masse len the division masses have become very numerous b protrudes upon the surface, and is converted into i i monadiform germ, provided with an endoplast, a beak, i a long tentacle, which is hardly to be distinguished from agellif orm cilium.
lie process of conjugation has been directly observed. 0 NoctiliteoB, applying themselves by their oral surfaces, lere closely together, and a bridge of protoplasm con- ting the endoplasts of the two becomes apparent. The taenia are thrown off, the two bodies gradually coalesce, ' the endoplasts fuse into one. The whole process ipies five or six hours. Spheroidal or encysted Noctllucce conjugate in a similar manner. In this case, the re- 3 nearest the endoplasts are those which become applied her. Whether this process is of a sexual nature, or 3 not clearly made out. Cienkowsky admits that it lasten the process of multiplication by monadiform described above.
Uuca is extremely abundant in the superficial waters
ocean, and is one of the most usual causes of the
>rescence of the sea. The light is given out by the
•al layer of protoplasm which lines the cuticle.
Tridinece (see Fig. 1./} form another aberrant group of
Uata, which lead to the Ciliata. The body is enclosed
case (sometimea produced into rays), which, at one
ents a groove-like interruption, laying bare the con-
>toplasm, in which lies an endoplast, and in some
itractile vacuole. One or more flagelliform cilia,
Y a wreath of short cilia, are protruded from the
, and serve as locomotive organs. The mouth is
1, whence, in some cases, an (esophageal canal is
ad terminates abruptly in the semi-fluid central
! the body, the food-particles being lodged in
H 2
100 THB AB&TOKT OT IHVEKTEBKATBD ANIMALS.
TBcnolea fontied at its extremity, ae in the CUiaia. 'So other mode of multiplication than that by fission has yet been observed in the Peridme(s ; but this fission is some- times preceded by the encloBure of the nninipi in an elongat«d crescent-shaped cyst.
(6.) The TENTACULiFKa*.— The Acindm (Pig. 9, D, E, F. G) have no oral aperture of theordinary kind, but filiform procesBea ot teutacula, which are usually aleuder, eimple, and more or less ri^nd, radiate from the surface of the body
Fig. 9.
rig. 9.
generttUy, or from one or more regions of that surface. At first sight, these tentacula resemble the radiating pseudo- podia of AcHnophryg, but, on closer inspection, they are seen to have a different cbaiwtter. Each, in fa«t, is a delicate tube, presenting a stractnreleBS ertemal wall, with a semi- flnid granular axis, and nsoally ends in a slight enlargement or knob. It may be alowly pushed ont or retracted, ordi- TSTBoly bent Bat* instfad of playing the part of mer«
THB INFUSOBIA.
101
snslle organs, these tentacles act, in addition, as
srs ; the Aeineta applying one or more of these organs
e body of its prey* — ^usually some other species of
loriam, — when the substance of the latter travels along
iterior of the sucker into the body of the Aeineta. Solid
is not ingested through these tentacles, so that the
d(B cannot be fed with indigo or carmine. In the
lor of the body there is an endoplastf with one or
contractile vacuolea, and it may be either fixed by a
or free.
e AcineUB multiply by several methods. One of these nple longitudinal fission, which appears to be rare Lg them. Another method consists in the development liated embryos in the interior of the body. These yos result from the separation of a portion of the plast, and its conversion into a globular or oval germ, b, in some species, is wholly covered with vibratile
tein (* Der Organismus der onsthiere/ i. 76) thus de- a the method by which an ta seizes its prey : *Mf an irium swims within reach e Aeineta, the nearest ten- are swiftly thrown towards d, at the same time, often le much elongated, bent, ^ularly twisted about. The like ends of these tentacles, I come into immediate con- rith the surface of the en- »d prey, spread out into and adhere fixedly to it. I many of the tentacles have kttacbed Uiemselves, the ira- led animal is no longer able ape, its movements become r, and at length cease, tentacles which have fixed «lves most firmly shorten bieken, and draw the prey r to the body . . . Suddenly, m aa the sucking disk has through the cuticula of the a very rapid stream, in- id by the fatty particles i it carries, seis along the
axis of the tentacle, and, at its base, pours into the neighbouring part of the body of the Aeineta. . . . The cause of the movement is unknown. It is not accom-* panied by any discernible move- ment of the walls of the tentacle."
t No endoplastule, such as ex- ists in other Infusoria, has been observed as yet in the Aeineta:. Under some circumstances, the Aeineta draw in their radiating processes, and surround them- selves with a structureless cyst ; but this process does not appear to have any relation to either mode of multiplication.
In Aeineta myataciyia and Podo- phrya fixa, a peculiar mode of multiplication by division occurs. At the free end of the body a portion becomes constricted off, together with part of the endo- plast, from the remaining stalked part. The tentacula are drawn in, and the segment becoming elongated, developes cilia over its whole surface and swims away.
V
102 THE AK ATOMY OF INYESTEBBATED ANIMALS.
cilia, while, in others, the cilia are confined to a zone around the middle of the embrjo. The germ makes its escape hj bursting through the body wall of its parent. After a short existence (sometimes limited to a few minutes) in the condition of a free swimming animalcule, provided with an endoplast and a contractile vacuole, but devoid of a mouth, the characteristic knobbed radiating processes make their appearance, the cilia vanish, and the animal passes into the Acineta state.
The AcinetcB have frequently been observed to conjugate, the separate individuals becoming completely fused into one, and their endoplasts coalescing into the single endo- plast of the resultant Acineta ; but it is not certainly made out, whether this process has, or has not, anything to do with the process of the development of ciliated embryos just described.
(c.) The CiLiATA. — The characteristic feature of the CUiata is, that the outer surface of the body is provided with numerous vibratile cilia, which are the organs of prehension and locomotion. According to the distribution of the cilia. Stein has divided them into the Holotricha, in which the cilia are scattered over the whole body, and are of one kind; the Heterotricha, in which the widely diffused cilia are of different kinds, some larger and some smaller; the Hypo- triehat in which the cilia are confined to the under or oral side of the body ; and the Peritricha, in which they form a zone round the body. The great majority of these animals are asymmetrical.
In the simplest and smallest CUiaJta, the body resembles that of one of the Fla^ellata in being differentiated merely into an ectosarc and endosarc, with an endoplaat and a contractile vacuole. In most, if not all cases, however, there is not only an oral region, through which the in- gestion of food takes places, but an oesophageal depression leads from this into the endosarc ; and it may be doubted whether, even in the simplest CUiaia, there is not an anal area through which the undigested parts of the food are thrown out.
THS IKFU80BIA. 103
The genus Colpoda, which is very common in infusions of haj, is a good example of this low form of ciliated Infu- sorium. It has somewhat the form of a bean flattened on one side, and moves actively about by means of numerous cilia, the longest of which are situated at the anterior end of the body. At the posterior end is the contractile vacuole, while a large endoplast lies in the middle, as Stein origin- ally discovered. Colpoda frequently become quiescent, retract their cilia, and surround themselves with a structure- less cyst. Each encysted Colpoda then divides into two, four, or more portions, which assume the adult form and escape from the cysts to resume an active existence.
Allman has described the encystment of a Yorticellidan, followed by division of the nucleus into many germs, with- out any antecedent process of conjugation ; and Everts has observed that the progeny of an encysted Vorticella take on the form of Trichodina grandineUa, The TrichodincB mul- tiply by transverse divisions, and then grow into Voi-ti- eellce,*
Encystment, whether followed or not by division, is very common among all the CilioUay and a species of Amphileptus has been seen to swallow — or rather envelope — a stalked 1>ell-animalcule (Vorticella) y and then become encysted upon the stalk of its prey, just as Vampyrella becomes perched upon the stalk of the devoured Gomphoneftna.
In the higher Ciliata, the protoplasm of the body becomes directly differentiated into various structures, in the same way as has already been seen to be the case in Gregarina giganiea, but to a much greater degree.
Thus, in the Per'itricha, of which the bell-animalcules, or VorticeUm (Fig. 9, A, B, c), are the commonest examples, the oral region presents a depression, the vestibule (Fig. 9, a), from which a permanent (Esophageal canal leads into the soft and semi-fluid endosarc, where it terminates abruptly ; and immediately beneath the mouth, in the vestibule, there is an anal region which gives exit to the refuse of digestion, but [resents an opening only when fsecal matters are passing
* AUman, 'Piesidential AJUress to the Linncan Society,* 1S7J.
104 THE ANA.TOJIZ C
INTIBTKBIU.TKI> AKIMALS.
out. Eic«pt where the ciliated circlet, or rather epiral, is situated, the outer wall of the bodj gives rise to a relativelj dense eulieula, and not nnfreqaently aecretes a transparent cap or case, foreshadowing the theca of hjdrozoal poljpes. Moreover, in the permanently iixed Vorticetia, the atalk of attachment may preaent a central muscnlar fibre (Pig. 9,/l, by the sudden contraction of wbich the body is retracted. the Btaikbcingat the Bame time thrown into aspiral. In the Pig. 10-
Fl)r. 10. „ „,..
from the dorent (ide ; a c, canrraclile apace; d d"
S(eln)
rer of th.
<: Chloi
ewrd rmm the venlnl Bide: a. depreMiod leading I A. moutb; c. gulleL; d, ODdoptut ; li', PDiiopLostiite : r, central pnit< plaam. In both thesa flgurea the circulstlon. C, /■arodiimMa diTidinjt traniversely
endop
contractile apace* ; b b,
holotrichous Paramteeiitm (Fig, 10) beneath the thin super- ficial transparent cuticle f i-om which the cilia proceed, there is ft very distinct cortical layer, fibritlated in a direction per- pendiinilartothe8nrface,and, in some species of this or other genera, as Stromindium and Polyhrieot (Butschli), beset with minnte rod-like bodies similarly disposed, which, nnder some circumstances, shoot out into long filaments, and have been
THE IKFTTSOBIA. 105
termed triehoeysts. In P. hurmria, minute green granules of chlorophyll axe dispersed through this layer, and Oohn demonstrated, in 1851, that these yield the same reactions as the chlorophyll grains of the Algse. In Balaniidium, NycMhervs, Spirostomum, and many others, the cortical layer is divided hy linear markings into bands, which there is reason to believe are rudimentary muscular fibres.
In many Ciliata, the endosarc appears to be almost fluid. The food, which is driven into the mouth and down the (BsophaguB by the constant action of the ciUa, accumulates at the bottom of the cesophagus ; and then, with the water which surrounds it, is passed, at intervab, with a sort of jerk, into the endosarc, where it lies, close to the end of the oesophagus, as a food vacuole, for a short time. But it soon begins to move, and, along with other such vacuoles formed before and after it, circulates in a definite course up one side of the body and down the other, between the cortical layer and the endoplast. This movement is particulai'ly free and unrestricted in Balaniidium ; in Faramcedurriy the tract through which the food vacuoles move is more de- finitely limited,* while, in Nyctotherusy it appears to be confined to a part of the body between the end of the gullet and the anal region, which, in this Infusorium, is seated at one end of the body. In fact, the finely granular endosarc of Nyctotherus so limits the passage of the food Tacuoles, that the tract along which they pass might pro- perly be described as a rudimentary inteatiual cansJ.
The oral cavity is usually ciliated : sometimes, as in Chi- lodon, it has a chitiuous armature, which becomes some- what complicated in ErvUia (Dysteria f) and the Didinium described by Balbiani.
TarqucUella (Lankester) has a plicated membrane around the mouth in the place of cilia.
The contractile vacuoles attain their greatest complexity
* In Paraauxcium bursaria, inch in a second.
Cohn observed that the circula- t Uuxley, *^On D^steria."
tion was completed in 1^ to 2 (^Quarterly Journal of Micro-
minates, which gives a rate of scop. Science/ 1857.) ^ totatioo of 9^ to liijsth of an
106 THS ANATOMY OF INYEBTEBBATED ANIMALS.
in the Paramceda, in which there are two— one towards each end of the body. Thej are lodged in the cortical layer, and, in diastole, a portion of their outer peripheiT' is bounded only by the cuticle, through which it is very probable that they communicate with the erterior. When the systole takes place, a number of fine canals, which radiate from each vacuole, are seen to become distended with clear watery fluid. These canals are constant in their position, and some of them may be traced nearly as far as the mouth ; so that the canals and vacuoles form a permanent water- vascular system. The endoplast is finely granular, like the substance of the endosarc. It is frequently said to be enveloped in a distinct membrane, but I am dis- posed to think that this is always a product of reagents. Attached to one part of it there is very generally (but not in the Voriicelke) a small oval or rounded body, the so-called " nucleolus " or endoplcisMe. The endoplast is commonly said to be imbedded in the cortical layer, but this is cer- tainly not the case in Colpoda, Paramcecium, Bcdantidivm, or Nyctotherus.
The outermost, or cuticular, layer of a large portion of the body becomes hardened and forms a sort of shell, in many of the free Infusoria, In the free marine Didyocystida and Codonellida of Haeckel, the body has a bell-shaped envelope, which in the Dictyocydida (see Fig. 1,) is strengthened by a silicious skeleton like that of a Badiolarian. In both genera the circular lip which surrounds the oral end is provided with numerous long flagelliform cilia.*
Most of the Ciliata, while in full activity, multiply by division; this is generally effected by the formation of a more or less transverse constriction, whereby the body becomes divided into two parts, which separate, each de- veloping those structures which are needed for its comple- tion. The endoplast, however, always elongates and divides, one portion going along with each product of fission. Neither budding nor longitudinal fission occurs among the free Infusoria, the appearances which have been regarded * Haeckel, * Zur Morphologie dcr InfuBoricc,' 1873.
THE INFT780BIA. 107
as evi'lence of these processes being due to the opposite operation of conjugation. M. Balbiani,* its discoverer, thus describes this process of conjugation in ParamoBcium burgaria : —
" The Paramcsda assemble in great numbers either to- wards the bottom or on the sides of the vessel in which they are contained. They then conjugate in pairs, their an- terior ends being closely united ; and they may remain in this state for five or six days or more. During this period, the nucleus and nucleolus become transformed into sexual organs.
" The nucleolus is changed into an oval capsule, marked superficiaUy by longitudinal strise. Sooner or later, it usually becomes divided into two or four portions, which grow in- dependently, and form many separate capsules. About the time of separation, each of these is found to be a capsule containing a bundle of curved rods (baguettes), enlarged in the middle, and thinner at the ends.
" The nucleus also becomes enlarged, and gives rise — in a manner not clearly explained — to small spherical bodies analogous to ovules.
" It is usually about the fifth or sixth day after conjugation that the first germs appear, as little rounded bodies, formed of a membrane which is rendered visible by acetic acid, and of greyish pale homogeneous or almost imperceptibly granu- lar contents, in which, as yet, neither nucleus nor contrac- tile vacuole is distinguishable. It is only later that these org^ans appear. The observations of Stein and of F. Cohn have shown how these embryos leave the body of the mother under the form of Acinetce, provided with knobbed tentacles and true suckers, by means of which they remain for some time adherent to her, and nourish themselves from her substance. But their investigations have not disclosed the ultimate fate of the young.
" I have been able to follow them for a long period after their detachment from the maternal organism ; and I have
• Rilhianl, "Note relative li rExistenre d'une Gendration Sexiielle chez leg iiifuaoiros." (* Juurnul do la Pii^'siu ogle,' toine i., 1833.;
lOS THE ANATOMY OF IKTBBTEBRATED AKIMALS.
been able to assure myself that, after baying lost tbeir ten- tacles, becoming clothed with vibratile cilia, and acquiring a mouth, which makes its appearance as a longitudinal groove, they return definitely to the parental form, develop- ing in their interior the green granules which are charac- teristic of this Paramoeciwm, without undergoing any more extensive metamorphosis."
In Figs. 19-22 of Plate lY., which accompanies his x>aper, Balbiani figures all the stages by which the acinetiform embryo becomes a ParamcBcium.
So far as the fact of conjugation, the changes in the " nucleolus," and the development of filaments in it, with the subsequent detachment, by division, of masses fi*om the " nucleus " are concerned, these statements have not been modified by M. Balbiani, while they are fully confirmed by the observations made by himself, ClaparMe and Lachman, Stein, Kolliker, and others, in Paramoecium buraarioy P. aurelia, and other ciliated Infusoria,
In the closely allied ParamoBcium aurelia, the occurrence of the various stages of conjugation, conversion of the " nucleolus " into bundles of spermatozoa, and subsequent division of the ** nucleus " is also established by the coinci- dent testimony of Balbiani and Stein. Balbiani affirms that, in this species, the clear globular bodies which result from the division of the " nucleus " pass out of the body without undergoing any further modification, and he considers them to be ovules. Stein also admits that he has never seen acinetiform embryos in this species.
But, as it would seem, on the strength of these nega- tive observations in Paraw/cecium awrelia, Balbiani, in his later publications, asserts that the " acinetiform embryos " observed not only in PararruBcium, but in Siylonychia, Stentor, and many other ciliated Infuaoriay are not embryos at all, but parasitic Acinetce; and he makes this assertion, without explicitly withdrawing the statement given above of his own obsen'ation of the passage of the acinetiform embryo of ParamcBciwm, buraaria into the parental form. Engelmann and Stein, on the other band, hold by Balbiani's
THE INFT7SOSIA. 109
original doctrine, and give strong reasons for so doing. Among the most forcible analogical arguments are those afforded by the process of sexual reproduction observed by Stein in the peritrichous Infusoria,
In the Peritricha (VorticeUidoe, OphrydidoB, Trichodidce) conjugation takes place by the complete and permanent fusion of two individuals, which are sometimes of equal dimensions ; though, in other cases, one is much smaller than the other, and, while it is in course of absorption, looks like a bud, and was formerly taken fur such (Fig. 9, A, g, h). The small individuals usually take their origin from a group of small stalked VorticelloR, which are produced by the repeated longitudinal division of a VorticeUa of the ordinary size. The result of the conjugative act is that the ** nuclei *' of the two individuals, either before or after their coalescence, break up into a number of segments. The segments may remain separate or coalesce into a single mass, called by Stein, placenta. In the former case, some of the segments become germ masses, while the others reunite to form a new " nucleus ; " in the latter, the placenta throws out a number of germ masses, and then assumes the form of an ordinary " nucleus." The germ masses give off portions of their substance, including part of their ** nucleus," and these become converted into ciliated embryos, which escape by a special opening. Knobbed tentacles, like those of the AdneUB, have not been, observed in the embryos of the Teritricha, nor has their development been traced out.
If the bodies regarded as acinetiform embryos of the CUiata are really such, they may be taken to represent the myxopod stage of the Catallacta, and the relations of the Aeineice to the CUiata would appear to be that they are modifications of a common type, differing from the Catal- laeia in having tentacula instead of ordinary pseudopodia. In the Acindos, the tentaculate stage is the more permanent, the ciliated stage transitory; while, in the CUiata, the ciliated stage is the more permanent, and the tentaculate stige transitory.
110 THE ANATOMY OF INYEBTEBRATED AKIHA1.S.
CHAPTER in.
THE POBIPBKA AND THE COELENTE&ATA.
1. The PoEiFESAor Sponoida. — It has been seen that, in the Protozoa, the germ undergoes no process of division analogous to the " yelk division " of the higher animals, and to the corresponding process by which the embryo cell of every plant but the very lowest becomes converted into a cellular embryo. Consequently, there is no blastoderm ; the body of the adult Protozoon is not resolvable into mor- phological units, or cells, more or less modified; and the alimentary cavity, when it exists, has no special lining. Moreover, the occurrence of sexual reproduction in most of the Protozoa is doubtful, and there is, at present, no evidence of the existence of male elements, in the form of filamentous spermatozoa, in any group but the Infusoria ; and even here the real nature of these bodies is still a matter of doubt.
In all the Jtfe^ozoa, the germ has the form of a nucleated cell. The first step in the process of development is the production of a blastoderm by the subdivision of that cell, and the cells of the blastoderm give rise to the histological elements of the adult body, With the exception of certain parasites, and the extremely modified males of a few species, all these animals possess a permanent alimentary cavity, lined by a special layer of cells. Sexual reproduction always occurs; and, very generally, though by no means invariably, the male element has the form of filiform spermatozoa.
The lowest term in the series of the Metaaoa is un- doubtedly represented by the Pori/cra or Sponges, which,
THE POBIFEBA. Ill
after oscillating between the vegetable and tbe animal king- doms, have, in recent times, been recognised as animals by all who have sufficiently studied their structure and the manner in which their functions are performed.
But the place in the Animal Kingdom which is to be assigned to the sponges has been, and still is, a matter of debate. It is certain that an ordinary sponge is made up of an aggregation of corpuscles, some of which have all the characters of Amoebce, while others are no less similar to Monads ; and therefore, taking adult structure only into account, the comparison of a sponge to a sort of compound Proiozaon is perfectly admissible, and in the absence of other evidence, would justify the location of the sponged among the Frotozoa,
Bat, within the last few years, the development of the sponges has been carefully investigated; and, as in so many other cases, a knowledge of that process necessitates a reconsideration of the views suggested by adult structure.
The impregnated ovum undergoes regular division; a blastoderm is formed, consisting of two layers of cells — an epiblast and a hypoblast, — and the young animal has the form of a deep cup, the wall of which is composed of two layers, an ectoderm and an endoderm, which proceed respectively from the epiblast and hypoblast. The embryo sponge is, in fact, similar to the corresponding stage of a hydrozoon, and is totally unlike any known condition of a protozoon.
Beyond this early stage, however, the sponge embryo takes a line of its own, and its subsequent condition differs altogether from anything known among the Coshewteraia ; all of which, on the other hand, present close and intimate resemblances in their further development, as in their adult structure.
It is not long since the only sponge of the structure and development of which we were accurately informed was the SpongUla fluviatilis, or fresh-water sponge, the sabject of the elaborate researches of Lieberkiihn and Garter. But, recently, a flood of light has been thrown
113 THE AKATOHT OF IKTESTSBSATED iXIUALB. Fie. ".
Fig. U.—Aietltaprimordialii (kftar Hwckel).
I., A m»turB ^KTfta.pwt of one itde of Uie body of «hiflit« removed; o, lh« exiulukt apaiturt ; p, lolMltst pore* Id Um wtll of tit*-, bod; ;
THI POBIFEBA.
113
Iflnn ; e^ eotod«nii ; y, ova. The trindiate splcula are seen
Bd in the ectoderm.
rtion of the endoderm, with two pores (/>) ; t, endodermal
bote round the margins of the pores have their cilia directed
I ; e. ectodermal syncytium ; y, ova ; «, sperm cells.
maaiform endodermal cell.
ndodermal cell, with retracted dllom, and having the charac-
m Amaba,
iliated embryo of Ateetta mirabilis,
same embryo in optical longitudinal section ; e, epiblast ;
ilaat ; v, blastoccele.
) morphology and phjsiologj of the marine sponges, irly of those sponges with calcareons skeletons, ire termed CalcupongioB, by Lieberklihn, Oscar , and especially HaeckeL It has become clear mgiUa is a somewhat aberrant form, and that Ismental type of Poriferal organisation is to be imong the CdUigponguB, In the least complicated ikareoos sponges, the body has the form of a cnp, ^tached by its closed extremity. The open extremity vuluntf and leads directly into the spacious ventri' cavity of the cup. The comparatively thin wall of LB composed of two layers, readily distinguishable structure — ^the outer is the ectoderm, the inner derm. The ectoderm is a transparent, slightly , gelatinous mass in which nuclei are scattered, ^h, in the unaltered state, shows no trace of the ) distinctness of the- cells which contain these nd is therefore termed by Haeckel a syncytiwm, stic and contractile, and sometimes exhibits an L to fibrillation.
idoderm, on the contrary, is composed of a layer istinct cells, each of which contains a nucleus and lore contractile vacuoles, and is produced at its ^mity into a long solitary cilium or flagellum. the base of this, the transparent outer portion of plasm of the cell is produced into an upstanding e a collar, so that each cell has a wonderful ace to some forms of flagellate Infusoria, Micro- ertures — the pores — scattered over the outer sur- e cup, lead into short passages which perforate the
114 THB ANATOMY OF INYBBTSBRATED ANIMALS.
ectoderm and endoderm, and thus place the yentricnlas in communication with the exterior. The working of the flagella of the endodermic cells causes the water contained in the gastric cavity to flow out of the osculum ; to make good this outflow, minute streams set in hy the pores, which have consequently been called inhalewl, while the osculum has been termed the eashalent aperture. It is said, however, that the direction of these currents is not invari- able ; and it is certain that the pores are not constant, but that they may be temporarily or permanently closed, and new ones formed in other positions.
The skeleton of the calcareous sponges always consists of a multitude of separate spicula, composed of an animal substance, more or less strongly impregnated with carbonate of lime, which is deposited in concentric layers around a central axis, formed by the animal basis. This skeleton is developed exclusively in the ectoderm, and is not supported by any framework of fibrous animal matter.
The calcareous sponges are frequently, if not always, hermaphrodite. The reproductive elements are ova and spermatozoa. There is some reason for assuming that the latter originate in metamorphosed cells of the endoderm, as they are found scattered between ordinary cells of the latter. The ova, on the other hand, occur sometimes between the cells of the endoderm, sometimes imbedded in the syncytium itsell But the question of i^e origin of the sexual elements in these and other animals, needs much further investigation. The spermatozoa are very delicate and have minute rod-like heads, with long flagella. The ova present the normal germinal vesicle and spot, but exhibit active amoeboid movements.
Impregnation is effected, and the first stages of develop- ment take place, while the ova are still imbedded in the body of the sponge.
Metschnikoff* has recently described the development
* « Zur Entwickelangs - ge Zoologie,' Bd. xxiv.) F. E. Schulxe, •ohiohte der Kulksohwl&mine.** so far as I follow Uaeckert ac- ('SSeltMhr. fOr WiMensohaftUohe ootmt of hia recent obaervatioiif
THB POBIFEBA.
115
«>f Sffcon eUiaiiMn, Tbe ovum, after impregnation, be- comes a morula, with a central cleavage cavity or blasto- code. But the blastomeres of the two halves of the morula take on different characters — those of the one half elon- gating and acquiring flagellif orm cUia, while those of the opposite half remain globular and develope no cilia. The latter now coalesce into a syncytium, and develope spicula, while the layer of ciliated cells becomes invaginated within the syncytium. More usually, however, it appears that a gastrula is formed by invagination of the morula, the ectoderm of which has the structure of the endoderm of the adult, while the cells of the endoderm, or lining membrane of the gastric cavity, are devoid of cilia. The embryo quits the parent, prox>elled by the flagelliform cilia which cover the outer surface of the ectoderm. After a time, it fixes itself by the closed end ; the flagella of the cells of the ecto- derm are retracted, the cells themselves become flattened and coalesce so completely that their boundaries cease to be distinguishable, and the ectoderm passes into the con- dition of a syncytium. At the same time, the cells of the endoderm multiply, elongate, and take on the form which characterises them in the adult. In this state the young sponge is termed an Ascula. The transition to the iinal condition is effected by tiie development of the spicula in the syncytium and the separation of some of the con- stituent cells of the syncytium to form the inhalent pores.
In the simplest Caldgpongioe, forming the f amOy to which Haeckel applies the name of Ascones, the wall of the ventri- culus is thin, and the pores open directly into the ventri- cular cavity; but in another family, the Leu^cones^ the syncytium becomes greatly thickened, and the pores are consequently prolonged into canals (which may be ramifled
(*I>ie Gsstmla nnd die Eifur- ehangder 'Dilere/ p. 158), agrees with Metsehnikoff as to the first slagci of dcveiopmeoty bat differs in reganl to tubseqiient stages. Haeekd withdiaws bis earlier ao-
count of the formation of the gas* trula by delamination, or splitting of the walls of an oval shut pla- nula-MC into two layers, and the subsequent opening of the planula at one end.
T 2
116 THE AKATOHT OF IKYEBTEBBATBD ANIKiXS.
and anastomose), connecting the yentriciilus with the ex- terior. The endodermic cells, which in these, as in the Atcones, at first form a continuous lajer, are eventuallj restricted to the canals, or even to local dilatations of these canals — the so-called " ciliated chambers."
The same relative disproportion of the ectoderm, with the consequent development of passages which traverse the mass of the spoBge, and are provided at intervals with ciliated chambers, is found in the silicious sponges, in which the spicula, if they possess anj, are formed hj a deposit of silez ; and in which, as a rule, the sponge corpuscles are supported hj a more or less complete skeleton of a tough animal substance, termed keratoae,
HaliMTca, however, is devoid both of skeleton and spicula, and the minute structure of the curious boring sponges — the Clionce — has yet to be elucidated.
Haliphysema and Chttirophysema, of Haeckel, appear to be sponges which get no further than the Gkutrula con- dition, and thus form a connecting link between the Sponges and the Hydrotaa,
The fresh-water sponge (SpongiUa) has been studied with extreme care by Lieberkuhn, and the following account, based upon the investigations of that author, is given for the use of the student to whom SpongiUa fluvialu is likely to be the most readily accessible of the sponges.
The fresh-water sponge grows on the banks of docks, canals, rivers, and on floating timber, in the form of thick encrusting masses, which usually have a green colour, and require a constant supply of fresh water for their healthy maintenance. The surface presents irregular conical emi- nences perforated at their summit like smsdl volcanic craters, and from these exhalent funnels, which answer to the oaeula of the CalcifpoTigias, currents of the water are continually flowing. Careful examination of the surface of the SpongiUa between the exhalent craters, shows that it is formed by a delicate membranous expansion, separating which from the deeper substance of the SpongiUa are a number of irregular cavities. In some cases, these run into one great water-
THK POBIFEBA. 117
cliamber. The superficial chambers, or chamber, communi- cate with the exterior bj pores, which perforate the mem- branous expansion, are similar to those in the outer surface of the yentricular wall of a simple calcareous sponge, and Bubserre the same inhalent function. On their inner face, or floor, the superficial chambers exhibit the apertures of innumerable canals, which trayerse the deep substance of the SpongiUa in all directions, and, sooner or later, unite into passages which lead directly into the cavities of the exhalent craters. Dilatations of the canals occur at in- terrals, and are lined bj the characteristic monadiform endodermic cells, which are restricted to the walls of these ciliated chambers. It is hj the working of the cilia of these cells that currents of water are made continuallj to enter bj the inhalent pores and to pass out hj the exhalent craters. The whole fabric is supported and strengthened bj a skeleton, which consists, in the first place, of bands and filaments of keratose, and, secondly, of silicious spi- cnla, the majority of which resemble needles pointed at each end, and contain a fine central canal filled with an unsilicified substance. The individuality of these animals is so little marked that two SpongillcBj when brought into contact, before long fuse into one ; while they may divide spontaneously, or be separated artificially into different portions, each of which will maintain an independent existence.
A process analogous to the formation of cysts, which is ao common among the Protozoa, takes place in the deeper substance of the body, especially in the autumn. A number of adjacent sponge corpuscles, losing their granular appearance, become filled with clear strongly refracting granules, the nucleus ceasing to be visible. The sponge corpuscles which surround these become closely appUed together, and secrete coats of keratose, which fuse with those of the adjacent corpuscles. In the intei*ior of each a singular silicious spiculum is formed, consisting of two toothed disks, like cogged wheels, united by an axis. At this ** aimphidMCua " enlarges, the protoplasm of the cor-
118 THE ANATOMY OF INYEBTSB&ATED ANIMALS.
puscle disappears, and at length nothing is left but the envelope of keratose, with the imbedded amphidisks, dis- posed perpendicolarlj to its surface. At one point of the spheroidal envelope, a email opening is left, and the so- called "seed" of the SpongiUa is complete. It remains throughout the winter unchanged ; but, with the return of warmth, the sponge corpuscles enclosed within the coat of the " seed," or more properly cyst, slowly escape through the pore, become perforated by inhaJent and exhalent apertures and canals, and develope the characteristic spicula of a young SpongUla,
This process of encystment, which may be regarded as a kind of budding, akin to propagation by bulbs among plants, has not been observed among maiine sponges.
Sexual propagation takes place in the same way as in the CalciepongioBf and the embryo passes through morula and planula stages. But the ciliated cells which form the outer wall of the latter, and constitute its locomotive apparatus, seem to vanish when the embryo fixes itself, and the body of the young Fibrospongia appears to be developed out of the inner cells, which, in the meanwhile, have become spiculigerous. However, the details of the mode of develop- ment of the FihrospongioB require further elucidation.
In both the marine and the fresh- water sponges the ingestion of solid matters — such as carmine and indigo — by the monadiform endodermic cells has been seen by several observers. According to Haeckel, the solid par- ticles, which usually are taken in between the flagellum and the collar, may also be ingested at other parts of the surface of the endodermic cell. In the course of such experiments, also, granules of the pigment may be found in the ectoderm, but whether they ent^r it directly, or secondarily from the endoderm is unknown. Sponges absorb oxygen, and give off carbonic acid with great rapidity ; and the manner in which they render the water in which they live impure, and injurious to other organisms, suggests the elimination of nitrogenous waste matter.
The syncytium may contract as a whole, and is liable
THS POBIFBBA. 119
itractions, as when the oecula or the pores shut nie contours of the cells of which it is composed le in the fresh state, and hence it appears as larcode" or transparent gelatinous contractile in which nuclei and granules ai*e imbedded here But Lieberkiihn has shown that, when the rhich SpongiOa lives is heated to the point at mc coagulation of the protoplasm of the cells ir boundaries at once become defined, and the onlj detach themselyes from one another. The is therefore formed bj the close union, and not lal fusion, of the cells of the bodj. 317 interesting fact that thread-cells, similar to 1 are so abtmdant in the CcdentertUat are said to »me singes. Eimer* finds these structures in the BenierifUB, The thread-cells are scattered >th endoderm and ectoderm, and abound on the e of the former, where it limits the canals of I, but do not occur on the outer surface of the The same observer states that he found partly smains of small crustaceans in the ventriculajr id passages of both silicious and calcareous
ifera present three principal modifications — wngioR, the CatcUpongioe, and the FihrospongixB, otpongicB being altogether devoid of skeleton ; wngicB possessing calcareous spicula, but no atose skeleton; and the Fibrogpongi<B having skeleton; and (usually) spicula of a silicious
0 these it is probable that the Clwnidce must be
1 fourth type, devoid of a fibrous skeleton, but silicious spicula of a very peculiar Idnd, by the ich they are able to burrow pai-asitically in the ollusks. FinaUy, Haliphysema and Qasirophysema te even simpler than the Myxoapongice.
don of the Myxospongiw contains only the gela-
lellen and Saamen bci See-Schwammen." (* Archiv f&r Mbe Anatomie/ viii., 1872.)
120 THB ANATOMY OF IKYSBTBBBATBD AIOMALS.
tinoiiB Halisarea, The Calcisp<mffUB, in addition to the two families of Ascones and Leucones, already referred to, include a third — ^the Sycones, which are essentiallj com- posite Asconea, The FibrospongioB present a great diversity of form and structure. They may have the form of flattened or globular masses, arborescent tree-like growths, flagellate expansions, or wide or deep cups. The sponge of commerce derives its value from the fact that its richly developed fibrous skeleton is devoid of spicula. On the other hand, in such sponges as Hyalonema and Eupledella, the silicious spicula attain a marvellous development and complexity of arrangement. In the latter genus, they form a fibrous net- work with reg^ar polygonal meshes. These appear to be the representatives of the VerUricvlUes, which were so common in the seas of the Cretaceous epoch.
8eepaffel2l.
Fig. 12.— A, Hypothetical section of a Spongilla: a, superficial layer; bj inhalent i^rtores ; c, ciliated chambers ; <2, an exhalent aperture ; e, deeper substance of the sponge. The arrows indicate the direction of the currents. B, a small Sponffilla with a dngle exhalent aper- ture, seen from above (after Lieberkfihn): a, inhalent apertures; c, ciliated chambers ; d, exhalent aperture. C, a ciliated chamber. D, a free-swimming ciliated embryo.
Sponges abound in the waters of all seas, but SpongiUa is the sole fresh-water form. CUonidce existed in the Silurian epoch, but the most plentiful remains of sponges have been yielded by the chalk.
The OcELBNTBBATA. — This group of the Metazoa con- tains those animals which are commonly known as Polypes, Jelly-fishes, or MeduscBy Sea anemones, and Corals. They . exhibit two well-marked series of modifications, termed the Hydrozoa and the Actinozoa,
The Htdbozoa. — The fundamental element in the structure of this group is the Hydrawtk, or PolypUe. This is essentially a sac having at one end an ingestive or oral opening, which leads into a digestive cavity. The wall of the sac is composed of two cellular membranes, the outer
TBB FOSIFEBA.
THE AKATOmr 0
' mVBBTXBBATKD AITDULS.
of wliich istennedtke Mfod«rm, and the umer the entoderm,
the former h&Ting the morphological valne of the epidermis
Ftg. 13.
Fig. 13.— DiagniM lIlmtntiTe of tbe mnlokl relatioiu of Uw Hgdn-
I. Hydra. 5. Sertuluiui. 3. Caljeophoridu. 4. PhfiDphoriilui.
5. Lucenuriao. o. Eetoderm. b. Endodenn. e. The dlgntiTe and tomattfl eavity. P. TentaclM. N. Neotooalyx. T. CcFnotare. B. HydniphjlllaiD.
C. BTdrothMia. B. Qjdiantlu G. Gonopbore. A. Air Vcriole
ooatalncdin W. PneniDMoplioTe. e. Dlgnttve and winalio oavlty. 1., n,. III., IV~ rnmmt tbs aacceaslTe BUca of deTclopment of a
Madndfon-
THE HTDBOZOA.
123
of the higher animals, and the latter that of the epitheliam of the adimentary canaL* Between these two lajers, a
♦ •• The bodj of ererj Hydro- loon is ewentially a 8«c composed of two membranes, an external and an internal, which have been con- venientlj denominated by the tamM ectoderm ai»demdoderm. The caTitT of the sac, which will be called the aomatie earify, contains a fluid, charsed with nutiitive matter in scuution, and some- times, if not always, with sus- pended solid particles, which per- form the functions of the blood in animak of higher organisation, and may be termed the tomatie Jimid. . . . Notwithstanding the extreme variety of form exhibited by the Hydrocoa, and the multi- plicity and complexity of the orxaos which some of them possess, they never lose the traces of this primitive simplicity of or- ganisation; and it is but rarely that it is even disguised to anpr considerable extent. . . . This important and obvious struc- tural peculiarity could hardlv es- cape notice, and 1 find it to have been observed by Trembley. Baker and Laurent, Corda and Ecker in Hydra; by Rathke, in Coryne ; by Frey and Leuckart, in Lmeentariai and it is given as a character of the hydroid polypes in general {Hytlrasy Corymida, and SerttU4rrkUt\in the second edition of Cnrier's * Le9ons.' I points it out as the general law of structure fif the hydroid polypes, I}ipf^€Ue and PhywophoridUtj in a paper ^ sent to the Linnean Society, from Australia, in 1847, but not read before that body till January, 1849 ; and 1 extended the general- isation to the whole of the Hy- drozoay in a 'Memoir on the
Anatomy and Affinities of the Medusctj* read before the Hoyal Society in June 1849.
** Professor AUman, in his valu- able memoir *On Gordylophora ' (PhiL Trans. 1855), has adopted and confirmed this morphological law, introducing the convenient terms * ectoderm' and 'endo- derm,' to denote the inner and outer membranes, and Gegenbaur (*Beitrage sur naheren Kennt- niss der Schwimmpolypen,' 1854, p. 42) has partially noticed its exemplification in ApoUnua and Hhizo§^iyai but it seems singu- larly enough to have failed to attract the attention of other excellent German observers, to whose late important investiga- tions I shall so often have occa- sion to advert. l*he peculiarity in the structure of the body walls of the Hydrozoa to which i have just referred, possesses a singular interest in its bearing upon the truth (for, with due limitation, it is a great truth) that there is a certain similarity between the adult states of the lower animals, and the embryonic conditions of those of higher organisation.
^ For it is well known that, in a very early state, the germ, even of the highest animals, is a more or less complete sac, whose thin wall is divisible into two membranes, an inner and an outer ; the latter turned towards the external world; the former, in relation with the nutritive liquid, the velk. The inner layer, as Remak has more particularly shown, un- dergoes but little histological change, and throughout life re- mains more particularly devoted
I'ObiervatSoDsnpan the ADAtomyof pborldc.' An abstract of this cesaj the Diphyte, and tne Cnitj of Orfpui- was published in the * froceeding* of taatkn oC the Diphyda and Pbyao- the Linnean Society ' for 1849.
124 THB ANATOMY OF INYBBTABBATED ANIMALS.
third layer — ^the mesoderm, — ^which represents the structures which lie between the epidermis and the epithelium in more complex animals, may be developed, and sometimes attains a great thickness, but it is a secondary and, in the lower Hydrozoa, inconspicuous production.
All the Hydrozoa are provided with tentcicula. These are elongated and sometimes filiform organs of prehension, which are generally diverticula of both ectoderm and endo- derm, but may be outgrowths of only one of them.
Thread-cells, ornemMtocystSf are very generally distributed through the tissues of the Ccetenterata, In its most perfect form, a nematocyst is an elastic, thick-walled sac, coiled up in the interior of which is a long filament, often serrated or provided with spines. The filament is hollow.
to the functions of alimentation, while the outer gives rise, by manifold differendations of its tissue, to those complex structures which we know as integument, bones, muscles, nerves, and sen- sorv apparatus, and which espe- cially subserve the functions of relation. At the same time, the various organs are produced by a process of budding from one or other, or both, of these primary layers of the germ.
*' Just so in the Hydrozoon : the ectoderm gives rise to the hard tegumentary tissues, to the more important masses of muscular fibres, and to those organs which we have every reason to believe are sensory, while the endoderm undergoes but very little modifi- cation. And every organ of a Hydrozoon is produced by bud- ding from one, or other, or both of these primitive membranes; the ordinary case being that the new part commences its ezistence as a papillary process of both membranes, including, of course, a divertiottlam of toe tomatic
>vi^.
** Thus there is a very real and genuine analogy between the adult Hydrozoon and the embryo- nic vertebrate animal ; but I need hardly say it by no means justi- fies the assumption that the Hydrozoa are in any sense 'ar- rested developments' of higher organisms. All that can justly be affirmed is, that the Hydrozoon travels for a certain distance along the same sreat highway of development as the higher animal, before it turns off to follow the road which leads to its special destination.'*
In this passage of my work on the * Oceanic Hvdrozoa,' (1859,) 1 eznanded the idea enunciated in the Memoir on the Medusse here referred to, that, ** the outer and inner membranes appear to bear the same physiological relation to one another as do the serous and mucous layers of the germ.'* The diagram (Fig. 13), exhibit- ing the relations of the different groups of the Hydrozoa, was pub- lished in the * Medical Times and Gazette' in June, 1856.
THB HTDBOZOA.
125
and is continuous with the wall of tlie sac at its thicker or basal end, while its other, pointed, end is free. Yery slight pressure causes the thread to be swiftly protruded, appar- ently by a process of evagination, and the nematocyst now i^peara as an empty sac, to one end of which a long fila-
Fig. 14.
Fig. 14. — Sacenlas of a tentacle with nematocysts of Athoryhia, A. peduncle or stalk, and B, involucrum of the sacculus C ; D. fila- menta ; d^ ectoderm ; e, endoderm ; /, nematocysts ; 1, small nema- tocysts of the filaments and involucrum ; 2, 8, larger nematocysts of the sac ; 4, largest nematocysts.
ment, often provided with two or three spines near its base, is attached. Many of the CkBlerUercUa, and notably the Fkyacdia, give rise to violent urtication when their tentacles come in contact with the human skin, whence it may be concluded that the nematocjrsts produce a like injurious effect upon the bodies of those animals which are seized and swallowed by the Polypes and Jelly-fishes.
As regards the existence of a nervous system in the HfdrotocLf Y&rj diverse opinions have been entertained, and it may be doubted if the problem has even yet received its final solution. I have already discussed Sileinenberg^s
!:!♦> THK ANATOMY OF IN VERTERKATED ANIMATS.
suggestion, that the branched prolongations of the iniior ends of the cells of the ectoderm in Hydra, which end in the longitudinal fibres which lie between the ectoderm and the endoderm maj be nerres in their earliest stage of differentiation. Haeckel describes a nervous system in Olo88oeodon and Carmarina, It consists of a circular band which lies on the inner side of the circular canal of the bell-shaped swimming organ of these Medusce, and presents a ganglionic enlargement at the base of each of the litho- cysts. Of these eight ganglia, the four which correspond to the openings of the four radial canals into the circular canal, are the larger. Each of these gives off four branches, one of which follows the course of the radial canal to the central poljpite or manubrium; two others go to the adjacent tentacles, and the last to the Hthocyst.*
There can be little doubt that the lithocysUy or sacs con- taining mineral particles, which are so frequently found in the Medusse, are of the nature of auditory organs; while the masses of pigment, with imbedded refracting bodies, which often occur associated with the lithocysts, are doubt- less rudimentary eyes.
The sexual reproductive elements are ova and spermatozoa — the ova being very often devoid of a vitelline membrane. The fully formed generative elements lie between the ecto- derm and the endodeim of that part of the body wall in which they make their appearance. In HydracHnia, as has already been pointed out, the ova appear to be modified cells of the endoderm, and the spermatozoa modified cells of the ectoderm ; but it remains to be seen how far this rule is of general application.
Usually, the region of the body in which the generative organs are produced undergoes a special modification before the reproductive elements make their appearance in it, giving rise to a peculiar organ, the gonophore. In its
* Hfteekel, * Beitrige sur important observations of Mr.
HatunccMhichte der Uydrome- Romanes on the Locomotor
duaea* The anatomical disposi- system of Medusse. (Proceedings
don of this nervous apparatus of the Boyai Society, December
•oeordt very well with the recent 1875.)
THE HTDBOZOA. 127
simplest condition the gonopbore is a mere sac-like diyer- ticnlam, or outward process of the body wall. Bat, from this state, the gonophore presents eyerj degree of complica- tion, nntil it acquires the form of a beU-shaped body, called from its resemblance to a Medusa or jelly-fish, a medusoid*
In its most complete form, the medusoid consists of a disk having the form of a shallow or deep cup (nectocalyx), from the centre of the concayity of which projects a sac termed the manubrium. The cavity of the sac is continaed into that of sundry symmetrically disposed canals, most commonly four in number, which radiate from the centre of the disk to its circumference, where they open into a circular marginal canal. A membranous fold, the velum, which contains muscular fibres arranged concentrically to its free margin, is attached to the inner circumference of the mouth of the bell, and projects, like a shelf, into its interior. Lithocysts are usually developed on the margins of the bell, which may also give rise to tentacles. The manubrium, opening at its free end, may become function- ally, as well as structurally, a hydrantb, and may serve to feed the medusoid when it is detached from the hydro- foino, or body of the hydrozoon. However complex its structure may be, the medusoid commences as a simple bud-like outgrowth, which thickens at its free end; the central part of this thickening becomes the manubriimi, while its periphery, splitting away from the manubrium, is converted into the disk (Fig. 13). A single prolongation of the somatic cavity is continued into the manubrium, while several, usually four, symmetrically arranged diverticula extend into the nectocalyx and become its radiating canals. The distal ends of these subsequently throw out lateral branches, which unite and give rise to the circular canal.
The lithocysts are usually, but not always, free and
* From the Imperfection of our stricted to what are known to be
knowledf e respecting the origin f^nophores developed by gemma-
of many of the medusiform tion,^*medu8a" may be employed,
IfydmzoOjit in difflealt to employ in a general sense, as the equiva-
anj lefminology with strict con- lent of the somewhat inconvenient
^stenoy. If ^medusoid" is re- vernacular term **Jelly-flsh."
128 THE ANATOMY OF IKYBBTSBRATBD ANIMALS.
prominent, and the one or many solid mineral bodies which thej contain are enclosed in special envelopes. Their structure appears to be more complicated in the GeryonidcB than in other MednssB. (Haeckel, 2. c.)
In some of these medasoid gonophores, the reproductive elements are developed while the gonophore is still attached to the hydrosoma, and then thejr always make their appear- ance in the wall of the manubrium. But, in other cases, the medusoid becomes detached before the development of the reproductive elements, and, feeding itself, increases largely in size before the ova or spermatozoa appear. Sooner or latter, however, the reproductive organs are developed, either in the walls of the manubrial hydranth, or in those of the canals of the nectocalyz of the medusoid.
In an early stage of its existence, every hydrozoon is represented by a single hydranth, but, in the great majority of the Hydrozoa, new hydranths are developed from that first formed, by a process of gemmation or of fission. In the former case the bud is almost always an outgrowth or diverticulum of the ectoderm and endoderm, into which a prolongation of the cavity of the body extends. Some- times the hydranth formed by gemmation becomes de- tached from the body; but, in many cases, the buds developed from the primary hydranth remain connected together by a common stem or ecenoaarc, and thus give rise to a compound body, or hydrosoma.
In many Hyd/rowa, the ectoderm gives rise to a hard cuticular coating, and in some of these {CampanuUmdoB, SerMaridcB, Fig. 13, 2), this cuticular investment, on the hydranth, takes the shape of a case or *' cell " — the hydroiheca — ^into which the hydranth may be more or less completely retracted. In other Hyd/rowoa, protective coverings are afforded to the hydranths by the development of processes of the body wall, which become thick, variously-shaped, glassy lamdlse. These appendages are termed hydrophyllia (Kg. 18, 3).
Again, certain groups (the Ccdycophcrida and most Thpophorida) are provided with bell-shaped organs of
THI HTSBOPHOKl. 129
ipnlaioii, prodnoed hy the metamoTphoms of laterul budi th» lufimmma, Thcae tMrfoetuyoet have Uie atructure k madiuoid, doroid of a maiinbriaiiL In others (Phyto- trida), one extramit; of tbe iyitoKimA ii dilated, con- na air onclosed wiUiin a sac formed hj an iiiTolatioii of I ttctodenn, ttnd oonetitateB a float or jmnMuilophora ; i]« in yet othen (Diteophora), the aboral end of the dranth ia dilated into a disk or MmbreUo, which ia ens- >tibl« of rhjthmical oontractile morementa, \>j which ) bodj ia propelled throng the water. Thus, notwith- nding ita different mode of development, it haa a cloae 3 to a medmoid. According to the eziBt«mce e of tliete Ttkrious append^^s, and the manner which tliey are diapoeed, the SijAmoa are distingoish* Le into three gronpa — 1, Uie Sydrophora; 2, the Di*eo- wa; 3, the Stphonopftoro.
The Htdbophora are, in all caeea but that of Bydra,
1 ramified h7droBome8, on which manj hjdranths and
iphorea Eire developed. The oomatic cavitj contained
he hjdroHoma alwajs retaina a free commnnicatioii
the gastric cavitiea of the hjdranths. In other words,
m mittooxle. The tentocola are either scattered over
rdranths (Coryne), or are arranged in one circle rooiid
oath [BertuiaTia) ; or in two circlee, one close to the
, and one near the aboral end (Ttibvlarui). Yery
Uj — e.g., in all Berialarida, Campawdarida, and
Hdce — there is a hard, chitinooa, cuticular skeleton.
« <rf Allman,) which freqnently gives rise to hydro-
ito which the hydrautha can be retracted (Fig. 13, z).
^nophorcfl present everj varietj, from simple
diverticula of the hjdroBomSi to free Bwimniiug
Is. The inner margin of the bell in these meda-
ilwBjB produced into a velum, and otolithic sacs
ipota are very generally disposed at regular in-
snnd the circumference of the bell. The great
of what were formerly t«rmed the naked-eyed
Jfm/KOphtkalMaia) are merely the free-swimming
180 THE AKATOITY 0» I]tVaSTBBBl.TKD AITDULS.
goDophowaof theBjdrojjfcora. Thus the medueoida known M BartiadtB ore the free ^nopliores of the Corynida ; ihe Aou^intMitUra and Lwice of the Ewdtfiulrula) ; many Ooeanitke proceed from Z^i&HlarM&B j ThaumanUdiB and .^^rwmdte from OainfMinMlarute. tn Bome Sy<lrqp&ora (e.9., CoV'I'a) t^ morgiiu ol the
Fig. 15.
I (atUr Gegenlwur).— A, Hjdnnth; e, lU !t»; D, month; te, tentuin; jI*, digeatiTe uTln,
le MUiutio cavity i, contained in tbs pednncla and
in tbe creeping atem, 8. B, gonangium containing two mednjifonD ■ooldi or gODC^ibom u ; the •omatie cavity If itia connecIioD witfa ttaal of the creeping atem. C, Bud.
hjrdrotheca are prolonged into triaagular processes, which eerre aa an openmlutn.
Gertam PbutttUarida are provided with promineikMa of the hjdrosoma sorroiinded b; a chitinona iiiTestmant, which ia open at the extremitj. The encloaed aoft ectoderm □Boallj ooDtaine manj thread-ceUa, and has the power of throwing out oontTKctile pseadopodial prooeaacM. These
THK trmBOFHOSA.
131
hare been termed nmtatophoret by Hr. Biuk.* la Ophiadtt Hincks) tbej are tentaculiform.
It freqnentlj happens that the ^nophoree are developed npon epecial stalka, each of which haa eeaeutiall; the ■Cnieturc of a moathleaa hjdranth. This ia termed a blaitottfU. In some fakatoatjrlea (Fig. 16), dnring the de- Tektpmcnt of the bnda of the gonophores, the eotodorm
Fig. !«.
aftitm into two lajers — am inner, which inreats the central axis formed b; the endoderm with the contained prolon- gation of the somatic cavit;; and an outer, chiefly, if not wholly, chitinons layer. Into tiie interspace between theee two, the budding gonophores prcrjeot, and may emerge from the enmmit of the goiumgnua, thm formed, either to derelope the reprodnctive elements, and Bhed them while atill attached, or to be set at liberty as free mednsoide (Figf. 16).
Allmiiii f has shown that, in Du:oryne fj„ am/eria, the gonophore contained in a gonsn^nm, somewhat like that of Lao- medea, is set free aa a ciliated bitenta- cnlate body, on the central axis of which the ora and spermatozoa are de- yeloped.
In the genua Aglaophenia [Plnmu- larida-), gronpe of gonangia are enclosed in a common receptacle [corbula, Allman), formed by the development and anion of lateral processes (comparable in some respects to the hydrophyllia of the Calyetyphorida)
• They are dexaibed onder the auni of "cUiata orguis," 4ad euniiared with the tentacle* of Diplmda Id m; Hamoir on the "AfinitiB of tha Medow." (Phil. Tnitt. iat9.)
t ' HoMwimph of tb* Gynrno- biwtte, or Tubnlarlui Hjdniiil*,'
MaiuBiform loold of Campami- laria (afta- Gegen- bur). A, necto-
briuni,aTiclMiag the dignliveeaTlly; o, moulb; i", ladial
much InfarQullon reapectiog cho
132 THS ABATOMT OT UimTKBKATED A1TIMAI.B.
bom th&t region of the hifdroeoiua, vhicli bears ike gono- phores.
Some medneoide, such ae Sariia prolifera and WUUia, Ute liTdroid stages of which ore not at present certami; known, bnt which are probably coiTniform, produce mednsoids similar to themselfes bj budding. The buds may bo de- veloped either from the mannbrium, or from the marginal canal of the neotocalTx, or from the baaes of the tentaonla, or even from their whole length.
Fif. IT.
Fig. 17.— Wabia, ap.— A, the nedDM, with budding stoloni. B, ons of the budi dmlopad on ■ itolon; k, nditl ouul of the noclo- mXjx ; «, manubrium. C. s atoloa; jr, it* free end bcMtwith Demi^ tocTsla ; b,e,d, budding raedufoLdi ; /, meduaold Dearly read; to be detached; t. Its manobrium ; d, lu nectocalfi ; A, a radial esoal.
In Angnst, 1849, while in the North Pacific, off tlie Luoisiade Archipelago, I took a species of WHlna (Fig. 17), in which stolons were developed at the bifurcation of each of the four principal radiating canals of the nectooaljx. Each stolon was terminated bj a knobbed extremity containing many uematooysts (0, g), and gave rise, on one side, ta a series of bnds, of which those nearest the free end of the
THB DI8COPHO&A. 1S3
stolon bad acquired the form of complete medosoida. They had four Tmbranched radiating canals and four tentacles ; but it is probable that they wonld aasmne the form of the parent stock after detachment.
In striking contrast with the complexity of these repro- doctiYe processes, the gonophore is represented, in Hydra, by a mere enlargement of the body wall, sitoated close to the bases of the tentacnla, in the case of the testes, and nearer the attached end of the body in that of the ovary. The OYaiy developes a single ovum, which, as Kleinenberg has shown, nnderg^oes division and invests itself with a chitinons coat while still attached to the body of the parent. This chitinons investment is more or less spinose, and is often confonnded with an egg-shelL It obviously answers to iheperUarc of a Tnbnlarian, and its presence in the em- bryo of the Hydra, in which no perisaro is developed by the adnlt, saggests that Hydra may not represent the simplest primaiy condition of a Hydrophoran, but may be a reduced modification of a Tubnlarian.
2. The DiscOPHO&A. — These "Medusffi" resemble the more perfect free medusoid gonophores of the Hydrophora, in so far as they consist of a hydranth or polypite attached to the centre of a gelatinous contractile swimming disk. But they differ from the medusoids of the Hydrophora, inasmuch as they are developed either directly from the im- pregnated ovum ; or by gemmation from a Medusa which arises in this way ; or by the transverse fission of the hydri- form product of the development of the impregnated ovum.
In some of these (e.^., Carmarvna, Polyxenia, JEginopsis, Traehynema), the disk is similar to the nectocaJyx of one of the medusoids of the Hydrophora ; and, like it, is provided with a velum. But in the rest (Lueemaria and the Steganoph- thalmata) the disk is either devoid of a velum, or possesses only a rudiment of that structure, and is termed an urn- brdla. The edges of the umbrella are divided into lobes by marginal notches in which the lithocysts are lodged. Moreover in these, the mineral particles of the lithocysts
134 THE ABATOXJ OF nrTaBTIBKA.TKD AITDIALS.
Rre nomerons, and not encloaed in Hepontte sacs. The Uthocjsts are often covered by liood-like prooeaaea of the nmbrella, whence they haye be^i teimed " oovered-ejed " or Steganopkthahaata.
Lucaraaria ia fixed by the aboral aide of its nmbrella (Fig. 13, s), hj means of & longer or Bhorter pedoncle. The nmbrella is divided into eight lobee, at the extremities of each of which there is a gronp of short tentacles. The
Ftg. 18.
II. Dndcr view of s MgBiMit of ths dJik, to it tbfl ndlBling c»n»lg ; tha kperture of ft gooltal cbamber and ths
EUlled genltBl membruie ihowipg tbroiuh ill ventral wall ; and ft tlia«7rt with Iti protective hood (a).
hjdranth stands np in tite centre of the nmbrella, and its oavitj commnnioateB with a central chamber, whence fonr wide chambers pass into the lobes. These chambers are separated by septa, the free central edges of which are beset with slender tentacles. The reprodnctive organs are doable radiating series of thickening* of the oral wall of each chamber.*
THB DI800PH0KA.
135
An the other Diseophorti, which are what are commonly known as " JeUy-fish," are free, and some attain a very large aie. In the adnlt (Fig. 18) the umbrella is thick and divided bj small marginal notches into as many (nsnaUy eight) lobes. At the bottom of each notch, often protected by special lobnles, is an oyal lithocyst, supported by a cylin- drical pednnde, the cavity of which is in direct commnni- cation with one of the radiating canals of the umbrella (Fig. 28, lY.). This canal communicates with the exterior on the aboral side of the base of the peduncle.* The thick mesoderm of which the great mass of the umbrella con- sists is composed of a gelatinous oonnectiye tissue, in the meshes of which is a watery fluid, containing numerous nucleated cells which exhibit amcsboid movements. On the oral &ce there is a broad cone of striped muscle, made up of fusiform fibres placed side by side. In Aurelia auriia, the angles of the four-sided hydranth are produced into long foliaceous lips, the margins of which are beset with minute solid tentacula (Fig. 18). The gastric cavity contained in the hydranth terminates, beneath the centre of the umbrella, in a somatic cavity which passes into four radially-disposed wide offshoots, or gewUal sinuses, the oral walls of which constitute the roof of the genital chambers (Fig. 18, IL). From their margins the narrow branching radial canab are given off. The peripheral ends of these unite when they reach the margin.
Each genital chamber is a recess, surroimded by a thick wall of the oral face of the umbrella, in the centre of
tenaehangen fiber niedere See- tUere' (1862), in his monograph on the genus, fully confirms this view, and Prof. H. J. Clark arrived independently at the same eonelnsion : ** Lueemaria the Cce- notype of the Acalephot" (Pro- ceedings of the Boston Society of Satxinl History, 1862). The Lmeemaria (^Carduella, Allman') afothifonmia of Sars differs much from the ordinAry LueermaritBj
especially in the position of the genital organs as longitudinal thickenings in the walls of the gastric cavity. See Allman, " On me Structure of Cardueila cyathi- formu** (Trans. Microscop. So- ciety, viii)
* The circular canal of the nectoodyx communicates ^th the exterior by apertures on the summits of papillose elevations in some medosoids.
136 TRX AITATOKT 0
nmSTIBE^TSD .UtUIALB.
which 011I7 a amall aperture ia left (Fig. 18, L a). The roof of this cavity is the floor of the genital sinos; it is mnch pUit«d and folded, and the genitiJ elements are developed in it. Its inner or endoderm&l wall ie beaeb with email ten- tacular filamenta (Fig. 28, m.). TheoTaortheepermatozoa
rwU*; c, tb
tli«ni ; 0, tha ]^lUn which nup«nd the braohiferoDi disk vbicb fortoi the floor of the lab-niiibreUar wvlt; ; I, ihort olavila teu- tMlei IwtwBen the af«I pone.
poaa out of the apertnrea of the genital ohaml>er8, and the ova are received into small pouches or folds of the lips, and there undergo the preliminaiy stages of their develop- ment.
In the jB&wsfomiiin (as was ori^nallj snggestAd b; Ton
TBI BHIZOBTOXmX.
Bmt and haa been proved b; L. Aguaiz and A. Brandt,*) tlw nutrgina of the lips of the hydraoth nnite, leaving only ■ mnltitade of nnall apertarefl for the ingeation of food on the long anna, which represent prolongatione of the lipa of the hydranth (Piga. 19, 20, 21). The polyatomatons con- ditwnt Ihna brought about, by the aDbdiviaion of a primi- tirely aiinple oral cavity, ia obvionaly quite difTerent In its natore from tbat which occurs in the Por^fnv,
— C^thta Bcrllata (f ).— A, p«rt of Uis umbrella, viewed from
below, to ihow the plaited KeolMl membrmna if) and the diridgd aUaohmmt of one of the jdTlan : d, plaoe of one of the lilhoejatj.
SODS of the onl porei (m) aurroundad by tentaculii (■•) ; p, oae the olaTftte teatsclea intenpened between the onl pines. C, one of the pedaoculkted lithocyite (t) In Iti notch (d) ttta trom beloir, with tiieovtit plate from which moscular fibre* (&3 take their origia; *, tba racllKtJDg ean&l with !(■ cnwl Utenl bruohes, g.
In most of the S}iiio$t<nnid<E, not only do the edges of the lips unite, but the opposite walls of the hydranth beneath the nmbrella are, as it were, poshed in, ao aa to form four chambers, the walla of which unite, become perforated, and thus give riae to a anb-nmbrellar cavity with a root formed • <Hte.del'Acsd.de9t.P«tenbon^,'ivi., IS70.
138 THB ANATOMY OF HTYBBTIBBATED AKDCALS.
by the umbrella and a floor, the hrachiferous disk, suspended
Fig. 21.
A i
Fig. 2\,—C«phea oceUata (0-— A, lithocytt enlarged with its hood (k) and the alx>nl pore of the canal (c) ; <£, the notch of the margin of the umhrella. B, the brachiferous dii k ^th the origins of the anns ; /, endoderm ; o, ectoderm. C, tentaculate lip of an oral pore en- larged; «, oral cayity; n, nematocysts.
by four pillars. In the roof the plaited genital membranes are developed. The floor (Fig. 21, b) gives off the subdivided
Fig. 22.
Fig. 22. — A, Dipkyei appendiculaia, — a, hydranths and hydrophvUia on the hydroBoma ; o, proximal nectocalyx ; c, aperture of distal neotoodvx ; dy somatocyst ; e, prolongation of the distal nectocalyx. by which it is attached to the hydrosoma ; /, point of attachment ot the hydrosoma in the cavity, or hvdrcecium, of the proximal necto- adjTz. B, the distal nectocalyx with the canal (through which the br&le a is passed), which is traversed by the hydrosoma in A. C, estremity of the distal nectocalyx, with its muscular velum.
arms, the free margina of which bear the oral pores, and
TBB BHUOBTOIEIDX. 13&
wlueb mn t»Tcned bj canals which unite, paw throagh the {nUani, Bod open into tlie central oavitjr of the umbrella.*
Fig. 23.— A, B, Diphyioold (Sphenoida), Istenl uid front Tiewi. C, DIph^Kxiid of jUyla iCMOotda), a, t, EonopboTe or nproducMve orgmn ; b, h^dnnlh ; c, phylloojit or cavity of hydrophjllliim, with iU proeeu (J). D, free gonopbore, its manabrliua (a) eonlaiuing
whichU ■H obulnad in the Soath Pacific BOv the LoulilMle Arohlpaluo, oa the 1 1th of July, 1849. ifie iboni florfaoe of ue ombrellB WH of ■ brownlih-gre]' colaor, Tarlcgated witli OT*i wbltc >pot) : til* oral Boiface, light brown with •ieht bloiib-grcen Uhm radia- tiiig tomid* tha Uthoeyiti: tba
bnehla gre; with browD dot*. Tlig brachU divide into two at their origia, and theo aubdivlde into an i^nityofamallbnacheB. The general oolour of the amallcr branches is light brown, tho ■mall IntersperMd elavate tea- tulea belag wblto. The long lanCacle* whioh terminal* each braohlum, aie bine and oylin- diieal at their origin, hut become
140 TBK ABATOKT 07 nTCBTKBRATZD ANIIUXS.
3. The SiPHOKOPHOBA. — In Uub group the hTdroBoma is alwajB free and flexible, the ectoderm dereloping no hard ohltinoaH eioBkeletoo, save in the cose of the pneomato- phoree of some apeciee. In moat, the hTdronthB are of eqoal size; bnt in TekUa and Porjnta, the hjdranth aitnated in the centre of the diacoidal bod^ is Tury mnch larger than the ittfb, which ocoapjr a oiromnferential zone around it ;
Fig. S4.
Fig-M.—AOiorybiaivaeta. A. Utenlvjew; B, from >batc; C, D, detaobcd hTdniphjUla ; a, poljrpita ; b, tentulo ; c, necnli of the taatoolM ; 4, hjrdntphyllik ; /, pnmmmtophore.
Bad the principal function of which ia to derelope the gonophores from their pedicles. In these two genera the tflntacnla are separ&te from the hydranths, and form the OBtermoBt circle of appendages.
The hjdranthB of the Siphonophora (Fig. 25, A) never poMew a oirclet of tentacnla roond the month, which.
Mgonal ftvUm on, wh«re Ibay anBata of Prdd umI Leraeoi ? uvilwdad with brown uiilnaai. The ladiTidaal figured wm a Ii It l*'""™^ wUh tha C^Am yomiKnait.
THS 8IPHONOPHOBA.
Ul
whok expanded, is trompet-sliaped. The endoderm of the hjdnuith is ciliated, and viUiu-like prominences project into its cavity. The interior of these frequently contains vaeaolar spaces (Fig, 24, B, 0). A yalynlar 'pyloms' separates the gastric from the somatic cavity in the Cai§caphorida. Long tentacles, frequently provided with unilateral series of branches, are developed, either one from the base of each hydranth, or, independently of the hydranths, from the ccenosarc In the CaJyeophoridcB and many PhysophoridcBj complex
Fig. 25.
fig. 25. — AthoiTfbia rxmaeta, — A, ft hydranth with villi (a). B, one of the villi in ita elongated state, enlarged. C, a small retracted villas, still more magnified, with its yacuolar spaces and ciliated surfkce.
organs, containing a sort of battery of thread cells, terminate each lateral branch of a tentacle (Figs. 24 and 26). Each consists of an elongated %acwlu9, terminated by two fila- mentous appendages, and capable of being spirally coiled up. In this state it is invested by an intH>2i4crum, which sur- rounds its base. The somatic cavity is continued through the branch, which constitutes the peduncle of this organ, into the saoculus and its terminal filaments. In the latter, it is narrow, and their thick walls contain numerous small
142 THE ANATOMY OF INTSBTSB&ATSD ANIMALS.
spherical nematocTsts. In the saocnlus, the cavity is wider. One wall is yerj thick, and multitudes of elongated nema- tocjsts, the lateral series of which are sometimes larger than the rest, are disposed parallel with one another, and perpendicular to the surface of the sac. Like the other
Fig. 26.
Fig. 26. — Athorybia rotoofo.— The ends of the tentacular branohee in variouB stages of development. A, lateral branch, commencing as a bad from the tentacle. In B, terminal papillae, the rudiments of the filaments, are developed at the extremity of the branch ; and, in C, the saccnlus is beginning to be marked off, and thread cells have appeared in its walls ; in D, the diyision into inyolucrum and sacculus is apparent : in £, the involucrum has invested the sacculus, the extremi^ of which is straighti while the lateral processes have curled round it
organs, each of these tentacular appendages commences as a simple diverticulum of the ectoderm and endoderm, and passes through the stages represented in Fig. 26.
In Phyacdia the tentacola may be several feet kng. They have no lateral brandies, bat the large nematocysts are
THB 8IPHOKOPHORA. 143
sitiuifted in transverse renif orm thickenings of the wall of the tentacle, which occur at regular intervals.
HydrophjUia aregeneralljpresent, and, like the tentacula, are developed either from the pedicle of a hjdranth, in which case they enclose the hydranth with its tentacle and a group of gonophores {CalycophoricUB), or, independently of the hydranths, from the coBnosarc (many Physophoridce),
The hydrophyllia are transparent, and often present very beautifully defined forms, so that they resemble pieces of cut ^ass. They are composed chiefly of the ectoderm (and mesoderm) but contain a prolongation of the endoderm, with a corresponding diverticulum of the somatic cavity. They are, in fact, developed as csBcal processes of the en- doderm and ectoderm ; but the latter, with the mesodermal layer, rapidly predominates.
^e gonophores of the Si^honophora present every va- riety, from a simple form, in which the medusoid remains in a state of incomplete development, to free medusoids of the Gymnophthalmatous iype. As an example of the former condition the gonophores of Athoryhia may be cited (Fig. 27) ; of the latter, the gonophores of Physalia, Porpita, and VeleUa,
In Atharybia, groups of gonophores, together with pyri- form sacs, which resemble incompletely developed hy- dranths {hydroeysis) (Fig. 27, A, a), are borne upon a com- mon stem, and constitute a gonohlastidium (Fig. 27, A). The groups of male and female gonophores (Fig. 27, A, 6, c) are borne upon separate branches of the gonoblastidium (androphores and ffynophores). Each female gonophore contains only a single ovum, which projects into the cavity of the imperfectly differentiated manubrium, and narrowing its cavity at different points gives rise to the irregular canals (Fig. 27, D, d). In the male gonophore, the necto- calyx is more distinct from the manubrium, and its ex- tr^nity has a rounded aperture (Fig. 27, E).
In the CaiyeophoridcB, as in the elongated Physophoridce, the development of new hydranths and their appendages, which is ooBBtaatly oocuiring, takes place at that end of the
144 THl ASATOKT OF IimBnBRl.TXD J
IiTdTCMoma, which oorrasponds to the fixed extremity of one
Fig. 37.
Fig. ST. — AMhaybia maaa. — A, gonablutidiDiii beulnB thrae hjdro- cnit, a ! nnopborv, b, tod two aiidniiiborM, c. S, fen»l« nno- pborei on Uudr common item or Ejnopboni, uowing the included oTum, 1, wid the Tmdi«l c«nftU, i, cTDf ™)*1* gonophona enlaigml : u, germuul vcalole : b, tJUUiu ; c, ndkl cankli of tho Impnfect neetoctiyx ; d, uatli of tbo nunabrial cstI^. B, malt gon^bore.
of the Hydrophara; and, if we conaidear this to be the
THB 8IFH0K0PH0BA. 145
proKunal end« new buds are developed on the proximal side of those already formed. Moreover, these buds are formed on one side only of the hydrosoma. Hence the appendages are strictly nnilateral, though they may change their position so as eventually to appear bilateral or even whorled. In the CdhfcopharidtB, the saccular proximal end of the ccenosarc (Fig, 22, A, <i) is enclosed within the anterior nectocalyx, at the posterior end of which is a chamber, the hydroecium (Fig. 22, A, c). The second, or posterior, nectocalyx is attached in snch a way that its anterior end is enclosed within the hydrcBcium of the anterior nectocalyx, while its contractile chamber lies on the opposite side of the axis to that on which the anterior nectocalyx is placed (Fig. 22, A). Bets of appendages (Fig. 22, A, a ; Fig. 23), each consisting of a hydrophy Ilium, a hydranth with its tentacle, and gonophores, which last bud out from the pedicle of the hydranth — are developed at regular intervals on the ccenosarc, and the long chain trails behind as the animal swims with a darting motion, caused by the simultaneous rhythmical contraction of its nectocalyces, through the wat^r (Fig. 22).
From what has been said, it follows that the distal set of appendages is the oldest, and, as they attain their full de- velopment, each set becomes detached, as a free swimming complex Diphyzooid (Fig. 23). In this condition they grow and alter their form aud size so much, that they were formerly regarded as distinct genera of what were termed monogastric Diphydcs. The gonophores, with which thcBe are provided, in their turn become detached, increase in size, become modified in form, and are set free as a third series of independent zooids (Fig. 23, D). But their manu- brium does not develop a mouth and become a functional hydranth; on the contrary, the generative elements are developed in its wall, and are set free by its dehiscence.
In the PhysapharidcBf the proximal end of the hydrosoma is provided with a pneumatophore. This is a dilatation, into which the ectoderm is invagiuated, so as to form a recep- tacle, which becomes filled with air and sometimes has a terminal opening, through which the air can be expelled
146 THB ANATOICT OF HTYBBTBBBATBD ANIMAS.
(Fig. 13, 4). It is sometimes small, relatiyelj to the hydro- soma (AgcUma, Phyaaphora), sometimes so Lurge (Atharybia, (Fig. 24), Physalia, PorpUOf Vdelia), that the whole hydro- soma becomes the investment of the pyrif orm or discoidal air-sac ; while the latter is sometimes converted into a sort of hard inner shell, its cavity being subdivided by septa into nnmerons chambers {PorpUa, Velella).
Nectocalyces may be present or absent in the Physo* pTioridcB. When present, their number varies, but they are confined to the region of the hydrosoma which lies nearest to the pneumatophore.
In the gre&t minority of the Hydrotoa, the ovum under- goes cleavage and conversion into a morula, and sub- sequently into a planula, possessing a central cavity en- closed in a double cellular wall, the inner layer of which constitutes the hypoblast, and the outer the epiblast.
In most Hydrophora the ciliated, locomotive, planula be- comes elongated and fixed by its aboral pole. At the oppo- site end, the mouth appears and the embryo passes into the g^trula stage. Tentacles next bud out round the mouth, and to this larval condition, common to all the Hydrophora, Allman has given the name of Aciinula,
Generally, the embryo fixes itself by its aboral extremity at the end of the planula stage; but, in certain Tubularidije, while the embryo is still free, a circlet of tentacles is developed close to the aboral end ; and this form of larva differs but very slightly from that which is observed in the IHgcophora.
In the genus Pelagia, for example, the tentacles are developed from the circumference of the embryo, midway between the oral and aboral poles; but it neither fixes itself nor elongates into the ordinary actinula-f orm. On the contrary, it remains a free-swimming organism, and, by degprees, that moiety of the body which lies on the aboral side of the tentacular circlet widens and is converted into the umbrella, the other moiety becoming the hydranth, or " stomach,'' of the Medusa.
THB DBTBLOPMBNT OF THE HYDBOZOA. 147
In Lncemaria, it is probable that the lai*va fixes iteelf be- fore or during the development of the umbrella, and passes directly into the adult condition. But, in most Discophora, the embryo becomes a fixed actinula (the so-called Hydra tuba or SetfphisUyma, Fig. 28, 1.) multiplies agamogenetically by budding, and gives rise to permanent colonies of Hydri- form polypes. At certain seasons of the year, some of these enlmrge and undergo a further agamogenetic multiplication by fiflsion (Fig. 28, II.). In fact, each divides transversely into a number of eight-lobed discoidal medusoids ('* Ephyr<E " or ** Medusas hifidtE** Fig. 28, II. and III.), and thus passes into what has been termed the Strohila stage. The Ephyrce becoming detached from one another and from the stalk of the SMbila, are set free, and, undergoing a great increase in nie, take on the form of the adult Discophore, and aoqtiire reproductive organs. The base of the Strobila may develope tentacles, (Fig. 28, II.) and resume the Scyphistoma condition.
MetBchnikoff * has recently traced out the development of Oeryonia {Carmarind), Polyzeniay Mginopsia, and other Diflcophora, which differ from the foregoing in possessing a relum; and in these, as in the TrachynevML eiliatum, observed by Gegenbaur,t the process appears to be of essentially the same nature as in Pelagia, The Scyphistoma of Aurelia, Cyanoea and their allies is probably to be re- garded, like the larva of Pelagia^ as a Discophore vnth a rudi- mentary disk ; in which case, the production of the Ephyra- forms of young Discophora will not be comparable to the development of medusoid gonophores among the Hydro- phcTCL^ but will merely be a process of multiplication, by transverse fission, of a true, though undeveloped, Discophore.
In the 8iphonophora,X the result of yelk division is the formation of a ciliated body consisting of a small-celled ectoderm investing a solid mass of large blastomeres, which
* ^ Stodien fiber die Entwicke- wechsel,' 1854.
hmg der Medusen tmd Siphono- X ^^ especially the late ob-
phoren." (* Zeitschrift fOr Wisa. servations of Metscbnikoff, ioc.
Zoolj* wriv.) eit.
f * Zor Ldire der Generations-
L 2
148 TBK AKATOKT or IirTBBTBBBATED ANIUALS.
Fig. as.
Flif. !B._FiKB. I. and II.— Cyama cBplllala (^tler Vki Beneden.*) 1. Tun) Hgdm Mv {ScgphiMoma atan), exhibiting their on!in>T]r
charooten. uid between tben tiro (a, &,) whioh we undergoing fiafion
(SfmUja itage).
* ' Becharolwi nr U Fuiu litlonla dc Bdfliiua. Palj>pe^' 1666.
THB DBVXLOPMENT OF THE HYDBOZOA. 149
n. The two Strobihty a and 6, three days later. In a, tentacles are dereloped, beneath the lowest of the Ephfra^ from the stalk of the StrobUOf which will persist as a Hydra tuba.
III. Half the disk of an Epk^a of AurtHa aurita, seen from the oral fiUM. The small tentacles which lie between the mouth and the baad of circular muscular fibres are inside the somatic cavity, wlMBoe sixteen short and wide radial canals extend to the periphery, where they are united by transverse branches. Eieht of the radial eaoala enter the corresponding l«»bes, and finally divide into three branehes ; one which enters the peduncle of the lithocyst, and two latarml e«ca. Badiating bands of muscular fibres accompany these
IT. flkloTlewof oneof the lithocysts with its peduncle. The arrow the direction in which the cilia of the exterior work.
eFentnaliy pass into the cells of the endoderm. This body does not take the form of an actinula. On the contrary, it i^pears to be the rale that buds from which a hydro- pbjUinm, a nectocalyx, a tentacle, or pneomatophore, or even all of them, will be developed, take their origin ante- cedently to the formation of the first polypite and of the gastric cavity.
As Metschnikoff well remarks, the mode of development of the Siphanophora is wholly inconsistent with the doctrine that the various appendages of the hydrosoma in these animals represent individuals. The Hydrozoa are not pro- perly compound organisms, if this phrase implies a coales- cence of separate individualities ; but they are organisms, the organs of which tend more or less completely to become independent existences, or zooids. A medusoid, thoiigh it feeds and maintains itself, is, in a morphological sense, simply the detached independent generative organ of the hydrosoma on which it was developed ; and what is termed the " alternation of generations," in these and like cases, is the result of the dissociation of those parts of the organism on which the generative function devolves, from the rest.*
In certain Discopliora belonging to the group of Traehy- nemata, a method of midtiplication by gemmation has been
* I have seen no reason to ture published in the ' Annals and
depart from the opinions on Mai^azineof Natural Uisiory' lor
the subkct of *' Animal indivi- June 1852. dualiqr'' enonclated in my lee-
150 THB ANATOMY OF INYEBTEBBATED ANIMALS.
observed, which is nnknown among the other Hydrozoa, It may be termed entogastric gemmation, the bud growing out from the wall of the gastric cayity, into which it even- tually passes on its way outwards; while, in all other cases, gemmation takes place by the formation of a diver- ticulum of the whole wall of the gastro-vascular cavity, which projects on to the free surface of the body, and is detached thence (if it become detached), at once, into the circumjacent water. The details of this process of entogastric gemmation have been traced by Haeckel* in Carmarina hiutcUa, one of the OeryonidcB, As in other members of that family, a conical process of the mesoderm, covered by the endoderm, projects from the roof of the gastric cavity and hangs freely down into its interior. Upon the surface of this, minute elevations of ^^th of an inch in diameter make their appearance. The cells of which these outgrowths are composed next become differentiated into two layers — an external clear and transparent layer, which is in contact with the cone, and invests the sides of the elevation ; and an inner darker mass. The external layer is the ectoderm of the young medusoid, the inner its endoderm. A cavity, which is the commencement of the gastric cavity, appears in the endodermal mass, and opens outwards on the free side of the bud. The latter, now f^th of an inch in dia- meter, has assumed the form of a plano-convex disk, fixed by its flat side to the cone, and having the oral aperture in the centre of its convex free side. The disk next increasing in height, the body acquires the form of a flask with a wide neck. The belly of the flask is the commencement of the umbrella of the budding medusoid; the neck is its gas- tric division. The belly of the flask, in fact, continues to widen out until it has the form of a flat cup, from the centre of which the relatively small g^astric neck projects, and the bud is converted into an unmistakable medusoid, attached to the cone by the centre of the aboral face of its umbrella. In the meanwhile, the gelatinous transparent mesoderm has appeared, and, in the umbrella, has acquired a gi'eat rela- * ' Beitrage zur Naturgeschichte der Hydromodusen,' 1865.
V088IL HTDBOZOA. 151
tire thickness. Into this, eight prolongations of the gastric cavity extend, and give rise to the radial canals, which become nnited into a circnlar canal at the circumference of the disk. The velum, tentacula, and lithocjsts are dereloped, and the bud becomes detached as a free swim- ming medusoid. But this medusoid is very different from the Carmarina from which it has budded. For example, it haa eight radial canals, while the Carmarina has only six ; it has solid tentacles, while the adult Carmarina has tubular tentacles ; it has no g^astric cone, and has differently dis- posed lithocysts. Haeckel, in fact, identifies it with Cunina rhododadyla, a form which had hitherto been considered to be not only specifically and generically different from Carmarina, but to be a member of a distinct family — ^that of the JBginidiB.
What makes this process of asexual multiplication more remarkable is, that it takes place in Carmarince which have already attained sexual maturity, and in males as well as in females.
There is reason to believe that a similar process of ento- gastric proliferation occurs in several other species of .^IginidoBf — .^gineta prolifera (Gregenbaur), EuryaUmia ruin- gino9um (Kolliker), and Cunina Kollikeri (F. Mtdler) ; but, in all these cases, the medusoids which residt from the gemmative process closely resemble the stock from which they are produced.
As might be expected, the Hydrozoa are extremely rare in the fossil state, and probably the last animal the dis- covery of fossil remains of which could be anticipated, is a jelly-fish. Nevertheless, some impressions of Medusae, in the Solenhofen slates, are sufficiently well preserved to allow of their determination as members of the group of RhiMOtiomid<B.* The apparent absence of the remains of Hydrophora in the mesozoic and newer palteozoic rocks, is very remarkable. Some singular organisms, termed Orap-
* *Haeckel, ** Ueber swei neae fossile Medusen nus der Fusille der Bhiivjttomidon." OJi^^l>ucii liir Mineralogie/ 1866.)
152 THE JLKATOICT OF IKYSBTBBBATED ANIMALS.
MUeSf which abound in the Silurian rocks, may possibly be Hydrozoat though they present points of resemblance with the Polyuoa, They are simple or branched stems, sometimes slender, sometimes expanded or foliaceous; occa- sionally the branches are connected at their origin by a membranous expansion. The stems are tubular, and beset on one or both sides with minute cup-shaped prolongations, like the thecffi of a Sertularian. A solid thickening of the skeleton may ha^e the appearance of an independent axis. Allman has suggested that the theciform projections of the Graptolite stem may correspond with the nematophores of Sertularians, and that the branches may have been terminated by hydranths. Appendages which appear to be analogous to the gonophores of the Hydrophara, have been described in some Graptolites.*
With a very few exceptions {Hydra, Cordylophora,) the Hydrotoa are marine animals ; and a considerable number, like the CahfcophoridcB and PhyBopharidce, are entirely pelagic in their habits.
The AcTiNOZOA. — ^The essential distinctions between the Aetinozoa and the Hydrozoa are two. In the first place, Uie oral aperture of an Aotinozoon leads into a sac, which, without prejudice to the question of its exact function, may be termed " gastric," and which is not, like the hy- dranth of the Hydrozoon, free and projecting, but is sunk within the body. From the walls of the latter, it is separated by a cavity, the sides of which are divided by partitions, the mesewteriea, which radiate from the wall of the gastric sac to that of the body, and divide the somatic cavity into a corresponding number of intermesenieric chambera. As the gastric sac is open at its inner end, however, its cavity is in free communication with that of the central space which communicates with the intermesenteric chambers; and the central space, together with the chambers, which
* Hall: *GraptoIites of the of the BriUsh GraptoUaUe,' Quebec Series of North America,' 1872. 1865. Nicholson: *Moziogiaph
THB ACTINOZOA. 158
ten collectively termed the " body carity " or " peri" il cavity,'* are, in reality, one with the dig^estive cavity, • in the Hydrotoa, constitute an erUeroecBle, Thus an noon might be compared to a Lncemaria, or still better Jatrduella, in which the outer face of the hydranth «d with the inner face of the umbrella ; under these utancee the canals of the umbrella in the Hydro- roold answer to the intermesenteric chambers in the >ioon.
nidly, in the AeHnoaoci, the reproductive elements are ped in the walls of the chambers or canals of the cosle, just as they so commonly are in the walls of the -vascular canals of the Hydroioaf but the generative I thus constituted do not project outwardly, nor dis- ) their contents directly outwards. On the contrary, •A and spermatozoa are shed into the enterocoele, and lally make their way out by the mouth. In this re- again, the Actiuozoon is comparable to a Lucemaria ed by the union of the hydranth with the ventral face umbrella ; under which circumstances the reproduc- ements, which, in all Hydrozoa, are developed, either waUs of the hydranth or in those of the oral face I umbrella, would be precluded from making their f any other route than through the gastro-vascular and the mouth.
he fundamental composition of the body of an ecto- uid endoderm, with a more or less largely developed erm, and in the abundance of thread-cells, the Acti- %gree with the Hydrozoa,
lost of the Actinoioa, the single polype, into which the 0 is converted, gives rise by budding to many zooids form a coherent whole, termed by Lacaze-Duthiers, \hodeme.
CosALLiGENA. — The Actinozoa comprehend two I — ^the Corcdligena and the Ctenophora, — ^which are different in appearance, though fundamentally similai* cture. In the former, the mouth is always surrounded
or more circlets of tentacles, which may be slender
164 THB ^NATOirr of DrVIBTBBRATED AITIHALB.
and conical, or ihort, broad and fimbriated. The month is lunall; eloDKated in one direction, and, at the eztremitiee of the long diameter, preaente folds which are continued into the gastric cavity. The arrangement of the parU of the bodj is therefore not so oompletdy radiate aa it appears to be. The enteroctele is divided into six, eight, or more, wide intermesenteric chambers, which oommonicate with the cavities of the tentacles, and Bometimes, directly with the exterior, by apertures in the parietea of the body. The mesenteries which separate these wide chambers are thin Fig. 89.
Fig. 29.— ParpendJcalir iMtlon of ^cftua hobatira (■ftrr Vnj and jleuclart} — a, aoulh; (, nitilo cavity; e. common aiity, into which the gailric uvity and tba inlermeaenWric chambtn opeti ; d, Intennesenteiio ohamben ; i, tblekaDCd fnt margin, oontaining threid-celli of,^ a mcaentery ; g, reproduoCiva organ ; A, tentacle, and membranons. Two of them, at opposite ends of a transrerse diameter of the Actinozoon, are often different from the rest. Each mesentcTy ends, at ite aboral ex- tremity, in a free edge, often provided with a thickened and folded margin; and these free edges look towards thecentre of an axial cavity,* inte which the gastric sao and all the intermesenteric chambers open. In the Coralligena, the outer wall of the body is not pro-
* Partiillv digested iubttia<!ei that It may functionally represent are often Aiuud In thii axial theilamachDrthecomir>enc«Df?nt ■liM*, and It U Dot improbable of tlw iniallne in lilgher anlmali.
THB CO&ALUOBNA. 155
▼ided with bands of large paddle-like cilia. Most of them aie fixed temporarily or permanently, and many g^ye rise by gemmation to turf -like, or arborescent, zoanthodemes. l^e great majority possess a hard skeleton, composed prin- cipally of carbonate of lime, which may be deposited in per- manently disconnected spicnla in the walls of the body ; or the spicnla may mn into one another, and form solid net- works, or dense plates, of calcareous matter. When the latter is the case, the calcareous deposit may invade the base and lateral walls of the body of the Actinozoon, thus giving rise to a simple cup, or theca. The skeleton thus formed, freed of its soft parts, is a " cup-coral," and receives the name of a eoraUUe,
In a zoanthodeme, the various polypes {anthoMOoids), formed by gemmation may be distinct, or their several enterocceles may communicate; in which last case, the common connecting mass of the body, or cceno»arc, may be traversed by a regular system of canals. And, when such compound AcHnoaoa devcdope skeletons, the coraUites may be distinct, and connected only by a substance formed by the calcification of the ccenosarc, which is termed eoenenehyma ; or the thecsB may be imperfectly developed, and the septa of adjacent corallites run into one another. There are cases, again, in which the calcareous deposit in the several polypes of a compound Actinozoon, and in the superficial parts of the codnenchyma, remains loose and spicu- lar, while the axial portion of the coenosarc is converted into a dense chitinous or calcified mass — ^the so-called aclerdbuse.
The mesoderm contains abundantly developed muscular fibres. The question whether the Coralligena possess a nervous system and organs of sense, hai^dly admits of a definite answer at present. It is only in the Actinidce that the existence of such organs has been asserted ; and the nervous circlet of Actinia, described by Spix, has been seen by no later investigator, and may be safely assumed to be non-existent. Professor P. M. Duncan, F.R.S.,* how-
* * On the Nervous System of Actinia.' (Proceedings of the Royal Sodety, October 9, I87;5.)
156 THE AKATOICT OF IirYSBTBBRATED ANIMALS.
ever, has recently described a nervous apparatus, consist- ing of fusiform ganglionic cells, united bj nerve fibres, which resemble the sympathetic nerve fibrils of the Verte- brata, and form a plexus, which appears to extend through- out the pedal disk, and very probably into other parts of the body. In some of the AcHnidcB {e.g.. Actinia mesemhry- anthemum), brightly coloured bead-like bodies are situated in the oral disk outside the tentacles. The structure of these " chromatophores," or " bourses calicinales," has been carefully investigated by Schneider and Botteken, and by Professor Duncan. They are diverticula of the body wall, the surface of which is composed of close-set "bacilli," beneath which lies a layer of strongly-refracting spherules, followed by another layer of no less strongly-refracting cones. Subjacent to these. Professor Duncan finds ganglion cells and nerve plexuses. It would seem, therefore, that these bodies are rudimentary eyes.
The sexes are united or distinct, and the ovum is ordi- narily, if not always, provided with a vitelline membrane. The impregnated ovum gives rise to a ciliated morula, which may either be discharged or undergo further development within the somatic cavity of the parent. The morula be* comes a gastrula, but whether by true invagination or by delamination, as in most of the HydroMa, is not quite clear. The gastrula usually fixes itself by its closed end, while tentacles are developed from its oral end. It can hardly be doubted that the intermesenteric chambers are diverticula of the primitive enterocoele; but the exact mode of their origin needs further elucidation.
Lacaze-Duthiers * has recently thrown a new light upon the development of the CoraUigena, and particularly of the AcHnicB {Actinia, SagarHa, Bunodes), These animals are generally hermaphrodite, testes and ovaria being usually found in the same animal, and even in the same mesenteries ; but it may happen that the organs of one or the other sex are, at any given time, exclusively developed. The ova
* ** Developpement dee Coralliaires." (* Archives de Zoologie ex- perimenule,* 1872.)
THE DEVELOPMBNT OF THE COSALLIOBNA.
157
undergo the early stages of their development within the body of the parent. The process of jelk division was not observed, and in the earliest condition described the embryo was an oval plannla-like body, composed of an inner coloured substance and an outer colourless layer. The outer layer (epiblast = ectoderm) soon becomes ciliated. An oval depression appears at one end, and becomes the mouth * and gastric sac, while, at the opposite extremity, the cilia elongate into a tuft. The ectoderm extends into and lines the gastric sac, while the interior of the coloured hypoblast becomes excavated by a cavity, the enterocoele, which communicates with the gastric sac. In this con- dition the embryo swims about with its oral pole directed backwards.
The oral aperture changes its form and becomes elongated in one direction, which may be termed the oral axis. The mesenteries are paired processes of the transparent outer layer (probably of that part which constitutes the meso- derm) which mark off corresponding segments of the enterocoele. The first which make their appearance are directed nearly at right angles to the oral axis near, but not exactly in, the centre of its length. Hence they divide the enterocoele into two primitive chambers, a smaller (A) at one end of the oral axis, and a larger (A') at the other. This condition may be represented by A -f- A' ; the dots indicating the position of the primitive mesenteries, and the hyphen that of the oral axis. It is interesting to remark that, in this state, the embryo is a bilaterally symmetrical cylindrical body, vnth a central canal, the ^ future gastric sac ; and, communicating therewith, a bilobed enterocoele, which separates the central canal from the body wall. In fact, in principle, it resembles the early
* Kowalewsky describes the finrmation of a gastrula hy inva- giaatioii in a species of Actinia •ad in CTeanthut, the aperture cf invagination becoming the Sonth (liofmann and Schwalbe, ^Jahretbericht/ Bd. U. p. '26 j).
In other species of Actinia and in AlcyoHium, the planula seems to delarainate. Ordinary yelk divi- sion occurs in some AmhozoOy while iu others {Afcyonium) the process rather resembles that which occurs in most Arthropods.
158 THB ANATOMY OF HTYBBTSBBATBD ANIMALS.
condition of the embryo of a Ctenophore, a Brachiopod, or a Sagitta,
Another pair of mesenteric processes now makes its appearance in the larger chamber A', and cuts off two lateral chambers, BB, which lie between these secondary mesenteries and the primary ones. In this state the eu-
teroccele or somatic cavity is four-chambered I ^^ "t- ^ A' Y
Next a third pair of mesenteries appear in the smaller chamber (A), and divide it into three portions, one at the end of the oral axis (A), and two lateral (CO). In this
stage there are therefore six chambers ( A rj-r-g A' 1 ; but
almost immediately the number is increased to eight, by the development of a fourth pair of mesenteries in the chambers B, B, which thus g^ve rise to the chambers D, D, between the primitive mesenteries and themselves. The embryo remains in the eight- chambered condition
A Qt-i-^ B ^ 7 ^^^ some time, until all the chambera
and their dividing mesenteries become equal. Then a fifth and a sixth pair of mesenteries are formed in the chambers 0, 0, and D, D ; two pairs of new chambers, E and F, are produced, and thus the Aetinia acquires twelve chambers
(-^ n E"^ D B '^ /* ^^® ^^ which result from the sub- division of the smaller primary chamber, and seven from that of the larger primary chamber. The various chambers now acquire equal dimensions, and the tentacles begin to bud out from each. The appearance of the tentacles, however, is not simultaneous. That which proceeds from the chamber A' is earliest to appear, and for some time is largest, and, at first, eight of the tentacles are larger than the other four.
The coiled marginal ends of the mesenteries appear at first upon the edges of the two primary mesenteries ; then upon the edge of the fourth pair, and afterwards upon those of Uie other pairs.
THE OCTOCOIIAUJL. 159
For the farther changes of the young Actinia, I mnst refer to the work cited. Sufficient has been said to show that the development of the ActinuB follows a law of bilateral symmetry, and to bring out the important fact that, in the course of its development, the finally hezamerous Anthozoon passes through a tetramerous and an octomerous stage.
Phenomena analogous to the "alternation of genera- tions," which is so common among the Hydrozoay are unknown among the great majority of the Actinozoa, But Semper * has recently described a process of agamogenesis in two species of Fungias, which he ranks under this head. The FungicB bud out from a branched stem, and then be- come detached and free, as is the habit of the genus. To make the parallel with the production of a medusoid from a hydroid polype complete, however, the stem should be notirished by a sexless anthozooid of a different character from the forms of FungicB which are produced by gemma- tion. And this does not appear to be the case.
In one division of the Coralligerui — ^the Octocoralla — eight enterocoele chambers are developed, and as many tentacles. Moreover, these tentacles are relatively broad, flattened, and serrated at the edges, or even pinnatifid. The Acti- nozoon developed from the e^g may remain simple (Haimea, Milne-Edwards), but usually gives rise to a zoanthodeme.
The CQBnosarc of the zoanthodeme in the Octocoralla is a substance of fleshy consistence, which is formed chiefly of a peculiar kind of connective tissue, containing many mus- cular fibres developed in the thickened mesoderm. The axial cavity of each anthozooid is in communication with a system of large canals. In Alcyonium, a single large canal descends from each anthozooid into the interior of the zoanthodeme, and the eight mesenteries are continued as so many ridges throughout its entire length,t so that these tubes have been compared to the thecal canals of the Mille-
* * Ueber Generations-Wechsel tributlon k rAnatomie des Alcy- beiSteiukorailen.' Leipzig, 1872. onaires." (* Journal d'Anatomie t Poodiet and Myevre» '* Con- et de la Fbysiologie/ 1870.)
Pig. 30.— Oanlliim nbnm (■»«- LuMBv-Diithien •}.
I. The and of ■ bnach vlth A, B, C, Uitm aDllixinoidi In dlOen degrcet rif pxpaiiilon ; i, thp moutti ; a, that ptrl i<f ihe ciraini wUoh rises Into a cup arouitd tlia liata of aacb ai "
< miMIn Natoralla dn CoitU,' IBtt.
THB ACTINOZOA. 161
II. Portion of a branch, the coenosmro of which hu been divided longitudinally and partially removed ; B, B\ B", anthosooids in sec- tion ; B^ anthosooia with expanded tentacles ; A, mouth ; m, gastric sac ; t, Its inferior edge ; j, mesenteries.
B*, anthosooid retracted, with the tentacles (d) drawn back into the intermesenteric chambers ; c, orifices of the cavities of the in vagi- Dated tentacles ; e, drcum-oral cavity ; 6, the part of the body which forms the projecting tube when the anthosooid is expanded ; a, fes- tooned edges of the cup.
B", anthosooid, showing the transverse sections of the mesenteries.
A, A, Coenosarc, with its deep longitudinal eanats (/), and superficial, imgolar, reticulated canals (A). P, The hard axis of the coral, with loogitiidinal grooves (ff) answering to the longitudinal v<;9sels.
ni., Iv. free ciliated embryos.
pores. In the red coral of commerce (Gordlliwn rubrumf Fig. 90), tbe large canals mn parallel with the axial skele- ton. A delicate network, which traverses the rest of the substance of the ccenosarc, appears to be sometimes BoHd and sometimes to form a system of fine canals opening^ into the larger ones. The anthozooids possess nnmerons muscles by which their movements are effected. The fibres are delicate, pale, and not striated. Nerves have not been certainly made out.
It is in these OetocoraUa that the form of skeleton which is termed a aclerohasef which is formed by comification or calcification of the axial connective tissue of the zoantho- deme, occurs. It is an unattached simple rod in Pennaiula and VeretiUum, but fixed, tree-like, branched, and even re- ticulated, in the Chrgonice and the red coral of commerce {CoraJUum), In the Alcyonia, or "Dead men's fingers,'* of our own shores, there is no sclerobase, nor is there any in Tubipcra, the organ coral. But, whereas in all the other OetocoraUa the bodies of the polypes and the coenosarc are beset with loose spioula of carbonate of lime, Tuhipora is provided with solid tubiform thecce, in which, however, there are no septa.
Dimorphism has been observed by Kolliker to occur extensively among the Pennatulidce. Each zoanthodeme presents at least two different sets of zooids, some being fully developed, and provided with sexual organs, while the others have neither tentacles nor generative organs, and
162 THE ANATOMY OF IKTBBTEBSATED ANIMALS.
exhibit some other peculiarities.* These abortive zooids are either scattered irregularly among the others {e,g, Sarcophyton, Veretillum), or may occupy a definite position {e.g. Vtrgtdaria).
In the other chief division of the CaraUigena — ^the Hexa- ecraUa — ^the fundamental number of enterocode chambers and of tentacles is six,t and the tentacles are, as a rule, rounded and conical, or filiform.
The Actinozoon developed from the egg in some of the Heooaeoralla remains simple, and attains a considerable size. Of these — the Actinidm — many are to some extent loco- motive, and some (Minyaa) float freely by the help of their contractile pedal region. The most remarkable form of this group is the genus CereanthtUy which has two circlets, each oomposed of numerous tentacles, one immediately around the oral aperture, the other at the margin of the disk. The foot is elongated, subcorneal, and generally presents a pore at its apex. Of the diametral folds of the oral aperture, one pair is much longer than the other, and is produced as far as the pedal pore. The lalra is curiously like a young hydrozoon with four tentacles, and, at one time, possesses four mesenteries.
The Zoan^idoR differ from the AcHnidib in little more than their multiplication by buds, which remain adherent, either by a common connecting expansion or by stolons; and in Uie possession of a rudimentary, spictllat, skeleton. In thd AntipaihidoR there is a sclerobasio dceleton. The proper stone-corals are essentially ActinioBy which Ibieoome converted into zoanthodemes by gemmation or fission, and develope a continuous skeleton.
The skeletal parts ^ of all the AciinoMoa, consist either of a substance of a homy character ; or of an organic basis impregnated with earthy salts (chiefly of Ume and mag- nesia), but which can be isolated by the action of dilute
* * Abhandlungen der Senken- they are either six or some mul-
bergiaohen N atunonohenden Ge- tiple of dz.
telltolMlt,' fid. vU. viii. 1 See KdlUker, *Ieoiiet Histo-
t That It to M7, in the adult, logioii,' 16^
THB ACTIHOZOA. 163
acids ; or finall j, of caJoareous salts in an almost crystalline state, forming rods or corpuscles, which, when treated with acids, leaye onlj an inappreciable and stractnreless film of organic matter. The hard parte of all the Aporo8a, Pefforaia, and 7}abHlaia of Milne-Edwards are in the last-mentioned condition; while, in the OetocoraUa, except TkfnporOf and in the Ani^paihidcB, and Zoanihidcs, among the HetoaeoraUa, the skeleton is either homy ; or ocmsists, at any rate, to begin with, of definitely formed spicBla, which contain an organic basis, and frequently present a laminated stmcture. In the organ coral {Tubipora), the skeleton has the character of that of the ordinary stone-corals, except that it is per- forated by numerous minute canals.
The skeleton api>ear8, in all cases, to be deposited within the mesoderm, and in the intercellular substance of that layer of the body. Eyen the definitely shaped spicula of the Oetoeorcdla seem not to result from the metamor- phosis of cells. In the simple aporose corals the calcifica- tion of the base and side walls of the body, gives rise to the cup or theca ; from the base the calcification extends upwards in lamellsB, which correspond with the interspaces between the mesenteries, and gires rise to as many vertical tepta,^ the spaces between which are termed loeulii while, in the centre, either by union of the septa or indepen* dently, a column, the eolumeUa, grows up. Small separate pillars between the columella and the septa are termed pahUL From the sides of adjacent septa scattered processes of calcified substance, or aignapHculmy may grow out toward one another, as in the Fungidm ; or the intermption of the cavitiee of the loculi may be more complete in consequence of the formation of shelves stretching from septum to septom, but lying at diiferent heights in adjacent loculi. These are inienepUd dissepimenU, Finally, in the TabukUa, horizontal i^tes, which stretch completely across the cavity of the theca, are formed one above the other and constitute idMar di$$epimeni9.
* Lsssse-Diitfaien' invettieations on Astrtsa ecilycularU prove th&t te septa b^gin to b« funned belbre the theoa.
164 THB ANATOMY OF IKYSBTEBRATED AKHCALS.
In the Aporosa the theca and septa are almoet inyariablj imperforate ; but, in the Perforata, they present apertures, and, in some Madrepores, the whole skdeton is reduced to a mere network of dense calcareous substance. When the HexacoraUa multiply bj gemmation or fission, and thus give rise to compound massive or arborescent aggregations, each newlj-f ormed coral polype developes a skeleton of ita own, which is either confluent with that of the others, or is united with them by calcification of the connecting sub- stance of the common body. This intermediate skeletal layer is then termed eoeneMiihyiMi,
The septa in the adult Hexacoralla are often yery numerous and of different lengths, some approaching the c^itre more closely than others do. Those of the same lengths are members of one ' cycle; ' and the cycles are numbered accord- ing to the lengths of the septa, the longest being counted as the first. In the young, six equal septa constitute the first cycle. As the coral grows, another cycle of six septa arises by the development of a new septum between each pair of the first cycle ; and then a third cycle of twelve septa divides the previously existing twelve interseptal chambers into twenty-four. If we mark the septa of the first cycle A, those of the second B, and those of the third 0, then the space between any two septa (AA) of the first cycle will be thus represented when the third cycle is formed— A G B C A.
When additional septa are developed, the fourth and following cycles do not consist of more than twelve septa each ; hence the septa of each new cycle appear in twelve of the previously existing interseptal spaces, and not in all of them ; and the order of their appearance follows a definite law, which has been worked out by Milne-Edwards and Haime. Thus, the septa of the fourth cycle of twelve (d) bisect the interseptal space A C; and those of the fifth cycle (e) the interseptal space B G ; the septa of the sixth cycle (f), A d and d A ; those of the seventh cycle (g), e B and B e ; those of the eighth cycle (h), d G and G d ; and those of the ninth cycle (i), G e and e C.
THB '^TABUULTA. 165
Henoe, after the f onnation of nine c jclee, the septa added between every pair of primary septa (A, A) will be thus arranged — ^A fdhOiegBgeiOhdf A.*
The stone-corals ordinarily known as MiUeporet are characterised by being traversed by numerous tubular cayities, which open at the surface, and the deeper parts of which are divided by numerous close-set transverse par- titions, or tabtUar diM^fnmenU, while vertical septa are rudimentary or altogether absents These were regarded as Anihosoa, and classed together in the division of Tahidaia, until the elder Agassiz f published his observations on the living MiUepora aleicomiM, which led him to the conclusion that the Tabulata are Hydrozoa allied to HydracUnia, and that the extinct Btigoaa were probably of the same nature.
The evidence adduced by Agassiz, however, was insuffi- cient to prove his conclusions; and the subsequent dis- covery by Yerrill that another tabulate coral, PociUopora, is a true Hexacorallan, while Moseley ^ has proved that Selio- pora ccarviea is an Octocorallan, gave further justification to those who hesitated to accept Agassiz' views.
The recent very thorough and careful investigation of a species of Millepora occurring at Tahiti,§ by Mr. Moseley, although it still leaves us in ignorance of one important point, namely, the characters of the reproductive organs, yet permits no doubt that Millepora is a true Hydrozoon allied to Hydr<MeHniaf as Agassiz maintained. The surface of the living MiUepora presents short, broad hydranths, the mouth of which is surrounded by four short tentacles. Around each of these alimentary zooids is disposed a zone of from five to twenty or more, much longer, mouthless zooids, over the bodies of which numerous short tentacles are scattered. Each of these zooids expands at its base
* That the order of oecarrence United States/ vols. ill. and iv.,
of the septa of various lengths, at 1 860-62.
the different stages of growth of % Moseley, ** The Stnictare and
a oorallite, is that indicated, seems Relations of the Alcyonarian,
to be clear, whatever may be the Heliopora earulea^* &c. (Proc.
exact mode of development of the Royal Society, Nov. 1875.)
seirta in each cycle. § Proceedings of the Royal
t *Matacal History of the Society, 1876.
166 THE ANATOMY OF nfYSBTEBRATED ANIMALS.
into a dilatation, whence tabular processes proceed, whicli ramify and anastomose, giving rise to a thin expanded hydrosoma. The calcareous matter (composed as usual of carbonate, with a small proportion of phosphate, of lime) forms a dense oontinuous crust upon tiie ectoderm of the ramifications of the h jdrosoma, that part of it which under- lies the dilatations of the zooids constituting the septa. As the first formed hjdrosomal expansion is completed, another is formed on its outer surface, and it dies. The ^ thecal " canals of the coral arise from the correspondence in position of the dilatations of the zooids of successiye hydrosomal lay^:^ and the tabula are their supporting plates.
Thus the group of the Tabulaki ceases to exist, and its members must be gprouped either with the HexacoraUa, the Odocoralla, or the ffydrozwi.
The Bugoaa constitute a group of extinct and mainl j Palffiozoic stone->corals, the thecsa of which are provided with tabular diss^iments, and generally have the septa less developed than those of the ordinary stone-corals. The arrangement of the parts of the adult Bugoaa in fours, and the bilateral symmetry which they sometimes exhibit, are interesting peculiarities when taken in connection with the tetramerous and asymmetrical states of the embryonic HewacoriiUa. On the other hand, some of the Btigoaa possess opercula, which are comparable to the skeletal appendages of the Alcyonarian Primnoa observed by Lindstrom, and the tetramerous arrangement of their parts suggests affinity with the OdocaraUa. It seems not improbable that these ancient corals represent an intercalary type between the HexacoraUa and the OetocoraUa.
All the Actinoaoa are marine animals. The AcHnicB, among the HexacoraUa, and various forms of Oetoeonilla, have an exceedingly wide distribution, while the latter are found at very great depths.
The stone-corals, again, have a wide range, both as re- spects depth and temperature, but they are most abundant
THE BSBF-BinLDDrO CO&AL8. 167
in hot seas, and many are confined to sncli regions. Some of these stone-corals are solitary in habit, while others are social, growing together in great fields, and forming what are called " coral reefs." The latter are restricted within that comparatively narrow zone of the earth's surface which lies between the isotherms of 60°, or, in other words, they do not extend for more than about dO° on either side of the equator. It is not conditions of temperature alone, however, which limit their distribution; for, within this sone, tiie reef-builders are not found alive at a gpreater depth than from fifteen to twenty fathoms, while, at the equator, an average temperature of 68° is not reached within a depth of 100 fathoms.
Not only heat, then, but light, and probably rapid and effectual aeration, are essential conditions for the activity of the reef -building Aaiinonotu But, even within the coral SBone, the distribution of the reef -builders appears to be singularly capricious. None are found on the west coast of Africa, very few on the east coast of South America, none on the west coast of North America ; while in the Indian Ocean, the Pacific, and the Caribbean Sea, they cover thousands of square miles. It is by no means certain, how- ever, that any one species of West India reef coral is identical with any East Indian species, and the corals of the central Pacific differ very considerably from those of the Indian Ocean.
Different species of Corals exhibit great differences as to the rapidity of their growth, and the depth at which they flourish best ; and no one must be taken as evidence for another in these respects. Certain species of Perforata (Mad/reporid(B and PoritidoR) appeal* to be at once the fastest growers, and those which delight in the shallowest waters. The AMtrteida among the Aporosa, and SerUxtopora among the TalnUaia,]iye at greater depths, and are probably slower of increase.
Under the peculiar conditions of existence which have just been described, it would seem easy enough to compre- hend, d priori, the necessary arrangement of cot«1 tq^^.
168 THE ANATOMY OF INTBBTSBSATXD ANDCALfi.
As the Teet'hxnlding Adinotoa cannot live at greater depths than twenty fathoms, or thereabouts, it is clear that no reef can be originallj formed at a greater depth below the surface, and such a depth usually implies no very great distance from land. Furthermore, we should expect that the growth of the coral would fill up all the space between the shore and this furthest limit of its gprowth; so that the shores of coral seas would be fringed by a sort of fiat terrace of coral, covered, at most, by a very few feet of water; that this terrace would extend out until the shelving land upon which it had grown descended to a depth of some twenty fathoms; and that then it would suddenly end in a steep wall, the summit and upper parts of which would be crowned with overhanging ledges of living coral, while its base would be hidden by a talus of dead fragments, torn off and accumulated by the waves« Such a "fringing reef" as this, in fact, surrounds the island of Mauritius. The beach here does not gradually shelve down into the depths of the sea, but passes into a flat, irregular bank, covered by a few feet of water, and terminating at a greater or lees distance from the shore in a ridge, over which the sea constantly breaks, and the seaward face of which slopes at once sheer down into fifteen or twenty fathoms of water.
The structure of a fringing reef varies at different distances from the land, and at different depths in its sea- ward face. The edge beaten by the surf is composed of living masses of Forties, and of the coral-like plant, the Nullipore ; deeper than this is a zone of Aporosa (AstrceidiB), and of Millepores (Seriatopara) ; while, deeper still, all living coral ceases; the lead bringing up either dead branches, or showing the existence of a flat gently sloping floor, the true sea-bottom, covered with fine coral-sand and mud. Passing from the edge of the reef landwards, the Foritidm cease, and are replaced by a ridge of agglomerated dead branches and sand, coated with Nullipore ; the floor of the shallow basin, or " lagoon," enclosed between the reef and the land, is formed by a conglomerate, composed of frag-
FBINOIKO BEBF8. ATOLLS. 16^
ments of coral cemented bj mud ; and, on tliis, Meandrince and FitnguB rest and flourish, exhibiting the most gaudj coloration, and sometimes attaining a great size. During storms, masses of coral are hurled on to the floor of the lagoon, and there gradually increase the accumulation of rocky conglomerate ; but in no other way can a fringing reef, which has once attained its limit in depth, increase in size, unless, indeed, the talus accumulating at the foot of its outer wall should esver rise sufficiently high to afford a footing for the corals within their prescribed limits of depth.
Such is the structure of a fringing reef; but the g^reat majority of reefs in the Pacific are very different in their character. Along the north-eastern coasts of New Holland, for instance, a vast aggregation of reefs lies at a distance from the shore which varies from a hundlfed to ten miles ; forming a mighty wall or barrier against the waves of the Pacific. At a few hundred yards outside this " barrier reef " no bottom can be obtained with a sounding line of a thousand fathoms ; between the reef and the main land, on the contrary, the sea is hardly ever more than thirty fathoms deep. Many of the islands of the Pacific, again, are encircled with reefs corresponding exactly in their character with the barrier reef; separated, that is, by a relatively shallow channel from the land, but facing the sea with an almost perpendicular wall which rises from a very great depth.
Finally, in many cases, especially among the single reefs, which taken together constitute the great Australian barrier, there is no trace of any central island; but a circular reef, usually having an opening on its leeward side, stands out in the midst of the sea. These reefs, apparently unconnected with other land, are what are called " Atolls."
How have these barrier reefs, encircling reefs, and atolls been formed P It is certain that the fabricators of these reefs cannot live at a greater depth than in the fringing reefs. How can they have grown up, then, from a tbowsasA
170 THB ANATOMY OF INTBBTEBSATED AKDCALS.
fathoms or more? Why do they take so generally the circular form? What is the connexion, finally, between fringing reefs and atolls P The only thoroughly satis- factory answer to these questions has been given by Mr. Darwin, from whose beautiful work on Coral Beefs I have borrowed most of the f or^;oing details. Consider for a moment what would be the effect of a slow and gradual submergence of the island of Mauritius — a sab- mergence, perhaps, of a few feet in a century (at any rate, not greater than the rate of upward growth of ooral) continued for age after age. As the edge of the fringing reef sank, new coral would grow up from it to the surface ; and, as the most active and important of the reef-buildera flourish best in the very surf of the breakers, so the margin of the reef would grow faster than its inner portion, and the discrepancy would increase as the latter, Rinking deeper and deeper, became further removed from, the iregion of active growth^ Nevertheless, th^ sea-bottom within the reef would constantly tend to be raised by the accumulation of fragments, and by the deposit of fine mud, in its sheltered and comparatively calm waters. On the other hand, on the seaward face of the reef, no possible extension could take place by direct growth ; and that by accumulation must be exceedingly slow, the incessant wash of tides, waves, and currents tcoiding incessantly to spread any talus over a wider and wider area.
Thus, then, the edge of the reef imceasingly compensates itself for the depression which it undergoes, while, inside the reef, only a partial compensation takes place, and, outside, hardly any at all. Continue the sinking process until its highest peak was but a few hundred feet above the surface, and all that would be left of Mauritius would be an island surrounded by an encircling reef; carry on the depression further still, and a circular reef, or atoll, alone would remain. But the region of the coral reefs is, for the most part, that of constant winds. During the whole process of growth of the reef, therefore, one of its sides — ^that to windward — has been exposed to more surf
ANCIBirr SEBFS. 171
tlian that to leeward. Not only will the (greater quantity of debria, therefore, have been heaped up bj stormB upon the windward side, but the coral builders tiiemselyes will here have been better fed, better aerated, and conaequentlj more active. Hence it is that, other things being alike, there is a probability that the leeward side of the reef will grow more slowly, and repair any damages less easily, than the windward side ; and hence, again, as a result, the known fact, that the practicable channels of entrance into encir- cling reefs or atolls are usually to leeward.
The winds and waves are singularly aided in g^rinding down the oorals into mud and fragments, by the Seari and HiMhiuruB which haunt the reefs; the former browsing upon the living Polypes with their hard and parrot-like jaws, and passing a fine calcareous mud in their excrements ; the latter, more probably, swallowing only the smaller frag- ments and mud, and, having extracted from them such nourishment as they may contain, casting out a similar product. It is curious to reflect upon the similarity of action of these wormlike Holothurica upon the sea-meadows of coral, to that which the Earthworms, as Darwin has shown, exert upon our land meadows !
In the FalsBozoic period, reefs like those which have just been described appear to have aboimded in our own lati- tudes ; and there is the most striking superficial resemblance between the ancient beds of calcareous rock which record their existence, and the masses of coral limestone, hard enough to clink with a hammer, which are now being formed in the Pacific, by the processes of accumulation of coral- mud and fragments, and their consolidation by percolating water. Closer examination, however, shows an important difference in the nature of the corals which compose the two reefs. The modem limestones are made up of Per- foraia, Millepores, and Aporosa. The ancient ones contain MilleporeB, but usually neither Perforaia nor Aporo»a, — ^both these groups being replaced by the Bugoaa, none of whose members (with some doubtful exceptions) have survived the Palfldozoio period. On the other hand, Paksoc^elus «xl<\
17:2 THE ANATOMY OF IN VERTEBRATED ANIMALS.
Pleurodictyon are the onlj genera, belonging to the Aporosa or PefforcUa, which have jet been discoyered in strata of greater than mesozoic age.
The CteKophosa.* — These are freelj swimming marine animals, which never give rise by gemmation to componnd organisms, and are always of a soft and gelatinous consist* ence, their chief bulk being made up by the greatly de- veloped mesoderm. Many are oval or rounded {Berde, PUw rohrachia, Fig. 31), while in others the body is produced into lobes {Callianira), or may even be ribbon-shaped {Cegtwn) ; but, whatever their form, they present a distinct bilateral symmetry, similar parts being disposed upon opposite sides of a median plane, which is traversed by the axis of the body. The mouth is situated at one end of this axis, which may be termed the oral pole. At the opposite, or aboral pole, there is no median aperture, but usually, if not invariably, a pair of apertures a short distance apart The faces of the halves of the body present four longitudinal bands of long and strong cilia, disposed in transverse rows, like so many paddles ; these constitute the chief organs of locomotion. Each half is also often provided with a long retractile tentacle ; and lobed processes of the body, or non-retractile tentacula, may be developed on its oral face. The mouth leads into a wide, but flattened, gastric sac, the aboral end of which is perforated, and leads into a chamber termed the infundihulwm. From the aboral face of this, a canal which bifurcates, or two canals, lead to the aboral apertures. On opposite sides of the inf undibulum a canal is given off towards the middle of each half of the body, which sooner or later divides into two, and these two again subdivide, so that four canals, which diverge and radiate towards the inner faces of the rows of paddles, are
* Allman (* Monograph of the part from the conclusion to which
Tubularian Hydroida,' 1871, p. 3) i woe led by the study of the
conaidera that the CUnaphora are structure of Fleurobrachioj many
more properly arranged among years ago, that the Ctenophora are
the Jfytm^xoa. I confess, how- peculiarly modified Adinotoa^ ever, that I tee no reason to de-
THE CIZMOPHOB^ 173
rentually formed. Having readied tKe Borface, eocli
adiatiiig can&l enters a longitudinal canal, which Tinderiiea
he row of paddles, and ma; give off branches, or unite
rith the o^er longitudinal canals in a circular canal at the
boral end of the bodj. In addition, two other canals, which
on parallel with each flat face of the gastric sac, open into
be infundibulum. And, when retractile tentacnla are pre-
ent. their cavities also communicate with the unie chamber.
The entire Byatem of canals is in free communication with
he gastric cavity, and corresponds with the enteroocele of
n AMnia. Indeed, an Actinia with onlj eight mesenteries,
nd these exceedingly thick, whcrebj the intermesenteric
hambera would be reduced to canals; with two nboral
ores instead of the one pore, which ciistg in Csreanlhtu ;
nd with eight bands of cilia corresponding with the
tduced intermesenteric chambers, would have all the
sential peculiarities of a Ctcnophoran.
The question whether the Clenophora pOBsess a nervoua
item or not ia still under debate. Between the aboral
irturea there ia a rounded cellular body, on which there
«at«d, in manj ciiscs, a sac cuntaining solid particles,
one of the lithocjeta of the medusiform Hydrotoa. I
10 reason to doubt that the rounded body is a ganglion
the sac a rudimentary auditory oi^an. Bands which
.te from the ganglion to the rows of paddles may be
^ded as nerves; though they may contain other than
ns stmctnres.*
ant originally cleicrihed
■" .Md rid|t, whei
IfMiglion. Aght- lon hu not bpon vcri- hnnd, htse denied ll
UdbI cordi proceeiierl in (_Plairolrracliia), but hi*
BtwequflnC inveelignlorB. ii*«™nces described (lliough thej
f laUIIne-KdnardBiful- rcslly eiiai) are jiiatly Inter-
otben (among vh-irn 1 prcled. And agiiQ, Ihough IV -
idamyicirhtbenerviua bndf.defcrlhedu an otolithic M
Hului of a gan[(lirin, uiidoubLMl])' eiiiti in the poiilii
railed whether it li ui Midllory Thtu prablemt 1i«>t« 'bma t«-
174 TRX UTATOHT OV imrXSTXBKATED JiKHTAXB.
Th« on, and spennatozoa ftre dereloped in the lateral walls <^ the longitudinal canak, which oorreapond with the bees of the meeenteriea in the CoraUiggtta, and tlie aezea are nBoallf united in the «ame individiuL
The deTelopment of the CUnopkorxt has recentlj been thorooghlj investigated hj Kowalewakj and bj A. Agatrn'' (' Uemoirs of the American Aoademj of Arts and Scienoee,' 1874).
Fig. 31.
IB of iU bnndiM, ill, cual nmnlng bj the ^de of tli*
The laid egg is contained in a spacioaa capenle, and con- aiats of an external thin layer of protoplasm, which, in lonie oanca, ia contractile, inveeting an innv venonlar aobetaaoe.
oeatl; idnrcBllB^cd with (MM nripUos of the Darroiu tytten tmttf aod br the lid of In* M- baa alrtmdr bcea ^WUd ('Ve> ftMd metbodt of Mo4«ta Ullo> p. 64).
Uifj, by D*. EtaMC^ wboM i—
THE OTBNOPHOKA. 175
After fecundation, the vitellas thus constitnted divides into two, f onr, and finally, eight masses ; on one face of each of these the protoplasmic layer accumulates, and is divided off as a blastomere of much smaller size than that from which it arises. By repeated division, each of these gives rise to still smaller blastomeres, which become distinctly nucleated when they have reached the number of thirty-two, and form a layer of cells, which gradually spreads round the large blastomeres, and invests them in a complete blastodermic sac. At the pole of this sac, on the face opposite to that on which these blastoderm ceUs begin to make their appear- ance, an ingrowth or involution of the blastoderm takes place, which, extending through the middle of the large yelk-masses towards the opposite pole, gives rise to the alimentary canaL This, at first, ends by a rounded blind termination ; but from it, at a later period, prolongations are given off which become the canals of the enterocoole.
At the opx>osite pole, in the centre of the region corre- sponding with that in which the cells of the blastoderm first make their appearance, the nervous ganglion is developed by metamorphosis of some of these cdils.
The invaginated portion of the blastoderm, which gives rise to the alimentary canal, appears to answer to the hyx)oblast, while the rest corresponds with the epiblast. llie large blastomeres which become enclosed between the epiblast and hypoblast in the manner described, seem to serve the purpose of a food-yelk ; and the space which they originally occupied is eventually filled by a gelatinous con- nective tiseue, which possibly derives its origin from wander- ing cells of the epiblast.
Ib those Ctenophora the bodies of which depart widely from tlie globular form in the adult state, the young undefgo a soH of metamorphosis after they leave the egg, and haive acquired all the essential characters of the group to whidi they belong.
As might be expected from their extreme softness and, perishable natorey no fossil Ctenophora toe known.
176 THE ANATOMT OF UfYEBTEBRATID AJTIXALS.
CHAPTER IV,
THE TUBBELLABIA, THE BOTIFEBA, THE TBEMATODA,
AND THE CE8TOIDEA.
The Tubbellabia. — The animals wluch constitute this gi'oup inhabit fresh and salt water and damp localities on land. The smallest are not larger than some of the hifu* soria, which they approach very closely in appearance, while the largest may attain a length of many feet. Some are broad, flattened, and discoidal, while others are extremely elongated and relatively narrow. None are divided into distinct segments, except the genns Alaurifia, in which there are four; and the ectoderm, which constitutes the outer surface of .the body, is everywhere beset with vibratile cilia* Bod-like bodies, similar to those met with in some Infusoria and in many Annelida, are often embedded in its substance, and in some genera (e.g,, Microatomum, Thy$anO' poon) true thread-cells occur. Stiff sets project from the ectoderm in some species.
The aperture of the mouth is sometimes situated at the anterior end of the body, sometimes in the middle, or towards the posterior end, of its ventral face. In many, the oral apei*ture is surrounded by a flexible muscular Up, which sometimes takes on the form of a protrusible proboscis.
A deflnite digestive cavity can hardly be said to exist in the lowest Turbellaria (e. g. Convoluta) in which the endo- dermal cells are not arranged in such a manner as to bound a central alimentary cavity, and the food finds its way through the interstices of an endodermal parenchyma. In the higher forms, the alimentary cavity, which may be simple or ramified, provided with an anal aperture or without one,
THS TUBBBUaAJUA. 177
is lined by the endoderm, between which and the ectoderm is an interspace more or less completely occupied by the connective and mnsoolar tissues of the mesoderm* Hence there is no definite perivisceral cavity.
The TurbeUaria possess vessels of two kinds. 1. Waier veaseUf which open externally by one or more pores, and are cDiated. When these vessels are present, there are usually two chief lateral trunks, from which many branches are given off. It is probable that the ultimate ends of these branches open into lacunar interspaces between the elements of the tissues of the mesoderm. 2. Faeud-luBmal vessels, which appear to form a closed system, usually consisting of one median dorsal and two lateral trunks, which anasto* mose anteriorly and posteriorly. The walls of these vessels are contractile and not ciliated, and their contents are dear, and may be coloured. These two systems of vessels have been shown by Schulze to co-exist in TetrasteTiMMi, The nervous system consists of two ganglia placed in the anterior end of the body, from which, in addition to other branches, a longitudinal cord extends backwards on each side of the body. In some cases, these lateral trunks exhibit ganglionic enlargements, from which nerves are given off; and they may become approximated on the ventral side of the body, thereby showing a tendency to the formation of the double ganglionated chain oharacter- istio of higher worms. Most possess eyes, and some have auditory sacs. The TwrheUaria are both monoecious and dicBcious, and the reproductive organs vary from the utmost simplicity of structure to considerable complexity. In most, the embryo passes by insensible gradations into the form of the adult, but some undergo a remarkable metamorphosis.
The TwrbeOMria are divisible into two groups. In the one, the Aproda, the digestive cavity is csBCfd, having no anal aperture ; in the other, the Proetueha, it is provided with an anal opening* The two groups form parallel series, in. each of which organisation advances, from forms which are little inore than gaatruke provided with reproductive OT^gBoi&v^
178 T0E ANATOMY OF INYXBTIBBATSD AjriMALS.
animals of relatively high organiBation. In Uie simplest of the Aprocia, such as Macrotlomwm,^ the oral opening is devoid of any protrusible mnscnlar proboscis, and the ali- mentary sac is a siipple straight bag. The male and female generative organs are tinited in the same individual, and each consists of a^i aggpreg^tion of cells ; which, in the former case, gradually enlarge, fil| with yelk-gprannles, and become ova ; while, in the latter, they are converted into spermatozoa. The generative cells are contained within a sac, which opens externally by a median pore on the oral face of the body, the male apertore being posterior to the female. The margins of the male apertare> are produced into a curved prominence, the penis.
Those 2Vir5e{toria which resemble if oorosfomtMfi in having a straight, simple digestiye cavity, are termed Bhabdoctda, They, for the most part, possess a buccal proboscis, which is capable of being protruded from, or retracted into, a chamber formed by the walla of the ciroum-oral region of the body (Fig. 32, c).
In some {e,g, Protiowwn) the anterior end of the body is provided with a second hollow muscular proboscidiform organ, which may be termed theyroiiM probototf.
In all the higher rhabdocoelous TwiMlaria^ the female generative apparatus becomes complicated by the presence of a special gland, the vUelUmwm (Fig. 32, m), in which an accessory vitelline substance is formed. There is a single or double germarium (Fig. 32, Q, having tieariy the same struc- ture as the ovary of MacrosUmwrn, and the ova are formed in it in the same way. When detached, however, they con- tain no vitelline granules; but the two vitellaria, which are long and simple or branched tubes, open into the oviduct ; and the vitelline matter which they secrete envelopes the proper ovum, and becomes more or lesb fused with it, as it passes into the uterine continuation of the oviduct connected with the outer, or vaginal, end of the uterus. There is usually a' spermatheca, or receptacle for the s^Uiinal fluid (IHg.
^ E. Van Beneden, ' Recherches tar la Coinpotition et la Significa- tion ds rCSnf,* ld70» p. 64^
a, Jfc),Bnd the e;^, after impregnfttioii, are eaoloMd within
I h»id ahell (Fig. 32, n). The teatee and vaaa defereutiA
Fij.3a.
Fig. »i.—OpiMamiam (ofW 8abulc«).— a, OentnU dsttoq* t7it«m; mBiBvtiauof ihemUr vmmI* waaeanolow to it| A,ni)Mth;c, proboauii ; d, tNla ; •, ran dalerentia ; /, vetianlm umliulli ; g, poili; il, tnafti Rpartnre ; J, v^^na; 4,tpennsthM>-, i,gertD>riaiDi -■'-"—• — -iterei ■with two on, aulowd within th^ bard
(f^g. 8S, d, a) geQerallj hare the fonn ot two long tabes. The peoia is often evenible and corered with epinea (Pi^.
a J).
180 THS AVATOMT OV nTYBBTXBBATSD AVIVAL8.
In some genera a difference is observed between the eggs produced in summer, which have a soft vitelline membrane, and those produced later. These so-called winter ova have hard shells.
The water-vascular system consists of lateral trunks, which open by a terminal pore, or by many pores, and give off numerous ramifications. They are not contractile, but their inner surface is ciliated.
Many of the Bhahdoccsia multiply by transverse fission ; and, in the genus Caienula, the incompletely separated animals produced in this way swim about in long chains.
The vitellus of the impregnated ovum undergoes complete yelk-division, and the embryos pass directly into the form of the parent ; but the precise nature of the steps of the developmental process require further investigation. How- ever, there seems little reason to doubt that the ectoderm and endoderm are formed by delamination.
In the remaining Aprocta, termed Dendrocceila, the diges- tive cavity gives off many ccBcal, frequently branched, pro- cesses into the mesoderm, one of which is alvrays median and anterior (Fig» 38) ; and the mouth is always provided with a proboscis. Some (Proeoiyla) have a frontfd proboscis, and others (BdeUwra) a posterior sucker. The Miimnla commonly known as PlanaritB belong to this division. Some are marine, some fresh* water, and some terrestrial.
In the fresh-water forms, the female reproductive appa- ratus has a distinct vitellarium, as in the higher Ehab- docoda, and there is only one common genital aperture. But, in the marine Plancma (Fig. 33), there is no vitel- larimn ; the ovaries and testes are numerous, and scattered through the mesoderm, being connected with the exterior by ramifications of the oviducts and of the vasa def erentia. A ramified gland, which secretes a viscid albumen or en- velope for the eg^, opens into the vagina, and the female is distinct from the male 'aperture, Fkmtwia dUnea is oniseznaL
In some of the PIonarMs there ure distinct water^vaaoalir
THX DIHDSOOdl^ 181
cuuJa of the ordinuy kind ; but in the land PlanariuiB* two nearlj simple oanalo, ooonpied by a spongj tdirae, and
, _, . ... , :, cciopb*e»l oriflce; d, itomaeb; r,
nmlfloatiom or gutrio rnca; /, giDglia; jr, teiU*'. A, tbIcuIb> MBlwdot; i, male ganllml canal and peai*; 4, ovidaota; I, ipSF' matbiwal dlUtatioD al tbelrjuncllon ; n, vulva.
the connexion of which with the exterior hu not been obwrred, occapj the place at the water Teaaeb.
* Uemtr, " Oa Un Anatomy and Hlitolon of tha Land Fknariana ^rCajiam.'^ (PUUnophloal Transaclioni, 1873.)
182 THE ANATOMY OF niYSBTBBRATBD AJTIMALS.
The fresh- water PlanaricB, like the Mhahdoecda, undergo no metamorphosis in the coarse of their development ; and the like is tme of some of the marine Dendroccela, Kefer- stein * has carefully worked ont the development of Le^Uh plana (Polyeeli8)4 The TiteUas undergoes division first into two and then into four equal blastomeree; next, from one snrf ace of these four blastomeres, four small segments are, as it were, pinched off< These divide rapidly, and form a blastoderm, which grows over the more slowly dividing large segments, and eventually encloses them. So far, the process is very similar to that which has been de^ scribed in the Ctenophora. But though Kef erstein describes and figures the various stages by which the globular ciliated embryo attains the form of the adult, neither his descrip** tion nor the figures enable one to say whether the aliment tary cavity arises by delamination or by invagination, nor to trace the mode of origination of the buccal proboscis, though this organ is one of the first to make its appearance^ and its aperture becomes the future mouth*
In some of the marine Planariit, however, the embryo, when it leaves the egg, differs very widely from the adult« Johannes Miiller described such a larva, in which the body is provided with eight lobes or processes, one ventral and median in front of the mouth, three lateral, and one dorso-median« The edges of these processes are fringed by a continuous series of cilia, which pass from one process on to another, so as to form a complete circlet round the bodyi The successive working of the cilia forming this lobed transverse girdle of the body produces the appearance of a rotating wheels as in the Botifera. The eyes are situated on the aboral face of the embryo, in front of the ciliated circlet, while the mouth opens immediately behind it. As development proceeds, the lobes disappear, and the body takes on the ordinary Planarian character.
* * Beitrige sur Anatolnit nnd EntwickelongscMchichte einiger See- ranarien,' 186S.
TKB PBOCTUOKA. 163
Aa will be seen, some of the Pnehteha ttRve ]axvm Fig. 31.
I, Mntnl canglla of thg ngrroui c, ftperture through which the prabaacii
--■'— of proboscla ; e, poster'
, '' /( ?• inleitine; h, ,__
tani i, wter-Twdi ; k, rh^thiuioaUy contnoling Teueii. (Aflw BoIidIm.) B. Anterior extremitv of the everted proUncis of Telra- Mimmii, cihibitiDg the prindp*) tad the reserve atlleu. (After
0 the pwieti
nmilarlj provided with a pne-ond ciUat«d zone; Mud Lbx^»
184 THE ANATOMY OF INYSBTEBRATED ANIMALS.
of ihe same fundamental type abound among the poljchsB- tou8 Annelida, the Echinodermatat and the MoUu9ca.
The lowest ProcUiehti, such as MierogtomMm, have no frontal proboscis (whence they are termed Arhynehia), and they differ very little from the lowest Bhabdocoday save in the fact that there is an anus, and that the sexes are distinct. But all the other Produoha {Bhfnehoecela, or Nemerteans) are provided with a frontal proboscis, which sometimes occupies the greater part of the length of the body (Fig. 34), It has special retractor mnscleB, and its internal surface is either merely papillose, or may possess a peculiar armature, consisting of a sharp chitinoos style (Fig. 34, B). There is no buccal proboscis, but the month leads into a long, straight intestine, with short, lateral, cnoal dilatations.*
The Proctttoha usually present only ihe pseud-hsmal vessels, though, as has been mentioned abore, Schulze found water-vessels co-existing with them in Teiinutmnfna (Fig. 34).
The nervous system of the Proetueha is like that of the Aproeta; but, in correspondence with the often extreme elongation of the body, the backwardly-prolonged cords are very stout. Moreover, the ganglia are united by an additional commissure over the proboscis, which thus tra- verses a nervous ring. In some, the lateral cords approach one another on the ventral aspect of the body, and gan- glionic enlargements appear where the nerves are given off, thus presenting an approximation to the double ganglio- nated chain of higher forms.
In addition to eyes, almost all the Proetueha possess two ciliated fosses, one on each side of the head (Fig. 34, 55), which receive nerves from the ganglia. Occasionally two otolithic vesicles are attached to the cerebral ganglia.
The Proetueha are almost always dioBoious. The simple reproductive glands are lodged in the intervals between ihe saccular dilatations of the intestine, and the ova and aper- matoKoa nsually make their vray out by the dehiscence of
* For the organisatSon of the toeh*s elaborate monogr^h latelly Rhynchoooele 7\irbeUarkL or pabliahod by the Bay Society. • Kemerteanii set Dr. C Mcln-
I rmaanaB^
185
th« mtegimiant. In gome, howerer, tbe embrjoa are derdoped in the onu-iao aaoa, or in the oaritf of the bodj. Jn mort of ^e Proetueha, the egg, after pMsiiig through the moraU stage, aoqnires an alimentar; caritji . Fig ST.. Fig. 37.
TT, Hiwij-tned Semeneta.
apgtirttaiij by deUminatioD, and paaHee, without other netuaorphoaiB than the shedding of a ciliated outer inreatmaat, into the form of the adult.
186 THI AKATOMT OV IKTSBTSftftATSD AVUIAUB.
Professor A. Agassiz* has desoribed a free-swimming larva, the broad anterior end of the body of which is sur- rounded by a zone of cilia, immediately ;behind which the mouth opens ; while, around the anal aperture, at the narrow posterior end, is a second circlet of cilia. IThis larra exactly resembles those forms of polychsfftous Annelidan lanrsB which are called Telatroeha, As in these Annelids^ the region of the body which lies between the two ciliated rings elongates and becomes segmented, while a pair of eyes and two short tentacles ore developed on the head in fix>nt of the prse-oral ciliated band. But, as development adyances, the segmentation becomes obliterated, the dliated bands and the feelers vanish, and the worm assumes the characters of a NemerteaUif
In species of the genus Lineua, the ciliated embryo which leaves the egg is speedily converted into a body like a helmet with ear-lappets, and having a tuft of cilia in place of a plume (Fig. 35). The lappets are fringed with long cilia, and between them, where the head would fit into a helmet, is the aperture of a mouth, which leads into a c»cal pouch- like alimentary cavity. This larva was named by Miiller, who discovered it, PUidium gyrans. On each side of the ventral face of the PUidiwm, two involutions of the in- tegument take place. Aggregations of cells in relation with these, and probably forming part of the mesoblast, appear, eventually enclose the alimentary canal of the Pilidiunif and give rise to an elongated veiiniform body, in which the characteristic features of a Nemertean soon become discernible (Fig. 36). The worm thus developed be- comes detached (Fig. 37) and falls to the bottom, canying with it the alimentary canal of the Pilidivm, and leaving the ciliated integument to perish.
In this remarkable process of development the formation
* " On the Young Stages of a an anncctent form between the
few Annelids." (Annals of the Turbellaria and oUier groupa.
j.yceum of New York, 1864.) See Schneider, " Ueber Baa and
f It is very probable, however, Entwickelung von Polygordins."
that this larva belongs to the genus C*Arohiv t^i Anau mid Pbytio-
/'oJj|yo9tit«i, which appears to be logic,' 1868.)
TUE ROTIFERA. 1^7
of the Nemertean body may be compared, on the one hand, to that of the segmented mesoblast in Annelida and Ardkropoda^ and, on the other, to that of an Schinoderm, EthinuB,) within its larva.
The Born^BBA. — ^The " wheel-animalcnles," as they were termed bj the older obserrers, on account of the appearance of rotation produced, as in many Annelid larvse, by the working of the yibratile cilia with which the oral end of the body is provided, were formerly included among the Infu- mria, Hoitevert they are true MetoKoa, as their vitelluB undergoes division into blastomeres, and the tissues of the body are produced by the metamorphosis of the cells into which the blastomeres are converted* They are free or ad- herent, but never absolutely fixed animals, and they do not multiply by gemmation or fissioui The oral end of the body is usually broader than the opposite e^remity, and presents the form of a disk, sometimes produced into ten- tacle-like prolongations (Fig. 39). The edges of this troehai dUk are fring^ With long cilia, but the general surface of the body, instead of being ciliated, as in the TurheUaria, is formed by a dense, genei^ally chitinous, cuticular layer, which is sometimes converted into a kind of shell and variously sculptured* Transverse constrictions, which are slight in the anterior part of the body, but may become more marked towards its posterior end, give rise to an im- perfect segmentation. The segments do not appear to exceed six, and the divisions are less marked in the tubi- colous than in the free Bot\fera, The mouth is a funnel- shaped cavity» situated in the middle, or on one side, of the troohal disk. The walls of this cavity are abundantly cili- ated, and at the bottom is a muscular pharjmx, or maHaa, provided with a peculiar armature* Sometimes, as in 8tephaMOcero9, a large crop<*like cavity lies between the moatli and the mastax, and the aperture of communication between this crop and the mouth is guarded by a valve formed by two broad membranous folds which project into the cavity of the crop. The armature of the maatax
188 THE ANATOMT OF IHYXBTBBKATSD AKIMAUB.
g^eraUj consists of four pieces — ^two lateral, the mai and two central, constituting the ineu$. The contract of the muscnlar masses, to which the mallei are attach causes the free ends of the latter to work backwarda f forwards upon the incus, and crush the prey which taken into the mouth.*
A short oesophagus, proyided with ciHa or -rihratile mc branee, leads into a digestiye cavity bounded by the eii( derm. The anterior or gastric part of this cayi^ is usiu dilated, and giyes off a large csBcum on each side. 1 posterior, narrower, intestinal part usually opens externa by a cloacal chamber; but, in some Rotifers (e,g. NoU nuUa), the alimentary cavity is a blind sac, devoid intestine or anus; and in the males, so far as they i known, the whole alimentary canal is aborted and rep sented by a solid cord.
A spacious perivisceral cavity occupies the inter between the walk of the alimentary canal and the parie of the body. The latter contains circular and longitudi] muscular fibres, which may be smooth or striated.
Opening into the cloaca there is usually a large th walled vesicle with rhythmically contractile walls ; and, connexion with this, are two delicate water- vessels, whi pass forwards, often giving off short lateral branches, a eventually break up into numerous ramifications in t trochal disk. The branches are open at the ends, where the cavities of the water- vessels are in communication w: the perivisceral cavity on the one side, and with the m rounding water on the other. Here and there, in the con: of the main trunks and at the ends of the branches, lo cilia, which, by their constant undulation, give rise tc flickering motion, are situated.
The nervous system is represented by a relatively lai single ganglion placed on one side of the body, near i trochal disk. One or more eyespots are sometimes seal
* See, for the ▼arioui forms of loguei of the Mmuducating A this apperatuB, Gone, **On the raius in the Matifera/* (,' Straetore, Fnnetioiia, and Homo- Trans. 1855.)
189
on the gaoglion, and there are other organs which appear to be sensor;. Sncb are the ciliated pit asd the spur-like proooM (mfaor) or processes, provided at the end with a toft of Betas, which oooor in many Rotifan, and are more or less cloaelj connected with the ganglion. In Bome there ia a sac filled with caloareoos matter (otocjst F) attached to the ganglion.
Fig. 38.
Fig 3S.— fljnUiu imla (•ftgr Cohn).— A, tvaala: a, uin«; i, «Mi- tnetUa *erielg; «, waUr veMeli; <, avar;;/, ganglion. B, mate; '>TMlola; ^tMlli;/,gaD)!lton;9,MCigMMiiplt.
Tb« ararimn and the testis are simple glands which open into Um doaca, and are always plaoed in distinct indifidnala. All the malea at present knowa difFer from the fomalsB in btttns mnoh sm^er, uid in tJieir digestive ^^ft| beins ■rwited in it« derrfopmait, ThA nulaa cop^Qsite m^^ ^bb
190 THI ANATOMY OV nTfSBTlBRATSD AKIMAL8.
females, and the eggs are sometdmes attached to, and carried about by, the latter — e.g, Brachionut,
In some Rotifers, the eggs are distinguishable, as in certain TwrbeUaria, into wwnmer and tinnier ova. The latter are enclosed In a peculiar shelL In Laeimdaria, it appeared to me that the winter oya were segregated portions of the oyarium, and that they were probably dereloped without impregnation. Cohn, on the contrary, has given reasons for believing that the summer ova are occasionally, if not always, developed without fecundation, and that it is the winter ova which are fecundated.
The egg undergoes complete yelk«division, and the embryo gradually passes into the adult form. The blastomeres are soon of unequal sizes, and the smaller, as an epiblast, invest the larger, which form the hypoblast.
Salensky's * recent observations on Braehiontu ureeolaris show that a depresfdon arises on one face of the epiblaqt and that the antero-lateral parts of this depres^on are con- verted into the trochal disk, while its mediaii posterior part grows out into the "foot;" and he points out the resem- blance of the embryo in its early stages to that of some Gasteropoda.
An involution of the epiblast at the bottoqi of the depres- sion gives rise not only to the oral chamber, but also to the mastax; eventually communicating with the gastro-intes- tinal division, which is developed out of the hypoblast. The ganglion is a product of the epiblast,
Some of the modifications of the general structure thus described which occur in the different groups of the Boti* fera are of considerable interest.
Thus, in the tubicolous forms, the body is elongated and terminated posteriorly by a discoidal surface of adhesion. The animals (of which a number are often associated together), fixed by this disk, enclose themselves in cases, the foundation of which is a gelatinous secretion. The intestine is bent upon itself (Laeinularia, Fig. 39, 11.), and opens upon the face of the body opposite to that upon which the
• «2«ltieMft mt WiM. ZoobsiV 1871
TBI BOTIFB&l. 191
ganglion ia plaoed. Tha p«tdiinole of attaohment ia theraf ore a prooeaa <rf the uaonl face of the bod^. In these Bot^wa the troohal disk ia sometimes prodaced into long ciliated t«nt«cala, vhioh surround the month symnietricallj (iSfe- phoMOMrM — Pig, 39, V.) or ita edges njay be provided with two oireleta of ailia, one in front of, and the other behind, the ani aperture ; and it may be bilobed or horaeshoe-ehaped, aa in Jfalieorfa and ZooHMtlarM* (Fig. 39, 1. II.).
In the free Botifers, the bod^ ma^ be rounded, sao-like, and deroid of appandagea, aa in the genas Ai^anehna,
rig. ».
which haa neither anas nor inteatine. In Albtrtia and latuUm, on the other hand, the bodj ia elongated and venni- foim. Koat of the free Sot^ferv (Fig. 38) are provided with a Mgouotad and sometimes telescopicsUj-jointed " foot," nsnallj terminated b; two stj'lee, which can be approximated or dirarioated like pincers, and serve to anchor the body. Thia toot is » median proceaa of that face of the body which ia o^oaite to that on which the ganglion ia placed, ao that
', LaetnloHn nttallM. {TnumeOaat
F tiM MlGiaaeaftcBV
192 THE AHATOKT OV INYXBTBBBATKD UriMAUB.
it is not the homologae of the peduncle of the tubiooloui forms.
Polyarthra and Triarthra possess long, sjmmetricaUy arranged, movably articulated set»; and PedoHon has median appendages proceeding from both the neural and the opposite faces of the body, as well as lateral appendages.
In most of the free Botifers the trochal disk is large; it may be bilobed or folded upon itself (Fig. 39, in.)» or its surface may give rise to ciliated processes (Fig. 39, lY.). In Alberiia and Notommata tardigradot however, the trochal disk is reduced to a small ciliated lip around the oral aper- ture; and there is no trochal disk in ApBUuB, Lindia, Taphrocampa, and Bal<Uro, Some few Botif era are parasitic. Thus Alberiia is an entoparasite, and Balairo an ectopara- site, upon oligochffitous Annelids.
Under the name of Oasterotritha, Metschnikoff and ClaparMe * include the curious aquatic genera ChoBtonoUu, Ichthydium, ChoBiura, Cephalidiwn, DoBydUis, TurhaneUa, and Hemddatys, the last of which alone is marine. These ^Tiinri<t1a haYC been united with the BoHfera, bnt they differ from them in the absence of a mastax and in the disposi- tion of the cilia, which are restricted to the yentral surface of the body. It appears probable that they form an annectent group between the Botif era and the TurbeUaria, which last approach the Botifera by such forms as DinophUtu.
The free Botifers present marked resemblances to the telotrochous larvse of Annelids. The joxmg LacinuUmtif for example, has a circular prsD-ora] disk proyided with two eye-spots and a second circle of cilia behind the month, and is wonderfully like an Annelid lanra (Fig. 39, 1.). The ap- pendages of TViarihra and Polyarthra may be compared to the lateral bundles of long set® of the lairs of 8pio and Nerine, and the pharyngeal armature is essentially Anne- lidan. On the other hand, in the sessile tubicolous BoUfera, the trochal disk assumes the characters of the lophophore in the Pohftoa, and of the tentacular circlet of the G^hyiean
• ClapsrMe and Metsohnikoff, 'Beitrigs mr KenntnlM der Eat- wiokelnngigeicbiohts der Chsetopods^* ISCa,
THX TBBMATODA.
193
Phonmia, Many years ago I drew attention to the points of resemblance between the BoHfera and the larrse of Echinoderms ("On Lcusinvlaria soeiaXU** I. c.) Of any SQch close and direct relations with the Crugtaeea, I see no evidence ; but Pedalion,* with its jointed setose appendages and cnrious likeness to some Nauplitu conditions of the lower Orutieicea, suggests that connecting links in this direction may yet be f onnd-f In fact, the BotifercL, aa low Metazoa with nascent segpnentation, naturally present re- semblances to all those groups, which, in their simpler forms, converge towards the lower Metaxoa,
The Tbbkatoda. — These are all parasitic, either upon the exterior (ectoparasites) or in the internal organs (endo- parasites) of other animals. Many are microscopic, and none attain a leng^ of more than an inch or two. Most have a broad and flattened form, one face being yentral and the other dorsal, and the body is never segmented.
In the adult, the ectoderm is not ciliated, but its out<?r- most layer is a chitinous cuticula. In most Tremaioday one or more suckers are developed upon the ventral surface of the body, behind the mouth. These are sometimes armed with chitinous spines or hooks; and setaB of the same character may be developed in other parts of the body, especially in the region of the head.
The mouth is usually terminal, but is sometimes ventral and sub-central; it is ordinarily placed in the centre of a muscular sucker, rarely proboscidif orm. The alimentary canal is never provided with an anus. Sometimes a simple
* Hudaon, " On a Vew Ro- tifer." (* Monthly MicroBCopical Journal,' 1871.)
t The lingular marine genas Eekimodere* (I>i:gardin) is perhaps such a link. These are minute wormlike animals, with a rounded head, followed by a number (10 or 11) of distinct segments, the last of which is bifurcated. Thara are no limba, but the head
is provided with recurved hooks, and the body segments with paired sete. The nervouM system appears to be represented by a single ganglion, which lies in the head and presents eye- spots. The development of £chinoderei is unknown. (See Greef, *Archiv fiir Naturge- schichte/ 186^.)
O
194 THE ANATOMY OF UnTEBTEBRATED ANIMALS.
sac, it is often bifurcated, and occasionaUj branched, like that of the dendrocoele TurheUaria. Sometimes {AmphUina^ Amphiptyches) the alimentary canal is absent ; and, accord- ing to Yan Beneden, it becomes aborted in the adnlt Distama JUicolle, The interval between the endoderm and the ectoderm is occupied hj a cellular or reticulated mesoderm, in which abundant muscular fibres are developed. The peripheral muscular fibres form an external circular and on internal longitudinal layer.
The water-vascular system is well developed, and may consist of — (1) a contractile sac, which opens externally and communicates with (2) longitudinal vessels with con- tractile non-ciliated walls, from which proceed (3) non-con- tractile and ciliated branches which ramify through the body and the ultimate ramifications of which probably end by open mouths, as in the Botifera,
Thei« is no pseud-heemal system. The nervous system has not been discovered in all ; but, when it exists, it has the same arrangement as in the aproctous TurbeUaria, Eye-spots have been observed, but no other sense-organs. With rare exceptions, the Tremaioda are hermaphrodite, and the reproductive organs are constructed upon the same type as in the rhabdocoele TurbeUaria, a large vitellarium being always present. The accessory vitellus is included, in the form of numerous pellets, along with the primitive ovum, and is absorbed pari passu with the development of the embryo.
Aspidogaster conchicola (Fig. 40) inhabits the pericardial cavity of the fresh-water mussel ; it is a very convenient subject for examination on account of its small size, and the ease with which it can be rendered sufficiently trans- parent for the display of the arrangement of its internal organs, by the judicious use of the compressorium. The flat oval body, rounded posteriorly, is produced in front into a truncated cone, on the face of which the mouth opens. The ventral sucker is very large, and its surface is Bubdiyided into rectangular areas. There is no peri- visceral cavity, its place being occupied by a mass of
ASPID0OA8TEB
195
gpaagj ceUnlar tjaaiie. Th« oral cavitj leads into an oral, thiclc-walled, maacnlar phaijngeal bnlb, whence an eIoiigftt«d pjrriform sac, which conHtitutea the rest of tbe alimentarf canal, ia continned. This occnpiea a grent part ot Ute bodj, and extends nearly to ita posterior end ; bnt there is no anne. A contractile racDole placed at the hinder extremitr of the body opens outwards b; Fig. 40.
Fig. 40. — AmiJogmItT anichicob.—A.. amngemei uid reproductlre organt ; prcifile or the •alTiiat ii b, tniucnlar phujnx ; c, glonu)! ; d, gennaiiuni :
or the alimcntiu^' lutliae. s.oiDDlb: T, iaienul viadefc-
a small pore (Fig. 41, a), and gives off two lateral con- tractile non-ciliated canals (b), which pass to the anterior end of the Tentrsl Backer and there end blindly ; bnt before reaching' thia termination each givee off a non-contractile ciliated Teasel, (Fig. 41, e) which, on arriTing at the pbarTni, tnnia bat^wards and ramifies through the body. The cilia " ' ' h towmrds the extremitiea of these vesBeU, — &e Vet-
196 THB AMATOKT Or IRTBBTIBKA^D AITQULS.
minationa of the corraspoadrng canals in the Botiftra being, on the contrary, richly ciliated. No nerrei have aa jet been found in JfpidtKfasfar.
As in most Tromatoda, the genitalia (Figs. 40 and 43.) fonn a loi^ part of the TiBcera, and the strncture of the com- plex hermaphrodite apporatoB is in some respects so pecu- liar, that it is needful to describe it in detail. It consiats of~l. The Kermarium. 2. Theritellariuni. 3. Theoviduct. 4. The ntcrus and vagina. 5. The common Teatibule. 6. The testis. 7. The Tasa deferentia, internal and eit«maL
Fig. 41.
, .. ._.or«l ollikled tnuki,
that of (he Icrt tidH shaded ; d, dilaUtioi of this trunk ; B, oua of larger, and C, one of the uoatler, ciliated Tneele,
8. The penis and ita sac. The ovary (d) is the anterior of two rounded maases lying in the sucker. At first sight it appears to be oval, but it is, in fact, pyrifarm, the larger end being anterior, while the posterior narrower extremity is bent backwards beneath the anterior end. Before it reaches the anterior extremity of the mass, however, it is bent sharply back again, parallel with itself, and so passes into the oviduct (Fig. 40, t). The ovary is eurrounded by u delicate, but sbwng, coat, enclosing a maae of tranapa- Tvat protoplaam. At the anterior end of the ovary miunte granules are scattered throngh this subotance, and are oc-
A8PIDOOA8TSB CONCHICOLA. 197
casionallj surrounded by a faint, clear area (Fig. 48, A 1). These are the radimentary germinal spots and Tesicles of the fatore OTa, the coarse of whose derelopment may be readily traced by working from the anterior to the posterior ex- tremity of the ovary. The germinal spots become larger, and gradnally assume the appearance of resicular nuclei ; while the clear area around them in like manner becomes larger, and acquires more and more the appearance of a cayity. While this cavity is small, it has no distinct wall, but, as it enlarges, the contour of the wall becomes dis- tinctly marked (Fig. 43, A 2, 3, 4). On examining the ovary close to the commencement of the oviduct, a division of the homogeneous protoplasmic basis or matrix of the ovary into areas surrounding each germinal vesicle becomes obvious. On the application of pressure, the matrix breaks up into masses corresponding with these areas in size, which are very flexible, but when left to themselves assume a rounded or oval form, and have all the appearance of perfect ova, except that they possess no vitelline mem- brane, and that the yelk, instead of being granular, is clear, and comparatively small. These primary ova, as they may be termed, become detached, and pass into the oviduct. Here they are fecundated, and, becoming surrounded by a great mass of accessory yelk, and a shell, gradually acquire the appearance of the complete ova.
The accessory yelk is the product of the vitellarium — a large double gland consisting of a number of oval, pyri- form, or irregular granular masses placed on each side, at the junction of the sucker with the body (Fig. 40, g).
These masses appear to be quite independent of one another ; nor do they at first present any obvious commu- nication with the genitalia ; but if the oviduct, just after it becomes free from the ovarium, be examined, it will be found to receive a short duct (Fig. 42, /), filled with strongly re- fracting granules of the same nature as those in the vitol- larinm. Tliis duct is enlarged posteriorly, and then divides into two ducts filled with the same matter, which take a direction towards the vitellarium, but can be trac^ tlq
198 THE AN4T0HT OV IBTBBTBBKATBD AmUHa.
fur^cr than tbej contain, grannlea (Fig. 42). Bj the carefnl application of pressure, howerer, the grannlea maj be forced from the Titellarinm, through ut anleriw and posterior branch npon eaoh side, into these dncta.
The OTidnct (Fig. 42. i) is richly ciliated intemallyi it is at first applied to the under surface of the orarinm, und when it becomes free it receives a canal (e), which nMy be traced back to the testis, and which would appear
Fig. *a.
TH darereoi ia leco behind lbs viteUu')4D ducu.
to correspond with the internal vas deferens of other Tremaloda described by Ton Siebold.* This canal, how- ever, presents no dilatation, or internal vesicnla seminalia. The oviduct next receives the duct of the vitellarium, and then becoming much convoluted (k], and rapidly widen-
■ The ceDneetion or thli duct Aipidagiuler, but I bsTS hsd no
witb tb« teitla ia th« Trtmatala oppoHuulty of re.eumlniiig tbl>
hu rscentlv been denied by uiimsl rince tfae DubllcslloD of
Rlleds (' HQlter't ArehlT,' 1871). Stleds't paper.
1 hul DO doubt of Its er' '
ASPIDOOASTEB CONCHICOLA. 199
ing, passes into the uterus (2), a wide tube, which nms forwards, disposed in many undulating curves (Fig. 40, Q, to terminate on the left side of the anterior part of the body, close to the male organs. Posteriorly, the walls of the uteiois are thin; but in its anterior, or vaginal, part they become thick and muscular. The genital vestibule into which the vagina opens is very small.
The testis (m) is an oval body of the same size as the ovarium, and situated just behind it. Minute water- vessels ramify upon it, as upon the ovarium ; and it contains a granular and cellular mass, but no spermatozoa. The external vas deferens (Figs. 40 and 42) is a delicate duct, which passes forwards and comes into contact with the ovarium, without, however, so far as I could observe, com- municating with it or with the oviduct; it then bends backwards and upwai-ds, passing between the anterior vi- tellarian masses into the fore part of the body. Here it suddenly becomes about twice as wide as before, and runs forwards, as an undulating thick tube, to the penis (Fig. 40, p), a shoi*t-and conical body, occupying the bottom of a large pyrif orm sac, which opens in common with the uterus. The spermatozoa are linear.
The development of the ova presents many very interest- ing peculiarities (Fig. 43). Above the junction of the duct of the vitellarium with the oviduct the contents of the latter were pale and clear, and presented no formed particles beside the primary ova which had just been detached from the ovarium (Fig. 43, C). Below the insertion of the vitel- larian duct, however, the oviduct was full of granules like those in the vitellarium, mixed up with ova in a more ad- vanced state. In the smallest of these (Fig. 43, D), the shell of the ovum had commenced, but was incomplete at one end. At the opposite extremity, it enclosed a mass of irre- gularly aggregated vitelline granules, which covered almost one-half of a round pale mass, not larger than one of the primary ova; in which, however, three nuclei (two of which were very close together, as if they had just divided) were to be distinguished. In more advanced ova the sheU. ^^^^
200 THE ANATOKT OV INTBBTXBBATBD ANIMALS.
complete, but either colourless or of a yerj pale brown hue. In some of these the primary ova contained many nadei and were imbedded in and surrounded by, a confused mass of accessory yelk granules ; while in others these grannies were aggregated into a number of regular spheroidal masses (Fig. 43, B).
As development proceeds, the accessory yelk-maases gra- dually disappear ; the primitive ovum, now become the ho-
Fig. 43.
^
Fig. 43,—Atpidogaster conchicola.—A, section of the ovary ; I, its an- terior end ; :^, germinal spot surrounded by a distinct wall ; S, 4, a complete germinal vesicle and spot; C, a primary ovum; D, young state of a oomplete ovum : the primary ovum partially surrounded by yelk granules and a shell ; B, complete ovum, with the accessory yelk aggregated into spheroids; £, vacuolated embryonic mass; F, embryo.
mologuc of the blastodermic disk or vesicle in other animals, to all appearance increasing at their expense. At the same time, clear rounded vacuoles in various numbers appear in its substance; but the nnclei of the germ, though Tery minute, can, with proper care, be readily detected between these. In the final stages the shell becomes browner, the ▼aonoles and granules disappear, and the substance of the embryo i^pears homogeneous. But if carefully examined,
THS BBTBLOPMENT OF ABPIDOOASTSB. 201
tbe mintite nuclei become -risible, especially if water be allowed to act on the tissue, and, if the shell be burst, and its contents poured out, thej readily break up into small but well-marked cells, each with its nucleus. At the same time, the embryo takes on a form not very distantly resembling that possessed by the adult; into which it eventually x>a8Be8 without any metamorphosis.*
Thus it appears that, in Aspidogcuter, the ovarium gives rise to primary ova, which pass down the oviduct and become fecundated, either by the spermatozoa conveyed by the internal vas deferens, or by those received by the vagina when copulation with another individual or, pos- sibly, self -impregnation, occurs; that, next, the essential part of the process of "yelk-division" takes place, the germinal spot dividing and subdividing, and the primary ovum becoming in this way converted into the spheroidal blastoderm; that, contemporaneously, the blastoderm be- comes invested by the accessory yelk granules poured in by the vitellarian duct, and by a shell ; that the accessory yelk arranges itself into spheroidal majBses, which probably supply the blastoderm with the means of its constant en- largement ; and that, finally, the accessory yelk disappears, and the blastoderm becomes converted into the embryo.
The modifications exhibited by other Trematoda concern the number of the suckers, of which there are usually several in the ectoparasites, but not more than one in the endoparasites ; their support on a chitinous framework, or the addition to them of spines or booklets, similar to those of Cettoidea or Acardhccephala : the bifurcation of the intes- tinal canal, and the ramification of its branches, so that the forms of the alimentary apparatus repeat the two extremes observed in the aproctous TurbeUaria; the existence of two nervous ganglia with a single transverse commissure
* The tabstsnce of this ac- Beneden has recently thrown
eoant of the structure and de- much light on the mode in which
velmmwiit of Atpittoga$Ur, with the ova of the Trematoda are
the illuttimtive figures, was pub- formed and developed in his
lished hi 1S56 in The Medical ^Recherches sur la composition
Timm and Omttite, M. £. van et la signification de VQE.ut?
202 THE ANATOKT OV nffYSBTSBBJLTSD AHIICALS.
in many ; and the occasional presence of sensory organs (eye-spots). The non-contractile canals of some genera are destitute of cilia, except at their inner terminations.
The variations of the reproductiTe organs are rather of position than of stmctore. Dioecioos Trematodes are very rare, the most important being the formidable Bllharzia, the male of which is the larger and retains the female in a gynceeophore, or canal, which is fonned by the
Fig. 44.
Pig. 44. — A, B, Monottomum mutabile ; A, the ciliated embryo (a) . enclosing the sooid, (b,) represented free in B (after SieboJd) ; C« BediOf or lying's yellow worm of Distoma patificvm^ oont^ning germs of other RedUe ; D, Redia containing CercancB (a) ; £, Ctr" carta; F, JJittomoy which results from the metamorphosii of the Cercaria, (After Steenstrup.)
infolding of the margins of the concave side of the body. BlViarxiahBa neither intromittent organ nor seminal pouch, and the history of its development has not been traced be- yond the escape of a ciliated embryo from the ovum. This parasite is found in the blood vessels of man, chiefly in those of the urinary organs, the ova escaping from the body through the ulcerated surfaces to which the parent
THB BEYSLOPMEKT OV THS TBSMATODA, 203
gives rise. In the ectoparasites, the embryo passes into a form identical with or closelj resembling that of the parent while still within the egg^ as in Aapidogaster, When this happens {e.g, Distoma variegaium, D. tereticoUe), the one end of the embryo is often provided with spines, and it is capable of slow creeping movements. But, in most of the endoparasites, the embryo leaves the parent as a morula, which is usually ciliated. Thus, in Distoma lanceo- latum, D. hepc^Hcum, and Monogtomum mutahile, the embryo which escapes from the egg has a ciliated investment, which propels it rapidly through the water, and may be provided with eyespots and water- vessels (Fig. 44, A). On becoming attached to the animal upon which it is parasitic, the embryo of Monostomnm gives exit to a larva, having the form of a cylindrical sac with two lateral prolongations and a taper- ing taiL The Bedia, as this form is called (Fig. 44, B, C), has a mouth and a simple csecal intestine, but no other organs. In its cavity a process of internal gemmation takes place, giving rise to bodies resembling the parent in shape, but destitute of reproductive organs, and furnished with long tails, by which they are propelled. These creatures, called Cercarice (Fig. 44, E), escape by bui^sting through the Bedia, and, after a free swimming existence, penetrate the body of some other animal, their tails dropping off. They then become encysted, and, under suitable conditions, assume the adult form, and develope reproductive organs ^Fig. 44. F).
The cycle of forms through which Distoma mUitare passes has been nearly completely traced, and may be briefly stated as follows. 1st. The parent form, whose habitat is the in- testines of water-birds, bears on its anterior extremity two alternating circles of larger and smaller booklets, and a few others, irregularly disposed. Rings of papillsB give the centre of the body an annulated aspect. The mouth, abnost terminal, leads into the long straight digestive caecum. The generative organs are similar to those of Aspidogaster ; the testes are, however, double, and lack the internal TAB deferens. The ova are few, eight or ten in ii.umV>^ic.
204 THE JLNATOICT OF INYBBTSBBATSD ANIMALS.
2nd. From each oyum issues a ciliated larva, showing the rudiments of, 3rd, a Bedia, but the mode of derelopment of the latter has not been f uUj traced. The perfect Bedia is found attached to the body of a water snail (Paludina), the cUiated inyestment having disappeared. It consists of a sac, within which is suspended a tubular bag, containing coloured masses, probably alimentary. Anteriorly, the head is represented by a kind of crown, in which no (bso- phagus exists as yet, and not far from the posterior ex- tremity the two lateral projections, characteristic of Disto- matous BedicR, appear. During the rapid growth of the zooid, the head becomes marked off by a constriction, and a mouth and gullet, with a pharyngeal dilatation, admit aliment to the digestiye sac. In the body cavity, external to this sac, vesicles appear, rapidly increase, and take the form of Cercarice ; the Bedia bursts, and these new zooids are set free. 4th. The Cercaria has a long tail with lateral membranous expansions, by means of which it swims after the fashion of a tadpole. The pharyngeal bulb is fol- lowed by an oBsophagus, which, opposite the ventral sucker, divides ; the two branches ending in a csscum on either side of the contractile vacuoles of the water-vascular system. These are median, the terminal quadrate chamber opening into an anterior ciixjular one, whence are given off the two main canals which traverse the body longi- tudinally, and are then lost. 5th. After swimming about freely for a while, the Cercaria fixes itself upon, or bores its way into, a Patuditia ; the tail dropping off, and the body coating itself with a structureless cyst, in which it remains quiescent, but undergoes some further advances in development, the coronal booklets making their appear- ance. 6th. When a Paludinat thus infested, is swallowed by a water bird and digested, the cysts are set free in the alimentary canal of the bird ; sexual organs appear within the included Distoma ; the body elongates and narrows an- teriorly ; the sucker moves nearer the head, and the coronal circlets reach their full development. The Distoma gra^ dually assumes the form of the parent, attaches itself by
TBI DBTBLOmUIT OT THX TBIIUTODA,
205
hookleta to the mtestmal walls, and aoqnires oomplete ml orgaiiB.* Thua the deTelopmeotal stages of Ditlcma Uar* maj be sninmed up, as : 1. Ciliated larrA. 2. Badia. Oareorta. 4. Cerearia, tail-less and encysted, or incom- baDUloma. 5. Perfect I>utofna.
^be stages of tranntioa vary in different genera. Thna, Bnl generatioBB of Bedim maj intervene between the Fig. «.
lit, — Bmefphalm votymorphut of the freih-water musvel- A, iBdIIed ■poroc;>t; B, portion of the mme more uimgniBed, a, iCer cokt, b, inner-, c, d, f^m mutes in coune of d*v«lopmeat ; , one of the germ muaes more highly miignified ; D, BucriAaiut,
h, mcken ; c, cleu uvily ; d, caudal ippendageg.
rd and fourth, stages ; or the mature animal ma;f appear lie close of this stage, baTing undergone no Cercarian xunorphosis.
Q Bncephahu ipohfmoTphm, a parasite of the fresh-water Mel (IHg. 45), two candal appendages, which seem to respond with the tail of the ordinaiy Cerearia, become • Vmi Bentden, ' BUmoire eur lei Ten Intettlokox '
206 THE AKATOICY OF nTYSBTSBBATED JLVmALS.
enormously elongated. They are converted into ramified tubes called tporoeytts, which sometimes occupy all the interspaces of the viscera of the musseL These develope new BueephaXi by internal gemmation. The Trematode condition appears to be the genus Qasterofdomwn, which inhabits fresh-water fishes.
The Sporocysts, BedisB, and Cercarise, free or encysted, are found almost exclusively in invertebrated animals, while the corresponding adult Trematodes are met with in the vertebrated animals which prey upon these Inveriebrata.
The singular double-bodied Diplonoon pcaradooeum has been shown by Von Siebold to result from a sort of conju- gation between two individuals of a Trematode, which, in the separate state, has been named Diporpa. The Diporpce, when they leave the egg, are ciliated and provided with two eyespots, with a small ventral sucker and a dorsal papilla. After a time the Diparpoe approach, each applies its ventral sucker to the dorsal papilla of the other, and the coadapted parts of their bodies coalesce. They acquire fully developed sexual organs only after this union.*
Cryrodactylus multiplies agamically by the development of a young Trematode within the body, as a sort of internal bud. A second generation appears within the first, and even a third within the second, before the young Oyro* ddctylus is bom.
The Cestoidea. — The Tape- worms are all endoparasites, and, in their adult condition, infest the intestines of verte- brated animals.
The simplest form known is CaryophyUcBus^f found in fishes of the Carp tribe. It has a slightly elongated body, dilated and lobed at one end, so as to resemble a dove,
* Zeller, ** Untersuchangen indebted for information reepeet-
iiber die Entwickelung des ing thi4 and other genera of
Diplozoon paradoxum." (*Zeit- C*stndea which have nol fallen
schrift f&r WiM. Zoologie,' 1872.) under my own observation. Alio
t See the * Memoire sur lea Yen Leurlcarr, * Die Menschlichen IntesUnauz,' 1858, by M. P. J. Parasiten,' van Beneden, to whion I am maoh * Entoaoa.'
TKK CBSTOmXA.
207
wlienoe tlie name of the genns. In atmctnre it reaembleB a l^emaitode, devoid of any trace of an alitnentarjr canal, bat prorided witli the characteristic water-rascnlar ajatem and with » single set of hermaphrodite reprodnotiTe organs. In. lagula, the bodj is much elongated, and, at the head end, eshibite two lateral depresnons. It ia not dWided into aegment«, bnt there are nnmerona aets of
Fig. «.
Flf. M.—Diagrmin of tbe Btrucluie of > Ceatoid worm, nilb only one kiinL Tba pomtion of the iiooki of & Tmia, and of one of Ifas pro- Vaoidei of ft Trtrarhynchai ie ladicaied. A, hesd Mid neck ; B, Mgment of the body correjponding with ■ pmglotlU ; a, nulrj/sn ,- b, iHtellar iidnei ( Tmia); c, <f. e", Bpinose evenibla pmboacis ( TttmHiftiduu]; d, sucker; t, KanKlioD (?) ; /, laleral, and g, circular wmter veuel ; A, ramificalioni of the water vessels ; *, anastoinoainj^ tmnk; i, eontraelile varuole; I, genila! Testibule; n, penia and vas defereni; a,Taglna, o, commoa avity ^nd erMieula trmiiuilit interior ; p. orarj; q, nt^Tu; r, vitetlarisn duct,
Mxnal organs arranged in longitudinal series. The open- inga of the genital glands are situated in the middle line of the body. These parasites inhabit Fiabes and Amphibians, M weD H wat^ bird>i, but thej attain their sexual state (mly im Um Utter.
ta the JXKxn typical Cetfoidea the body is do&gate^, «ii^
^»
208 THE ANATOICY OF niYBBTBBSATBD AKIMALS.
presents, at one end, a liead provided with suckers, and veiy generally with chitinons hooks, either disposed circularly around the summit of the head, or upon proboscidiform tentacles, which can be retracted into, or protruded from, the head. Sometimes the head is produced into lobes ; and very generally, when lobes or tentacles exist, they are four in number, and are disposed symmetrically round the head. A short distance beyond the latter, the slender body widens and becomes transversely grooved, so as to be marked out into segments. Longitudinal water-vessels run parallel with one another through the body, and are connected by transverse trunks in each s^^ent, and by a circular vessel in the head. In Bothriocephalus lotus, the principal trunks are occupied by a spongy reticulated tissue.
In most of the tape- worms, innnmerable, solid, strongly refracting corpuscles are scattered through the substance of the body (Fig. 48, A). It is probable that these are more or less calcified connective tissue corpuscles. Similar bodies which occur in some Trematoda were found by Clapar^e to be lodged in dilated ends of the water- vessels, but it would appear that they arc not so situated in the Cestaidea,^
The distance between these transverse grooves, and their depth, increase towards the hinder end of the body ; and each segment is eventually found to contain a set of male and female organs. The genital organs are constructed upon the same general plan as those of the Trematoda, but the uterus, as it fills with ova, usually takes the form of a ramified sac. At the extreme end of the body, the segments become detached, and may for some time retain an independent vitality. In this condition each segment is termed a pro- glottis ; and its uterus is full of ova.
The embryo is developed in these ova in the same way as iu the Trematoda ; and, as in the latter group, it may either be ciliated (as in Bothriocephalus) or non-ciliated, which
* Sommer and Landois, '^Ueber logie,* 1872.) Lcuckart, however,
den Bau der geschlechtareifeii maintains the contrary opinion,
Glieder tou Boihrhcephalu* la- *Die Menschlichen Parasiten,' p.
tus:* C Zeitschrift fur Win. Zoo- 175.
THE CBSTOIDBA.
209
last is the more usual case. The embryo is a solid morula, on one face of which four or six chitinous hooks, disposed sjmmefcrically on either side of a median line, are deve- loped.
If the egg is placed in appropriate conditions, the hooked embrjo emerges from the shell, and rapidly increases in size. After a time, a cayitj appears in the midst of the cella of which the momla is composed, and a chitinous cuticula is developed upon the outer surface of the embrjo. Bamified water- vessels make their appearance in the wall
Fig. 47.
Fig. 47. — Dimgrams illustrative of the relation between Tttnia, (^ati- cercuMj Cetnunut^ and JSchinococcus.—AL^ B, young Tctnia in the ScoUt stage, the latter with an enlarged recejakaadnm ScolicUy into which the head and neck are withdrawn in C, Cy^ticercu* ; D, CanttruM ; £, hjpothetical condition of Echmooocauy in which ^ Tttnia heads ' ' are developed only on the inner surface of the primary cyst; F, Eekimoeoeeus with secondary cysts ; G, embryo Tcmia (afier Stein).
of the spheroidal sac thus formed, and in some cases open by an external pore. There is, therefore, a very close resemblance between this cestoid embryo and the sporocyst of a Trematode.
When the saccular embryo has attained a certain size, a tiiiokening and invagination take place, usually at one (Tcenia), sometimes at many {CcenuruSy Echinococcus), points of its walL The invagination of the wall elongates inwards, and becomes a cecum, the cavity of which opens outwards. At the bottom of the interior of this csDcum, and thecetoT^
210
or iimKTXBfti.nD axwals.
on what is morphologically if« external anrface, the hooks of those Bpecies which poweas them are developed, whilo. upiiu the aide-walla, elerationa ariae, which become con- verted into anckers. The cscnm ia next evaginated or tnmed inside out, aild the embiyo has the form of a phiiU, u(
Fig. 48.
Vlg.4B,— Eclimiinifeutrftermmm.—\, "Tenta hekd," or Stvltii a, hDoka; b, gucken; c, cilia In w&tervewela; d, onal, ilmiglj nfnel- Ing pvllclea, B, ilDgle hooka; C, portion of Ibe elailic cyM, a; with ihe inner membrmnou* primary eyit, t ; eand *, Scolka derelup- ing from ici inaaraurface; li, a Beoondary eyat.
which the evaginated cfficom forma the neck. Round its apex are the hooka, and below these the enckera, forming s complete Ceatoid head ; while the sac answers to the bodj of the phiaL The original hooks of the embijo are cast off in tlie o&orae of this process.
THE DBYELOPMENT OF THE GESTOIDEA.
211
If the eggs of the Tape- worm have passed into the alimen- tary canal of an animal in which the worm is nnable to attain its sexual condition, the hooked embryo, as soon as it is hatched, bores its way through the walls of the alimen- tary canal, and eyentnally becomes lodged in the connective tissue between the muscles, or in the liver, or in the brain or eye. Here it goes through the changes which have been described, and, generally, the sac undergoes very great dila- tation. The region of the wall of the sac to which the cestoid head is attached becomes invaginated, and thuR is enclosed within a chamber, the parietos of which are really constituted by the outside of its own body. In thin condition, the animal is what is termed a Cystic worm, or bladder- worm ; and when there is only one head it is a CysHeercus. In the genera Ccenvrus and Eehinoeoecus the cystic worm has many heads ; and, in Echinocoeeus, the struc- ture of the cystic worm is still further complicated by its proliferation, the result of which is the formation of many bladder- worms enclosed one within the other, and contained in a strong laminated sac or cyst, apparently of a chitinons nature, secreted by the parasite (Fig. 48).
In the Cystic condition, the Tape-worms never acquire sexual organs ; but, if transported into the alimentary canal of their appropriate hosts, the heads become detached from the cysts, and, rapidly growing, give rise to segments, which become sexxisl proglottides. The Tape- worms are rarely met with in both the cystic and cestoid conditions in the same animal ; but the cystic form is found in some creature which serves as prey to the animal in which the cestoid form occurs. Thus : —
Ctstic Fobm.
Cysticercus celluioscB, (Muscles of the Fig)
Cysticercus f
(Muscles of the Ox) Oystieercus pisiformis. (Liver of the Babbit)
Cestoid Form.
Tasnia solium,
(Man)
Tcenia inedMcansUata.
(Man)
T^jenia serrata,
(Dog, Fox)
212 THE ANATOMY OF INTKBTBBSATBD AJmCALS.
Cystic Fobm.
Cystieercua JasciolarU,
(Liyer of Bats and Mice)
Ccenurus eerebralis,
(Sheep's bram)
Echinocoeetu veterinorum.
(Liver of Man and of
domestic Ungolata)
Cbbtoid Form.
Tcenia erauicoUls.
• (Oat)
TcBwia ecenwrus,
(Dog)
Tcenia Echinoeoeeus,
(Dog)
The embryo of Tcenia cucumerina passes, in the body of the Dog-louse (Trichodeetea eanis), into a OyaHcereoid, or minute unjointed and sexless Teenia, without anj terminal dilata- tion. The dog devours the louse and the Gjsticerooid becomes a Tcenia cticvmerina in his intestine. The eggs of the Tcenia, contained in fsBces adherent to the hair of the dog, are in turn devoured by the louse, and thus the " vicious circle " of parasitism is maintained.
The cystic Tetraphyllidea frequent osseous fishes, their sexual maturity being attained in the bodies of Flagio- stomes. The head is provided with four suckers, or lobes, which may be stalked and unarmed, as in Echeneibothrium, or furnished with booklets as in Acanthohothriwm ; while, in Teirarhtfnchus, four proboscidiform tentacles, thickly set with booklets, are retracted into sheaths alongside of the suckers (Fig. 46).
The DiphyUidea have two suctorial disks, two armed rostellar prominences, and a collar of booklets on the neck.
The migrations of the Psettdophyllidea are chiefly from fishes and amphibians to water birds, one genus (Bothrich cephalus) containing species which enter the human body, probably in the flesh of fresh- water fishes. The head has neither suckers nor lobes, but is deeply grooved on either side. In Bothriocephdlvs the genital apertures are in the middle of each segment. The embryo is ciliated, and swims actively in water. Becent experiments tend to show that the development of the embryo in this genus may take place directly, or without the intervention of a CysHeercue stage.
THB CE8TOIDEA.
213
It is obyions that the Cestoidea are very closelj related to the Trematoda, In fact, inasmuch as some of the latter are anenteroos, and some of the former are not segmented, it is impossible to draw anj absolute line of demarcation between the two groups. It would appear that the Ceatoidea are either Trematodes which have undergone retrogressive metamorphosis and have lost the alimentary canal which they primitiYely possessed ; or that they are modifications of a Trematode tjrpe, in which the endoderm has got no further than the spongy condition which it exhibits in ConvohUa among the TwrbeUaria, and in which no oral aper- ture has been formed; or lastly, it is possible that the central cavity of the body of the embryo Tcsnia simply represents a blastocoele.
If the Cesioidea are essentially Trematodes, modified by the loss of their digestive organs, some trace of the diges- tive apparatus ought to be discoverable in the embryo tape- worm. Nevertheless, nothing of the kind is discernible, unless the cavity of the saccular embryo is an enterocoele. And if this cavity is a blastoccele, and not an enteroccele, it may become a question whether the tape- worms are any- thing but gigantic morulse, so to speak, which have never passed through the gastrula stage.
214 THE AHATOMY OF HmSTUBiLTBD AJKHUlLB.
CHAPTER V.
THE HIBUDINEA, THE OLIOOCHiBTA, THE POLYCHiBTA,
THE OEPRTBEA.
The Hibudinea. — The Leeches are aquatic or fcerrestrial, more or less distinctlj segmented, vermiform animals, most of which suck blood, though some deyour their prej. The ectoderm is a cellular layer, covered extemallj bj a chiti- nous cuticula, and except in Malaeobdella, devoid of cilia. Yerj commonly it is marked by transverse constrictions into rings, which are more numerous than the true »omite$f as indicated by the ganglia and the segmental organs ; and simple glands may open upon its surface. One or more suckers, which serve as organs of adhesion, are developed upon it. In some (Acanthobdella) bundles of sets are pre- sent; in others (Branchellian) the sides of the body are produced into lobe-like appendages; but none have true limbs, unless the lateral appendages of HisiriohdeUa are to be considered as such ; nor are the anterior segments of the body so modified as to give rise to a distinct head.
The mouth is generally situated at the anterior end of the body ; the anus at the opposite extremity, on the dorsal side of the terminal sucker. The buccal cavity may be armed with several serrated chitinous plates, as in the Medicinal Leech, where there are three such teeth. By their aid the Leech incises the skin and gives rise to the well-known triradiate mark of a leech-bite. The buccal cavity usually opens into a muscular, sometimes protrusible, pharynx, from which a narrow oesophagus leads into a stomach, which is frequently produced into lateral csdca. In the Medicinal
TBW HQtTmniEA.
Leech (Fig. 49), for example, Uiere are eleven paira of each cnca, in- creaaing in length and capacity from before backirardB. From the stomach a narrow intestine le«ds to the anna. In Malaeobdtlla the alimentaiy canal is a aimple tnbe bent sereral times npon itself. The alimentary canal ia lined bj the cella of the eadoderm, and the space between tbem and the ecto- derm iaoccnpied by the mesoderm, which contains abandant connec- tive and mnacnlar elements, and is excavated by the blood-chaimela, which sometimes have the form of wide sinnsee, bat in other cases are comparativelj narrow vessels a* with definite wslls. ' ^
In the lower Himdinea, as Clep- Z tine, the einnses and vessels appear tu form one continnoas sj-atem of cavities containing a fluid which must be regarded aa blood. Bat in the Leech, a distinct paendhn- mal vaacnlar sjstem has attained a great degree of deBnition and compleiitf : it consists of (1) a median dorsal trunk ; (2) a median ventral tnmk, in which the gan- glionic nerve-chain lies ; (3, 4) two wide lateral longitndinal tmnks {Tig. 50). These anastomose with one another, and give off nnmerous branches, which open into a rich capiUaij net-work, sitaated in the moscolar layer of the mesoderm,
* * Dia Mentchlloben Fanslten.'
Ji
rm
216 THl AKATOVT OV IWBKTXBKATKD AHnULS.
and on tbe segmental and reprodnctiTe orgaiu. The fluid contomcd within these veeeeU has a red ooloor, and con- tains no corpoHcleB. More or fewer of the aegments of the bodf are piorided
Fig. SO.
■urfMeaf ■ portion of one-half of the bodv i* deplctedi a, a, tli* venlraJ trunk ; t, (■, (• the \mXen\ trank and id brincha ; /,/', Um donal trunk and iu bnneho ) g, the slender tiaoiccrH tmnkt which branch out at each end ; h, i, the trsniTene rantral bnm^M of the Uteral tniak ; 4, I, the brancb to (he («ti> (c), and tha Mg- menlal orjian (rf) -, «, branch from the dilatation on th< teadi to tba parietal plainaea ; b, b, va* defereni.
with what are termed tegmttdai organt. Theae are tnbei which open extemalljr on the ventral wall of the bodj, while at their other extremitiea they either open into the nnnaw bj ciliated monUia (CIspMMtf), or form a closed
THB HIBIJDIirBA. 217
and more or less reticulated non-oiliated coil (JETirudo). These obviouslj answer to the ciliated water-yessels of the TwrheUaria and Trematoda.
The nerrous system consists of a cerebral mass in front of the month, proceeding from which, on each side, is a oommissure connecting it with a ventral cord on which ganglia, corresponding in number with the somites of the bodj, are developed. In MaiacobdeUa, these cords are lateral and wide apart, but, in all the other Hirudinea, they come close together behind the month, and occupy the middle line of the ventral face of the body. In the Leech, according to Leackart, there are originally thirty pairs of post-oral gan- glia, but the seven posterior and the three anterior 'pam coalesce, so that only twenty-three pairs are distinguishable in the adult. Nerves are given off to the pharynx and intestines, and the former develope special ganglia.
Simple eyes are usually present on the anterior or oral segment, and receive nerves from the supraoesophageal ganglia. In the Leech these eyes are situated in the fii'st three segments. Cup-shaped depi'essions of the integu- ment of the anterior segments of the body, lined by peculiar glassy cells and in relation with nerves which tenninate in fine filaments, have been discovered by Leydig in several of the Hirudinea,*
The elongated spindle-shaped muscle-cells of the body are abundant, and are disposed in a superficial circular, and deep longitudinal layer, while dorso-ventral bands pass from the dorsal to the opposite body- wall.
Malacobddla and Sistriobdella are dioBcious, but the other Hirudinea are hermaphrodite. The male organs consist of numerous testicular sacs, situated on each side of the body, and connected by a vas deferens, which usually opens into a sac, terminating in an eversible penis. The spermato- zoa are often enclosed in a case or spermcUophore, The female organs, much smaller than the male, consist of ovarieB, with ovidticts opening into a vagina. The vaginal orifice is behind that of the penis. In the Leech the eggs * * Archiv fur Anatomie uud Phyttiologie,' 1861.
218 THE AKJlTOVY OF IHyXBTBBKATBD ▲KIMALB.
are enclosed in a sort of cocoon, formed bj a yiscid secre- tion of the integument.
The observations of Bathke and Lenckart on the de- velopment of Nepheli8f Clepnne, and Hirudo show, that, after the division of the vitellus into a few equal-sized large blastomeres, small blastomeres are separated from the large ones (as in the Ctenophora and Polycelis), and the rapidlj multiplying small blastomeres form an investment to the slowly-dividing large ones. This investment is the epiblast, and becomes the ectoderm, while the included larger blastomeres are eventually converted into the cells of the endoderm. At one end of the body the oral aperture appears, in some cases {e.g, Nephelis) surrounded by a raised lip, as in the embryo Planarian ; and the embryo passes into the G^trula stage. The body now elongates and, on the ventral face, the mesoblast makes its appear- ance as a layer of cells, sometimes divided into two longi- tudinal bands, separated by a median interval. Three pairs of segmental organs, which have only a temporary existence and have been regarded as primordial kidneys, are developed at the posterior end of the body. The mesoblast next becomes divided transversely into the number of somites of which the body is eventually composed; the division first making its appearance on the ventral face of the body. A pair of ganglia, probably derived from the epiblast, is developed in each segment.
Thus, in the Leeches, the segmentation of the body is the result of the segmentation of the mesoblast, which becomes the mesoderm of the adult. And it is this seg- mentation of the mesoblast, and consequently of the meso- derm, which constitutes the most important difference be- tween the Leech on the one hand, and the Turbellarian and Trematode on the other.
On the other hand, in the development of a mesoblast which undergoes division into segments, the Leeches exhibit the fundamental character of all such segmented Livertebrates as the chsetophorous Annelida and the Arthro- poda.
THE OLIOOGHJBTA. 219
The OLioooHiBTA. — ^The Earthworm {Lumbriew) and fresh-water worms (Nais, TuhifeXf Chostogcuier), which are included under this name, are closelj allied with the Leeches in the essential points of their structure and development, much as they differ from them in habit and appearance.
They have elongated, rounded, segmented bodies, often divided by many superficial transverse constrictions into rings, which, as in the Leeches, maybe more numerous than the proper somites. There are no limbs, but each segment is usually provided with two or four sets of longer or shorter chitinous setee, which are developed and lodged in integumentary sacs. The outermost layer of the ecto- derm is a non-ciliated chitinous cuticle.
The mouth is situated close to the anterior end of the body, but a "cephalic lobe*' not unfrequently projects beyond it on the dorsal side. The anus is at the opposite extremity of the body, and the straight alimentary tract which connects the two and is lined by the endoderm, is usually divided into a pharyngeal, cesophageal, and gastro- intestinal portion, the latter often being produced laterally into short ceeca. The mesoderm presents well-developed transverse, longitudinal, and dorao- ventral muscular fibres, as in the Leeches. It is excavated by a spacious perivisceral cavity, which contains a colourless corpusculated fluid, and is divided by thin but muscalar mesenteries, which stretch from the intestine to the parietes, and thus break up the perivisceral cavity into partially separate chambers. Li addition, there is a system of pseud-hflsmal vessels, like those of the Leeches, provided with contractile walls, and containing a red non-corpusculated fluid. No communi- cation has been ascertained to exist between these vessels and the perivisceral cavity ; but there can be little doubt that, as in the case of the Leeches, they must be regarded as a specially differentiated part of the general system of the perivisceral cavity.
In the majority of the segments there are, as in the HtrudineOf paired segmental organs; these are ciliated and their inner ends open into the perivisceral chamb^T.
220 THB ANATOMY OF HrYXBTSBSATSD AHIKALS.
The nervous sjstem consiBts of proB-oral or cerebral ganglia, continued backwards, on the Tcntral aspect of the bodj, bj commissures on each side of the oasophagas into a double chain of closelj united post-oral ganglia.
Large tubular fibres are imbedded in the neurilemma of the ganglionic chain on its dorsal face. In the Earthworm there are three of these — one median and two lateral — ex- tending along the whole length of the ventral end, but not into the oesophageal commissures.* The nature of these structures is unknown.
These animals are hermaphrodite. The generative organs are situated in the front part of the bodj, the male organs being anterior to the female. In the aquatic Oligoehaia (NaU, Tubifex) the genital glands have no proper ducts, but the segmental organs of the segments in which they are contained convey the generative products outwards. In the terricolous forms {Lumbricuti) the vasa deferentia are continuous with the testes, which are very large. The ovaries, on the other hand, are minute solid bodies attached to one of the mesenteries, and the oviducts are separate tubes with funnel-shaped mouths, which open into the cavity of the segment.
In Nats and Ch/Btogaster, agamic multiplication occurs by the development of posterior segments of the body into zooids, which may remain associated in chains for some time, but eventually become detached and assume the parental form. Schulze has observed that when a Naia has divided into an anterior and posterior zooid, the last somite of the former gradually enlarges, and becomes divided into new somites, the anterior of which give rise to a head. A new zooid is thus developed between the previously existing ones. This process is repeated in what was the penultimate, but is now the ultimate somite of the anterior zooid; and again in the penultimate somite when it has, in the same way, become terminaL
* Claparede, * HiBtologitohe Untenttchungen uber den RegcjUKurmy* 1869.
THB 8TBUCTT7BX OF THX XABTHWOBK. 221
Ls the Earth-worm ia a very accessible subject, it maj be
ifal to the stadent to be famished with an acconnt of
ae of the chief points of its organisation more in detaiL
rhe exterior of the body of an Earthworm (Lwrnhrieut
'e&tru, rvbeUus or communis) shows a namber of dose-set
nsverse grooyes which divide its body into nnmerons
now rings or segments.* The most anterior segment is
all and conical, and presents, on its under surface, a
xrestion which is the oral aperture. The anus is at the
poeite end of the body. Behind the mouth, the successire
;ments rapidly attain their average size ; but, in a full-
)(wn worm, a part of the body into which more or fewer
tlie segments between the twenty-fourth and thirty*
fch indusively (29 P-^, L. terregirit ; 24-29 P, L, rvbeUw ;
•32, L* eommunis) enter, is swollen, of a different colour
«n the rest, provided with abundant cutaneous glands,
I veoeires the name of eingulum or cUteUvm,
u the dorsal median line there is a series of small aper-
m o>r pores, one for each segment except the most anterior,
th lead into the perivisceral cavity; while upon the
ral surface of the anterior part of the body the eight
;ures of the organs of generation are situated. Of
, four, situated two on each side, between the ninth
mth, and the tenth and eleventh segments, are the
igs of the receptaeula seminU. The openings of the
fiducts are on the fourteenth segment; those of
> vasa def erentia on the fifteenth. Besides these, all
l^ents, except some of the most anterior, exhibit
of minute openings appertaining to the segmental
and they are further perforated by the four
inal double rows of setae, which project slightly
he surface of the integument, and offer a certain
9 when the worm is drawn from tail to head
he fingers.
ly is invested in a thin and transparent but dense
ettSon how far all of the development of the Earth- ite represent somites worm is in favour of their being open. The history true somitet.
222 THE ANATOMY OF INTXSTEBBATXD ANIMALS.
euticula, perforated by excessively minute vertical canals. Within this lies a thicker layer, consisting of a regulated nucleated protoplasm, the meshes of which are filled with a transparent gelatinous substance. This layer probably represents both the dermis and epidermis, and has been termed the hypodermis. Internal to it lies a thick layer of circular muscular bands, in the interstices of which pigment granules occur ; and, still more internally, is a much thicker coat of muscular fibres, which are disposed longitudinally.
The cavity circumscribed by this longitudinally fibrous muscular layer is lined by a kind of connectiye tissue. Corresponding with the divisions between every pair of segments (except in the most anterior part of the body), this connective tissue is continued transversely towards the axis of the body, and passes into that which forms the wall of the intestine ; while, on the ventral side, it forms an arch over the ventral nervous cord, and the vessels which accompany it. In the interior of each of these mesenteric septa, radiating and circular muscular fibres are abundantly developed, and the former are connected externally with the superficial layer of transverse muscles.
The perivisceral cavity is thus divided into nearly as many short chambers as there are segments ; each chamber communicates with the exterior, directly by the dorsal pore and indirectly through the segmental organs, while fluid may pass from one to the other by the supra-neural archways.
The short and curved setse project much further into the interior of the body than they do on to its exterior. The free apices of each pair are situated close together, while their inner ends diverge from one another. Each is enclosed in a sac in which it is developed, and to which the muscles, by which it is protruded, are attached. There are eight setsB to each somite, one pair not far from the ventral median line on each side ; and the other pair placed in the same transverse line, but further outwards.
The mouth leads into a muscular pharynx, with a com-
THB BTBUCTUBX OF THE XABTHWOSM. 223
paratirelj small internal cavity, which reaches as far back as the seventh segment. From this a narrow OBsophagas is oontinned as far back as the fifteenth or sixteenth segment ; and presents three pairs of lateral glandular diverticula, which contain a calcareous matter,* in the region of the twelfth and thirteenth segments. Posteriorly, the gullet opens into a crop, which is succeeded, about the eighteenth segment, by a thickened and muscular gizzard.
Upon this follows the intestine, which has the appearance of a simple tul>e ; but is in reality complicated by the invo- lution of its wall, along the dorsal median line, into a thick fold, which projects into the interior of the intestinal cavity, and is the so-called typhlo8ole. The exterior of the intestine and the cavity of the typhlosole present a coating of yellowish-brown cells.
The MegmentcU organs are greatly convoluted tubes, situated one on each side of every segment except the first, and attached to the posterior mesenteric septum of the segment. Each canal communicates internally, by a wide funnel- shaped ciliated aperture, with the perivisceral cavity, while externally, it opens by a minute pore, which is usually dose to the internal pair of setse.t
A colourless fluid, containing colourless corpuscles, and answering to the blood of other invertebrated animals, occu- pies the perivisceral cavity ; but, in addition to this, there is a deep red|fluid, devoid of corpuscles, which fills a very largely developed system of pseud-hsamal vessels. These consist of longitudinal and transverse principal trunks, and of very numerous branches which proceed from them and ramify in aU parts of the body, except the cuticle and hypo-
The longitudinal trunks are three : one eupra-irUegtinal, which lies along the dorsal aspect of the alimentary canal ; <nie wb'inietiinal, which corresponds with this on the
* Hie nature of this substance tale,' 1873.^
has recently been discussed by t Gegenbaur, ''Ueberdiesoge-
M. E. Perrier, " Etude sur un nannten Respirationsorgane des
genre noaveao des Lombriciens.'* Regenwurms. ' (* Zeitsohrift fur
(•Axohivet de Zoologie ezp^men- Wits. Zoologiey' 1852.)
224 THE ANATOlfY OF INYBSTXBBATXD AHIXALS.
ventral aspect of that canal ; and one tub^neural, which lies beneath the ganglionic cord.
The supra-intestinal and sub-intestinal vessels are con- nected in the greater number of the segments bj pairs of eommismral transverse trunks, which embrace the in- testine, and give off numerous branches to it. The supra- intestinal and sub-neural vessels give off transverse trunks into the mesenteric septa, which branch out into the mus« cular layers, and some of which anastomose so as to form a second set of transverse communications. More- over, the sub-neural trunk and the sub-intestinal trunk respectively 3end branches to each segmental organ, upon which they are distributed, and, anastomosing, give rise to another series of communications between the longi- tudinal trunks.
In the seven most anterior segments, the longitudinal vessels break up into a network, and there are no distinct transverse commissural vessels. Behind these, and in the region of the generative apparatus, the commissural vessels are greatly dilated, and form from five to eight pairs of so-called hearts which are attached to the anterior faces of as many mesenteries. These contract from the dorsal towards the ventral side.
The nervous system consists of two cerebral ganglia lodg^ above the pharynx in the third segment, and united by commissural cords with the anterior ganglia of the chain, which extends through the whole length of the body on the ventral wall of the perivisceral cavity.
There are no eyes, nor are any other organs of special sense known.
The Earthworm is hermaphrodite. The testes are two pairs of large sacs, each of the anterior pair being bilobed. The testes of opposite sides are united in a common median reservoir, situated in the tenth and eleventh segments, from which, on each side, ducts take their origin. The two ducts of the testes of the same side unite into a single vas deferens, and these two vasa deferentia open externally on the ventral aspect of the fifteenth segment The ovarii
THE DETSLOPMBKT OF THE EABTHWOSM. 225
are two minute solid bodies, not more than -^^ of an inch long, attached to the posterior face of the mesenteric septum which separates the twelfth and thirteenth segments. Thej therefore lie in the cavity of the latter. The ovidncts are quite distinct from the oyaries, and open internally by wide funnel-shaped apertures, situated in the cavity of the thirteenth segment. From these funnel-shaped ends the oviducts are continued, as slender tub^^ through the mesenteric septum which separates the thirteenth from the fourteenth segment, and open on the ventral face of the latter.
Four globular spermathecse, or receptacles of the sper- matozoa, are situated, two on each side, in the tenth and eleventh segments, and open on the ventral face between the ninth and tenth, and the tenth and eleventh segments respectively. These are filled when copulation takes place, during which process, the two worms are said to be bound together by a tough secretion of their clitella.
The development of the Oligochcsta has recently been carefully investigated by Kowalewsky. The eggs of the Earthworm are laid in chitinous cocoons or cases, which are probably secreted by the clitella. In addition to the eggs, the cocoons enclose an albuminous fluid, and packets of spermatozoa. The viteUus is invested by a membrane, and contains a germinal vesicle and spot. Complete yelk- division takes place, and eventually the blastoccele becomes reduced to a mere cleft. The blastomeres are disposed in two layers — one consisting of small, and the other of large blastomeres. The embryo thus formed becomes concave on the side formed by the large blastomeres, until it assumes the form of a sac, ciliated externally, with an opening, the future mouth, at one end ; the cavity of the sac being the primitive alimentary canal, and the layer of large blastomeres, the hypoblast. Between the two, a mesoblastic layer appears, but the exact manner of its origin is not known. On one face of the saccular embryo the mesoblast becomes divided into a series of quadrate masses, like the protovertebne of a vertebrate envbrj^^
226 THB AKATOlfY OF nTYBSTEBRATSD AHIMALS.
disposed gymmetrically on each side of a median line, which corresponds with the future ventral median line of the body. Along this line, the epiblast becomes thick- ened inwards, and the thickening is converted into the ganglionic chain. At the same time, each quadrate mass of the mesoblast is excavated bj the development of a cavity in its interior, whereby it becomes converted into a sort of sac. The adjacent anterior and posterior walls of successive sacs unite, and give rise to the mesenteric septa, while their cavities become the chambers of the perivisceral cavity. The segmental organs commence as cellular out- growths from the posterior face of each septum thus formed, and only subsequently become excavated and communicate with the exterior.
The development of tlie Earthworm, therefore, closely resembles that of the Hirudinea, and more especially that of the Medicinal Leech, in which the digestive cavity of the embryo would seem to be formed, as in the Earthworm, by a process which is, in a sense, invagination. It would appear that the first formed aperture is the mouth ; while the anus is a secondary perforation ; and the segmentation of the body commences in the mesoblast.
In the fresh- water Oli^ochcetay Euaaees and Tubifem^ the vitellus also becomes divided into large and small blasto- meres. The latter extend over the larger blastomeres, and form the epiblast ( = ectoderm). A mesoblast ( = mesoderm), divided into two broad longitudinal bands, is developed, and the oral cavity is said to be formed by invagination of the epiblast between the anterior ends of the two bands of the mesoblast. In this case, the mouth in these genera is a secondary formation. The innermost layer of large blasto- meres becomes the hyx)oblast ( - endoderm).*
The PoLTCHiETA. — ^Except that the PolychoBta are almost invariably dioBcious and marine, while the Oligochceia are
* Kowmlewikv, <* Embryologische Studien.*' (* Hem. de PAcad. de St Petenboorg/ 1861.)
POLTKOE SQITAMATA. 227
moiioBcioQS, and inhabitants either of land or fresh water, it is hard to say what absolute characters separate these two groups. The lowest forms of the Polyclueta, such as CapUella and PolyophthalmuSy might be regarded as marine dioecious Naidoe, But, in the higher Polychceta, each segment of the body developes lateral processes — the parapodia, or rudi- mentary limbs, which are usually provided with abundant strong setsB ; a distinct cephalic segment, the prcMtomium, appears in front of and above the mouth, and bears eyes and tentacles ; while those parapodia which lie in the vicinity of the mouth may be specially modified in form and direction, foreshadowing the jaws of the Arthropoda, Ciliated, some- times plumose, processes of the dorsal walls of more or fewer of the segments may perform the office of external hranchice ; and, occasionally, the dorsal surface gives rise to flat shield-like processes, the so-called elytra.
The following detailed description of a very common species of Polyrwe, will give a fair conception of a poly- chsetous Annelid, in which the highest degree of complexity of organisation known in the group is attained : —
Polynoe aquamaia is an elongated vermiform animal, about an inch long, the body of which is divided into a succes- sion of portions, for the most part similar and equivalent to one another, but presenting peculiar modifications at the anterior and posterior extremities. Each such portion is properly termed a somite; while the term "segment" may be retained to indicate generally a portion of the body, without implying its precise equivalency to one somite or to many. Thus, then, the body of the Polynoe is composed of a series of twenty-six " somites," terminated anteriorly by a "segment," the proestomiumi (* Kopf-lappen,* Grube) and posteriorly by another, the pygidium, which may or may not represent single somites.
If one of the somites from the middle of the body (Fig. 51, 0, D) be. examined separately, it will be found to be trans- versely elongated, so as to be about three times as broad as it is long, and to be slightly convex above and below, prc- MOting a deep, median, longitudinal groove inien.ot\3«
228 THE AITATOICT OT nmBTIBKATBD AHIMiUB.
Laterally tlie somite is prodaced into two thick proceaaes, the "parapodia."
Bacb parapodiom diridea attta esbremity into twopoiliona,
a anperior and an inferior, which may be denominated r«-
Fig. 51.
Fig. il.—Fol^r tqatuniOa.
A. Vleved from above ■ad enlarged : a, b, r, etc. u Id Fig. 53 B ; r, elytra ; /. apace lefl between the two potCerloT elytra ; g, febe
and fimbria of the elytra.
B. Poaterior extremity, inferior Hei tubercle ; c, c", notopodlal and neoropodii
C. Section of half a aomlte with elyr
pygidial drri; h, iDferfor DOtopodium; t, unriv
apectively the notopodaim(Fig,Sli)aixdiheneiiirop<>ditim(k), the one ooonpTisg the " hemal," or dorsal, the other iho ■' neonl " or rentral aspect, The latter ia, in thia ap«ciea
FOLYKOE 8QIXAMATA« 229
SO much the larger, that the notopodinin appears like a mere tubercle projecting from its tipper surface. In other An* nelidOf however, and in the young state of Polynoe, the notopodium is as large as the neuropodium. Both divisions of the parapodia are armed with peculiar stiff hair-like appendages (g), composed of chitin, and developed within diverticula of the integument, or irichophores, in which their bases always remain enclosed. These can be pro- truded and retracted by muscles attached to their sacs, and they vary exceedingly in form. Three distinct kinds are observable in Polynoe alone. The notopodium and the neuropodium carry each a single, sharp, style-like aciefUum, the greater part of the length of which is imbedded in the parapodium and its divisions, while the point just projects at about the centre of the latter. The neuropodial is very much longer than the notopodial aciculum.
Superiorly, the notopodium carries two transverse rows of more slender organs of a similar nature, the $eUB : the proximal set are much shorter than the distal, but even the latter do not attain a length of more than -^ of an inch (Fig. 62, G).
The proximal set are somewhat knife-like in shape if viewed in profile, consisting of a comparatively short, straight ** handle," by which they are imbedded in their sacs, and of a thick, rounded, curved blade, tapering to a fine point at its extremity. Close-set transverse ridges, finely serrated at their edges, and inclined obliquely to the stirface of the blade, traverse its convex anterior circum- ference, leaving the back free. The distal set® (Fig. 52, G) have a very similar sbructure, but they are much elongated and very slender. The handle is longer ; and the blade, little curved and simply set on at an angle with the handle, is produced at the end into a long and delicate filament. The base of the blade (E) is beset with incomplete ridges, like those of the short setee, but towai*ds the middle (F) these ridges appear to encircle the blade completely, assuming the aspect of so many closely imbricated concentric scales, before finally becoming obsolete upon the extremity of the seta.
230 THE
or IHTXBTKB&A.TED AXUCAXS.
The nenropodial actculnm needs no special notice, except that the eitremitj of it» trichophore projects as a sort ai papilU, less obviooB in fnll-grown apecimens, which dindes the neoropodium into an upper and a lower portion, the former containing about half as manj eeta aa the latt«r.
Fig. 52.
Tig-iO—Polyndt ,
A^t jtron viewed from »bov( G, F, parts of the blute of fr«e (Xlremlty of knotopodi
The apertures of the tricfaophores are placed between lobe- like prolongations of the neuropodinm, to which the special term of kUna (Gmbe) may be applied. In this species &eij present no remarkable pecoliaritj beyond their in- eqnali^.
POLYKOB BQITAMATA. 231
The neuropodial set® (Fig. 52, C, D), although at first sight very different from the notopodial setsB, are, in tmth, constructed on essentially the same plan, the blade being short, while the handle is proportionally elon- gated. The blade is subcylindrical at its base, pointed and slightly curved. Eight or nine transverse ridges ex- tend around about two-thirds of the circumference of its proximal half; the basal ridges are narrow, and merely serrated, but towards the apex the ridges become deeper, and the serrations pass into strong teeth ; at the same time, one side of the ridge is elongated into a strong point.
Attached to the under surface of the parapodium by a somewhat enlarged base, with which it is articulated, is a smooth, conical, very flexible filament, — the neuropodial cirrus (Pig. 51, c') ; it hardly reaches to the end of the neuro- podium. Again, springing from the neural surface of the somite, close to the parapodium, there is a small pyriform tubercle (h), divided by longitudinal grooves into about eight segments. This is possibly connected with the repro- ductive function.
The appendage of the notopodium or rather of the noto- podial side of the parapodium and somite, varies according to the particular somite which may be examined. In some somites this appendage is a cirrus (Fig. 51, D, c) similar to the neuropodial cirrus, but much larger, equalling the semi- diameter of the body in length, and presenting an enlarged pigmented bulb of attachment to which the filament of the cirrus, which is cylindrical for about two-thirds of its length, and then becomes enlarged and suddenly tapers to its extremity, is articulated.
In the other somites the notopodial appendage is a large, thin, oval plate — the elytron (Fig. 51, 0, c). It is attached by a thick peduncle, and has its long axis directed obliquely outwards and backwards. The surface of the elytron (Fig. 52, A) is covered with an ornamentation of larger or smaller tubercular prominences, granulated and ridged upon their surface. A part of the inner and anterior edge of each elytron overlaps or is overlapped by its fellows for
232 THE AKATOMT OF niTBBTEBRATXD ANIMALS.
a certain extent of its oircnmference, which is so far smooth, but in the rest of its extent it is fringed with coarse brownish filaments or fi/mbruB, which arise from the npper snrface just within the edge, and are obviouslj oatgrowths of the same order as the tubercles.
Such is the structure of one of the middle somites of Polynoe agrtiamaia. The anterior and posterior somites, with the exception of the first and second, present only minor differences, as in the proportions of the setsB, or in the figure of the elytra. The first somite, which contains the mouth, is the peridofnium (** mund-segment " of Grube). The parapodia of this somite are narrow and elongated (Fig. 63, B, C, m); they are obscurely divided at their extremity into a rudimentary neuropodium and notopo* dium, and give attachment to a pair of large peristomial ciiri (c' e) (" cirrhes tentaculaires," Audouin and Milne- Edwards ; " Fiihler-cirren,'* Grube), of the same structure as the notopodial cirri, which stretch forwards by the sides of the mouth.
The apex of a single small aciculum issues rather aboye the point of division of the peristomial parapodium, and two minute curved setsB accompany it. These, have been generally overlooked ; * but they seem to demonstrate, in a very interesting manner, the nature of the appendages of the peristomial segment.
The second somite differs from the rest only in the great elongation of its neuropodial cin*us, which is directed for- wards and applied against the mouth.
The peristomium and the pnestomium together are ordi- narily confounded under the common term of " head.'* The latter (Fig. 53, B, G, I) is an oval segment flattened supe- riorly, placed altogether in front of and above the month, presenting on its postero-lateral edges four dark spots, the ejes, and possessing hve cirriform appendages, two pairs and a single median one. The latter (a), or the prcestomial
* At lesst, in the descrip- valuable paper, *'Ueber die Ent-
tions of the adult Fofyn6§, 'Ihey wickelung und Metamorphose der
are particularly mentioned, how- Polynoen." Q M filer's Arohiv,'
ever, by Max MCOler in hit 1851.)
FOLTHOI B4ITAIU.TA. 233
feiUaofe (" antenne mMiane," Milne-Edwftrds) is eimilar in Btmctnre to an ordinorj ciiroe. Of the other t^pend^es, th« upper one apoa each aide (sapero-lsteral pnestomial eirnw, " antanne mitojenne ") also reeemblee an ordinal; cirnu (b) ; but the lower (infero-lateral pmBtomial cirmB, 'antenne extome") {V) ia mach larger, and ia capable of extreme elongation and contraction,* while the ordinary Fig. S3.
Fig. Sa.—Polfn6t tjuamala.
A. Pnlerlor eitremltj; from sbove: c, Dotopodiml clrma of tut ■otnite : df pjgidiftL cirri ; r, BtiUA.
B. Anterior eitremltj from kbove: a, pneitoml^ tenUole; b, *u- poior and 6' inferior pittatomial cimw; c, c', natopodial uid neuro- podlal cirri; e, peduncle of first elytron : /, prKilomium : a, par*' podium of periKomluni. C. Inferior view of anterior extremity, lettan u before.
cirri are merely flexible. Although at first sight probable, jet it would appear from Max Muller's accoant of the deve- lopment of Polynoe, that these two appendages do not, like the two periatomial cirri which tbej essentially resemble,
• I have never olwerved any InvaginalioD aurli ■■ ii alaled to seeur by Audouin and Uilae- p. 10.)
Edward*. IBM. ('Blittrir* Natn- relle du Littoral de la Fiance,'
234 THE ANATOMY OF IHYBBTEBBATED ANIMALS.
correspond with the notopodial and nenropodial cirri of a single parapodium, inasmuch as they arise from perfectly distinct portions of the prsDstomium. It is yery possible that each represents the appendage of a somite, and in this case the prsBstomium would be composed of at least two somites. Whether the prsBstomial tentacle indicates another, or whether it is merely an appendage of such a nature as the labrum or the rostrum of a Crustacean, there is no evidence at present to show.
It is highly interesting to remark, that thus, in the Polynoe, as in the Arthropoda, the " head " results from the modification of a number of somites, some of which lie in front of, and others behind, the mouth. The move- ments and evident extreme sensitiveness of the inferior prsBstomial cirri during life indicate that they perform the functions, as well as occupy the position, of an- tennae.
The hindermost segment of the body, or pygidium (Fig. 51 6, Fig. 53 A), is narrow, and divided at the end into two supports for the pygidial (d) cirri, which are as long as the three last somites, and resemble the notopodial cirri in form and structure. They extend directly backwards, almost parallel with one another and with the notopodial cirri of the last somite, which are thrown backwards and downwards (Fig. 53, A, c). It seems probable that the pygidium represents only a single somite.
The anus is not terminal; as in many Annelids, but is seated in the middle of a strongly raised papilla (Fig. 53, A x\ which projects from the dorsal surface of the penulti- mate somite ; its sides are produced into about fourteen folds. The two last elytra have their edges excavated, 80 as to leave a space over the anus (Fig. 51, A/).
The notopodial cirri and the elytra do not coexist upon the same somites; and the order of arrangement of the elytrigerous and cirrigerous somites is very curious. The Ist or peristomial somite is cirrigerous, and so are the 3rd, 6th, 8th, 10th, 12th, 14th, 16th, 18th, 20th, 22nd, 24th, 25th, and 26th; while the 2nd, 4th, 5th, 7th, 9th, 11th, 13th,
THX POLYCHiBTA. 235
15tli, 17th, 19th» 2 let, and 23rd somites bear elytra, making twelve pairs in all.
In no polychsBtoos Annelid is the stmctiire of a somite more complex than in Polynoe ; and there are but very few parts not found in Polynoe to be met with in other Annelida. The careful study of this species, therefore, furnishes us with an almost complete nomenclature for the external organs of the whole group ; and it will be found that the other forms of Annelida differ mainly in the greater or less development and modification of the organs which have just been de- scribed. A large proportion of the Polychceta are like Polynoe, free and actively locomotive animals, which rarely fabricate tubular habitations, and are therefore termed ErranHa ; they possess a prsestomium, usually provided with eyes and feelers, and have many parapodia, which are not confined to the anterior region of the body. They very generally have a proboscis, provided with chitinous teeth.
The singular genus, Tomopteris, is a transparent pelagic Annelid, with numerous parapodia, each terminated by two lobes representing the neuropodium and notopodium, but with setae, two of which are very long, only in the cephalic region.
The sedentary Annelids (Tubicola) fabricate tubes, either by gluing together particles of sand and shells, or by secret- ing a chitinous or calcified shelly substance, in which they remain (e. g. ProtuUit Fig. 54). The prsestomium is small or wanting; none have a proboscis; there are no cirri; and the parapodia are short or rudimentary. The branchise are developed only on the anterior somites, and the latter are often markedly different from those which constitute the posterior part of the body.
In some (SerptUidcB) a tentacle is enlarged and its end secretes a shelly plate which serves as an operculum, and shuts down over the mouth of the calcareous tube inhabited by the animal, when it is retracted. The dilated end of the opercular tentacle sometimes serves as a chamber in which the young undergo their development, (species of 8pirorbi»).
The alimentary canal of the polychsetoufi AntieUda x^x^-^
Fig. b4.—Pmiila Dyitcn. A, the lexnat, mabire lalinal, extracted lirom III calcanou* tube; a, bredcUai plamca- b, hood-Uke u- pantion of the tntecior end of the body: c, the month; 4, the ■tumach: *, the uiiu; /, (he t«atei: f, the oTk. B. « PrvMa in dmnearproUfamaoa; i, ttaebnnoliueof theiaoid..
THB POLYCHJBTX. 287
presents any marked distinction into stomach and intestine, and is almost always of the same length as the body, ex- tending, without folds or convolutions, from its anterior to its posterior extremity ; but in Siphonostomum {Chlorasma), PeeHnaria and others, it is more or less convoluted. It is attached by membranous bands, or more complete mesen- teries, to the walls of each somite, and very commonly presents a dilatation between eveiy pair of mesenteries. In most PolychcBia, the intestine acquires in this way merely a moniliform apx>earance, but in Polynoe, Aphro^ dUe, ^aUon, and their allies, long csca are given off upon each side of the alimentary canal, and, sometimes becoming more or less convoluted, terminate at the upper part of each segment (Fig. 51, D) close beneath, or in the branchiee, where such organs exist.
The anterior portion of the alimentary canal is, in a gpreat number of the Polychosta, in fact in all the typical ErrarUia, so modified as to constitute a distinct muscular pharynx, the anterior portion of the wall of which can be everted like the finger of a glove, from the aperture of the mouth, and the posterior portion protruded, so as to form a proboscis. In Polynoe squamata, the proboscis is one-fourth as long as the body, and its walls are very thick and muscular. At its anterior extremity it is sur- rounded with a circle of small papillae, immediately behind which are four, strong, pointed and curved homy teeth, implanted in the muscular wall (Fig. 52, B). Each tooth has a little projection upon its convex edge, which is con- nected by a short strong ligament with the corresponding projection of another tooth ; and the one pair of teeth, thus connected, works vertically against the opposite pair. In Nereis, there are two iK)werful teeth which work horizon- tally, besides minute accessory denticles. In SyUie, the chitinous lining of the pharynx is produced into a circle of sharp teeth anteriorly, and there is, in addition, a much stronger triangular median tooth. In Olycera, which pos- sesses a pair of teeth, the extremity of the protruded proboscis is covered with very remarkable papillaa. Th;^
238 THE ANATOMY OV UrTEBTKBRATED ANIMALS.
most complex arrangement of teeth, however, ia that pre- sented by the EunicidoB, In Euniee, there are altogether nine distinct pieces : two large, flat, more or less* calcified portions united together below, and three cutting and tearing teeth on the right side working against four on the left. As has been already stated, the tubicolar Annelids possess neither proboscis nor teeth.
No special hepatic gland appears to exist in the Annelida, unless the intestinal cseca perform that function, and the secretion of the bile is doubtless effected by the glandular tract, which extends for a greater or less distance in the walls of the aHmentary canal. A pair of glandular caeca, the function of which is not known, is appended to the base of the proboscis in Nereis,
The general cavity of the body, or periyisceral carity. which is included between the parietes of the alimentary canal and those of the body, is filled with a fluid which contains corpuscles, which are usually, as in the Inveriebrata in general, colourless. They are red, howcTer, in Olyeera, and in a species of Apnewmea (De Qaatrefages). The para- podia, the cirri, the branchisB, and all the other important appendages of the Polychceta contain a cavity continuous with the perivisceral cavity, and are therefore equally filled with the blood. The circulation of this fluid is effected partly by the contraction of the body and its appendages, partly by the vibratile cilia, with which a greater or less extent of the walls of the perivisceral cavity is covered.
In a great number of the Polychceta no part of the body is specially adapted to perform the function of respiration, the aeration of the blood probably taking place wherever the integument is sufficiently thin; and, even when dis- tinct biunchisB ordinarily exist, members of the same family may be deprived of them. In Polynoe sqvamatcty ciliated spots which appear to represent branchis, may be discovered on the dorsal side of the bases of the parapodia, at any rate, in young specimens. In some species of Polynoe the parapodia give rise, at oorresponding points, to large, richly ciliated, malleif orm toberdes, in which the csdca of
THE POLYCH.STA. 239
the alimentary canal terminate. In Sigalion, a filiform, ciliated branchia depends from the upper part of the somite, beneath the elytron ; and besides this, curious little ciliated palettes are arranged upon the dorsal surface of the para- podia, and upon the sides of the anterior somites. But the best developed branchise among these Annelids are pos- sessed by the AmphinomidoB, and the EunicidcB among the Erraniia; the TerehellidoB, and the SerpulidcB among the Tubicola, In the three former families the branchiffi are ciliated branched plumes, or tufts, attached to the dorsal surface of more or fewer of the somites. In the last (Fig. 54) they are exclusively attached to the anterior seg- ment of the body, and present the form of two large plumes, each consisting of a principal stem, with many lateral branches. The stem is supported by a kind of internal skeleton, of cartilaginous consistence, which sends off pro- cesses into the lateral branches.
I have been unable to find any pseud-hsemal vessels in Polynoe squamatay and as Claparede * could discover none in the transparent P. lunulatay it is safe to assume their non- existence. Clapai*ede, in fact, denies them to the whole of the AphroditidcB,
When it is present, the pseud-Lsemal system varies very much in the arrangement of its great trunks; but they commonly consist of one or two principal longitudinal dorsal and ventral vessels, which are connected in each somite by transverse branches. Where branchise exist, loops or processes of one or other of the great trunks enter them. The dorsal and the ventral trunks are usually rhythmically contractile, and contractile dilatations at the bases of the branchiae (Etmice), in x>ortions of the lateral trunks {Arenicola), or in those which supply the proboscis (Eunice, Nereis)^ have received the name of " hearts." The direction of the contractions is usually sach that the blood is propelled from behind forwards in the dorsal vessel, and in the opposite direction in the ventral vessel ; but the course which it pursues in the lateral trunks
* • Ann^lides Chetapodes da Golfe de Millet,' 1868, p. 65.
240 THE AHATOMT OF IHYBBTBBIU.TBD AKIMALS.
is probably verj irregular. In ChlaroBma, in wliicli eyen the smallest ramifications of the vessels are contractile, I have observed csecal branches depending into the peri- visceral cavity in which the contained fluid underwent merely an alternate flux and reflux. Ramified csBca of a similar kind appear to exist in the oligochsetous genera, Euaxes and Lumhriculus. The principal trunks give off a great number of branches, which ramify very minutely in some Annelids {Eunice) and may give rise to retia mirabilia (Nereis) ; but in many {e.g. Protula), there are hardly any branches and no minute capillary ramifi- cations.
In many Polycliceta no segmental organs have yet been discovered, and in others they appear to be represented by mere openings in the parietes of the body. I have observed short ciliated canals opening externally upon the ventral surface at the bases of the parapodia in Phyllodoce viridis, and there are indications of the existence of similar organs in SyUis vitttUa. True segmental organs have, however, been found by Ehlers and ClaparMe in many Polychata, In some cases their walls are thick and glandular, and they probably have a renal function. In addition, they frequently play the part of oviducts and spermiducts. Whether the ciliated canal extending along the ventral surface of the intestine, which I have described in Proitda^ is a structure of the same order or not, I am not prepared to say.
The nervous system of the Polychceta usually consists of a chain of ganglia, — one pair for each somite,— connected together by longitudinal and transverse commissural bands, which diverge between the cerebral ganglia and the suc- ceeding pair, to allow of the passage of the oesophagus. The most important differences presented by the nervous systems of the Polychcsta, result from the varying length of the transverse commissures. In Vermilia, SerptUa, SabeUa, these commissures are very long, so that two distinct and distant series of ganglia appear to run through the body, while, in Nepihys, the two series of ganglia are fused into a single cord enlarged at intervals. Every transitional
THE POLTCHiETA.
241
ccmdition between these is observable in Tcrchellay Aonia^ Glycera, Phyllodoce, and Aphrodite. In most Folychceta a ▼erj extensive series of visceral nerves supplies tlie alituen- tary canal.
The recog^sable organs of sense in the Annelida are eyes and auditory vesicles. The former are usually very simple, consisting qf an expansion of the extremity of the optic
Fig. 55.
—c
Fig. 55. — A. Anterior end of tlie nervous Bystcm of Polynbe squanmta (after De Quatrefoges) : ct, cerebral ganglia ; 6, oesophageal commis- ■ures ; c, longitudinal commissures of the ventral ganglia.
B. Anterior end of the nervous system of SabeUa fiabeUata (after De Quatrefages): a, cerebral ganglia; h, oesophageal commissures ; r, longitudinal commissures of the ventral ganglia. Those of opposite tides are uuited by long transverse commisburcs.
nerve, imbedded in pigment, and provided occasionally, but not invariably, with transparent spheroids or cones. Aleiope and Torrea have very well developed and large eyes. The eyes are usually confined to the anterior extremity of the body, and to the prsestomium where it exists ; but, in the re- markable genus PolyophthalmtiSf De Quatrefages discovered^
-1- THE ANATOiMY OF INVKRTKBRATED ANIMALS.
— I
]>odide8 the ordinarj cephalic eyes, a double series of addi- tional visual organs, one pair being allotted to each somite. In Branehiommat eyes are situated at the ends of the bran- chial plumes. Ehrenberg has described two caudal eyes in Amjphieorat and De Quatrefages has shown that similarly placed eyes exist in three other species of PolyehoBta, two of which are closely allied to Amphicara, while ^he other is an errant form, related to Lumbrinereis. These curious worms are said to swim about with the caudal extremity forwards.
Auditory sacs, containing many otoliths, have been ob- served upon each side of the oesophageal ring in Areni- cola, and similar organs have been noticed in other Tubicola ; but hitherto their existence has not been certainly deter- mined in the ErratUia.
The genitalia of the polychstous Annelida are exces- sively simple in their structure; indeed, special repro- ductive organs can hardly be said to exist in most, the generative products being merely developed from some part of the walls of the perivisceral cavity, in which they eventually freely float, making their way out in a manner which is not quite understood at present ; probably, how- ever, through temporary or permanent apertures at the bases of the parapodia. In many, the segmental organs appear to serve as excretory ducts. As a rule, the poly- chsetous Annelids are dioecious ; but some {e.g. ProitUaf Fig. 54) are hermaphrodite. The ova undergo their development within the body of the parent in some species of Eunice ; in pouches attached to the body in Ikoogone ; in masses of gelatinous matter which adhere to the tubes of the vermi- dom in Protula ; beneath the elytra in Polynoe cirraia ; in the cavity of the opercular tentacle in some Spirarhee ; while, in other cases, they appear to become, almost imme* diately, free ciliated embryos.
The vitelluB undergoes division, and is converted, as in the case of the Oligocha^ and Hirttdinea, into blastomeree of two kinds. This contrast between the two components of tlie emibiyo commences with the division of the vitellos into
THB DEYBLOPMBNT OV THE POLTCH.£TA. 243
two, inasmuoli as the first fiflsnre is usually so directed as to divide the yelk into unequal portions. Both subdivide, hat the smaller much faster than the larger ; so that the former becomes converted into very small blastomeres, which gradually envelope the larger blastomeres resulting from the subdivision of the latter. The larger included Uastomeres are destined to form the alimentary tract; the smaller peripheral ones, on the other hand, give rise to the ectoderm, and to the nervous ganglia.* As in the OUffochada and Hirvdineay again, the mesoblast forms a thick band on each aide of the median ventral line, and ita transverse division originates the segmentation of the body. But, generally, the development of the proiosomites, as these segments might be called, does not occur until some time after the embryo has been hatched. The somites increase in number by tibe addition of new ones between the last and the penultimate somite.
The embryos of the PolychcBta differ from those of the OligochcBia and Hirudinea in being ciliated. In some cases, the cilia form a broad zone which encircles the body, leaving at each end an area, which is either devoid of cilia, or, as is frequently the case, has a tuft of long cilia at the cephalic end. Such larvsB are termed Atroeha.
In other embryos the cilia are arranged in one or more narrow bands, which surround the body. A very common arrangement is one in which a band of cilia encircles the body immediately in front of the mouth, the region in front of the band bearing eyes, and becoming the prsesto- mium of the adult (e.g, Polynoe). In such embryos, there is very commonly a second band of cilia around the anal end of the embryo, and a tuft of cilia is attached to the centre of the prsestomium. These larvse are called Telotrocha, In other cases, one or many bands of cilia surround the middle of the body, between the mouth and the hinder extremity. These are Mesoirocha.
In the telotrochous larva of PhyUodoee a shield-shaped,
* ClaparMe and MeUcbnikoff, * Beitrige sur Kenntnifs der £nt- wickeluugsgeichiohte der Cbaetopoden,' 1^68.
244 THE ANATOMY OF INYESTBBBATBD ANIMALS.
mantle-like, elevation of the integument covers the dorsal region of the body behind the prsB-oral ciliated ring. In the larvae of the Serpulidce a process of the integument grows out behind the mouth, and surrounds the anterior part of the body of the larva like a turaed-back collar. It persists, as a kind of hood, in the adult.
Some lai*v8B are provided with set® of a different cha- racter from those which are possessed bj the adult, and which are cast off as development advances.
Many PolychcBta multiply by a process of zooid develop- ment, which, in some cases, appears to be a combination of fission with gemmation ; in others, to approach very nearly to pure fission or pure gemmation. The result is, not infre- quently, the formation of long chains of connected zooids.
The method of multiplication which De Quatrefages ob- served in Syllis prolifera, is nearly simple fission, the animal dividing near its middle, and the posterior division ac- quiring a new head.
In Myrianida, Milne-Edwards has described the occur- rence of a sort of continuous budding between the ultimate and penultimate segments, in which region new segments are formed until the zooid has attained its full length.
Frey and Leuckart and Krohn have shown that Autolytus prolifer multiplies in a somewhat similar manner; but, instead of each new zooid being formed at the expense of an entire somite, it is developed from only a portion of one. Finally, I found in Protula Dysteri that, when the Proiula had attained a certain length, all the somites behind the sixteenth became eventually separated as a new zooid ; but the development of the latter is not mere fission, inasmuch as one of the earliest steps in the process is the enlarge- ment of the seventeenth somite, and its conversion into the head and thorax of the bud (Fig. 54, B). Sars has described a similar mode of multiplication in his FUograna impUxa, a very closely allied form.
In SyUis and in Proiula, the producing and the produced zooids alike develope generative products, but in Autolytus,
AOAMOGBKBSIS AMONG POLTOHJETA. 245
Krohn has shown that the primary producing zooid remains sexless, the secondary produced zooids having a somewhat different form, and sdone giving rise to ova and sperma* tozoa.
In some species of the genus Nereis, the worm, after the development of its genital organs has taken place, takes on the characters of what was formerly considered a distinct genus, Heteronereis ; and the males and the females of the same species of Nereis have even been regarded as different species of Heteronereis,*
The series of forms represented by the TurbeUaria, the Hirudinea, the Oligoehceta, and the Folychasta, illustrates the manner in which a type of organisation, which, in its simplest condition, exhibits but little advance upon a mere GJastrula, passes into one in which the body is divided into many segments, each provided with a pair of appendages or rudimentary limbs^
The segmentation, or serial repetition of homologous somites, extends to the nervous system, and, more or less, to the vascular and reproductive organs, in the higher forms of these " Annulose " animals ; from which a further exten- sion of the same process of segmentation, with a fuller development of the appendages and a more complete appro- priation of some of tbem to manducatory pui*po8es, leads ud to the Arthropoda,
The Gbphtbea. — These are marine vermiform animals without distinct external segmentation or parapodial ap- pendages. The ectoderm has a chitinous cuticle, and is often provided with tubercles, hooks, or setae of chitiu (EehiuruSf Stemaspis). No calcareous skeleton is found in any of the Gephyrea, The integument frequently contains numerous simple glands, the apertures of which perforate the cuticle. In one genus {Stemaspia), two shield-shaped plates, fringed with set®, are developed upon the hinder part of the
* Ehlen, "Die Gatiung UeteronertUr (^Gdttingen Nachrichten/ 1867.)
246 THE ANATOICT OV INYBRTBBIULTED ANIMALS.
Tentral surface of the body. There are external circular, and internal longitudinal muscular fibres beneath the ecto- derm. An inner layer of circularly disposed muscular fibres may be added. The oral end of the body may haye the form of a retractile proboscis {Priapulus), or be provided with tentacular appendages. These may be arranged in a circle round the mouth, and short {Sipun- cuius. Fig. 56, I. t), or long {Phoronis), or there may be a single long, sometimes bifurcated and ciliated, tenta- cular appendage {Bonellia). Filamentous appendages, which are probably branchiie, are g^ven off at the hinder end of the body in Stenuupis and Priapulus, The endo- derm is usually ciliated throughout. The intestine is straight in most genera, but is coiled and bent upon itself, so as to terminate in the middle of the body, in Sipunculus (Fig. 56, I.). In PhorowU the anus is close to the mouth. The anal aperture is always situated upon the dorsal aspect of the body. There is a spacious perivisceral cavity undivided by mesenteries, which in some cases {Prui- pulus, 8ipuneiUu8) opens externally by a terminal pore. In EehiuruSf Bonellia, Thalcissema, a pair of tubular, some- times branched organs, which are ciliated internally, and communicate by ciliated apertures with the perivisceral cavity, open into the rectum. These appear to represent the water- vessels of the Botifera and the respiratory tubes of the HolothuricB.
A pseud-hsBmal system exists in most {8ipunculu8, Sier- nntpisy Bonellia^ EchiuruSy and Phoronia), and, when fully developed, consists of two longitudinal trunks — one dorsal, or supra-intestinal, the other ventral, with their terminal and lateral communications. The pseud-hsemal fluid is colourless, or may have a pale reddish tinge, in most. In Plwronis it is said to contain red corpuscles. In 8ipwi- cuius, the cavities of the tentacles communicate with a circular vessel provided with csecal appendages ; and this circular vessel is said to open into the pseud-hsmal vessels.
The nervous system presents a collar, which surrounds the ooeophagus, and from which a simple or ganglionated
FiE. M.— S'>niinJ« nxihi (kfter Kefe-steia ud Ehlera; ■ 1. lln uimkl lalil open lunsitudiully -) n.s. T; (entkoln; r, the four TCtraclOT muielti of ihe probaicis ; r, the pilnia ■[ which th«j *cra attsched tu the walls of tba bodj; a, cetophaput; i, Intei- tlne; a, anui; J, J', loop> of [he intestine ; i, jr, ippanduM of the rveluin; i, fuilform mmcla; w, ciliated gronve on the inner tide of tb« tnt«atine ; q. anil muaclea ; i, ueol eUndi ; (, cKca which open on each aide of the nervoua cord, anil are general/; eoutidered to be lealea j p, pore at the hinder end of ihe biidy ; x, nervoui cord, which end* in a liibed ganglionic matt, cline to the mouth, and preaenia an enlargement, ^, ai its posterior end ; m, m', ■■', miUBlei
U. AlarvalKipunculuiatKiul^ofaoinchlong.— a, moulh; a.galiet; (, cecal gUnd; >, intettlne wiib musea of faltj' cell>; ", anu>; ir, ciliated groovfl of (ho Iniealine; g, hraJn with two pair* of red eye- ■poKi a, nervout curd, p, pore; (, (, aa-called teatei; W W, circlet of Cilia.
giving off lateml branches. The ventral cord containB it • 'Zoologiiche Beitnisc,' ISIil.
24& THE ANATOMY OF IMYBRTBBEATED ANIMALS.
t
central canal, and the collar usually presents a cerebral ganglionic enlargement. Budimentcurj eyes are sometimes connected with the cerebral ganglion.
The sexes are distinct, and the reproductive elements are developed either from the parietes of the perivisceral cavity or in simple caocal glands. In 8ipunculu8, the ova and spermatozoa float freely in the perivisceral cavity.
The actively locomotive embryo of Sipunculua (Fig. 56, EI) is surrounded by a circular band of cilia placed immediately behind the mouth (w, w), and resembles a Rotifer or a mesotrochal Annelidan larva. As development advances it loses this apparatus, and passes gradually into the adult form. In Phoronis, the embryo is also mesotrochal, but it has two ciliated bands, one circular, round the anus, and the other immediately behind the mouth. The post-oral band of cilia is produced into numerous tentaculiform lobes, and fringes the free edge of a broad concave lobe of the dorsal side of the body, which arches over the mouth. In this state the embryo is the so-called Actinabrocka.* An invagi- nation of the ventral integument of the larva connects itself with the middle of the intestine, and then, becoming evaginated, pulls the intestine, in the form of a loop, into the ventral process thus formed, which gives rise to the body of the Phoronis, while the tentacles of the larva grow into those of the adult. Schneider has suggested that the bell-shaped larva, with long setfle, termed MUraria by Miiller, is the embryo of Stemaspia,
The affinities of the Gephyrea with the Turhellaria, with the Annelida, and with the Bati/era, are unmistakable. In fact, it may be doubted whether Siemtupia should not be associated with the Polychcdta, and BoneUia is in many re- spects comparable to a colossal Rotifer. Their usually assumed connexion with the Echinodermaia is more ques- tionable. The circular canal which communicates with the cavities of the tentacles in Sipunculua has been compared to the ambulacral system of the Echinoderms, but the
* Schneider, ** Uehor ilio Metamorphoie der Actinotrocha branehiuta^** C Axchiv tur Aiiat./ 1862.)
THE OBFHYBBA.
249
manner of its development is not yet safficientlj nnderetood to justify the expression of an opinion on this subject. Krohn has described a bilobed organ on the ventral face of the gullet of the larva of Sipunculus, which opens extemallj in front of the ciliated band by a narrow ciliated duot * (Fig. 56, II. s). It has a striking similarity to the " water-vessel " of the larva of Bcdanoglosmu, which, how. ever, lies on the opposite side of the body.
• ** Ueber die Larve des Stpunculut nudus** C Archiv fQr Anat.,*
250 THE ANATOMY OV nTYSBTSBBATBD ANIMALS.
CHAPTER VI.
THE ABTH&OPODA.
The segmentation of the body, that is, its division into a series of somites, each provided with a pair of lateral appendages, which is so characteristic a feature of the higher Annelids, is exhibited in a still more marked degree by the Arihropoda. In these animals, moreover, the appen- dages themselves are usually divided into segments, while one or more pairs of the appendages in the neighbourhood of the mouth are modified in form and position to sub- serve manducation. Segmental organs, at least in their Annelidan form, are wanting in the ^r^ropoda, and neither in the embryonic nor the adult condition do they ever possess cilia.
The process of yelk-division may be complete or incom- plete, but no known Arthropod ovum gives rise to a vesicu- lar morula, nor is the alimentary cavity ordinarily formed by invagination.* The precise mode of origin of the meso- blast has yet to be worked out, but the perivisceral cavity appears always to be developed by its splitting. In other words it is a 9ch%zoccele.
As with Annelids, the segmentation of the body results from the subdivision of the mesoblast by transverse con- strictions into protoaomUes ; and there is every reason to believe that the ganglionated nervous chain arises from an involution of the epiblast.
* The recent observations of Bobretzky on the dt-velopment of OnUcuttLnd AHaeu* {Hofmiknn and Schwalbe, * Jahresbericiite/ Bd. ii., 1875J however tend to
show that the hypoblast arises by a sort of modified invagination of the primitive blastoderm. And in other Arthropoda there are indications of a similar process.
THE ABTHBOPODA. 251
The neural face of the embryo is fashioned first, and its anterior end terminates in two rounded expansions — the proeephdlie lobes — which are converted into the sides and front of the head. The appendages are developed as paired outgrowths from the neural aspect of each somite, and what- efver their ultimate form, they are, at first, simple bud-like processes. Very generally, a broad median prolongation of the sternum of the somite which lies in front of the mouth, g^ves rise to a labrum ; while a corresponding, but often bifid median elevation, behind the mouth, becomes a meioHoma,
In many Arthropods, the hsmal or tergal face of the body grows out into lateral processes, which may either be fixed, or more or less moveable. The lateral prolongations of the carapace in the Crustacea and the wings of Ineecta are structures of this order.
In a number of Insects belonging to different orders of the class, an amnionic investment is developed from the extra-neural part of the blastoderm by a method similar to that which gives rise to the amnion in the higher Verte- brcUa,
In aU the higher Arthropods, a certain number of the somites which constitute the anterior end of the body coalesce and form a head, distinct from the rest of the body; and the appendages belonging to these confluent somites undergo remai'kable modifications, whereby they are converted into organs of the higher senses and intc» jaws. In many cases, the somites of the middle and posterior parts of the body become similarly differentiated into groups of polysomitic segments, which then receive the name oithorax and abdomen. The somites entering into each of these groups may remain distinct or may coalesce. The tergal expansions of the somites of the head, or of both head and thorax, may take the shape of a broad shield, or carapace. This may constitute a continuous whole (e.g, Apu9, Astacus) ; or its two halves may be moveably connected by a median hinge, like a bivalve shell (Cypris, Limnadia) ; or finally, the tergal processes of each side may remain dis-
252 THE ANATOMY OV IinrBSTEBSATBD ANIMALS.
tinct from one another and freely moveable on their respec- tive somites (wings of Insects).
Limbs, or appendages capable of effecting locomotion, are always attached either to the head or to the thoraX,* or to both. Thej may be present or absent in the abdominal region. In adult Ar<ichnida and Inseda, there ^re no abdominal limbs, unless the accessory organs of gedStation, the stings of some insects, and the peculiar appendages of the abdomen in the Tkyaanwra and CoUembola be such.
The alimentary apparatus presents very wide diversities in form and structure, and in the number and nature of its glands. The anus, which is very rarely absent, is situated in the hindermost somite.
In like manner, the blood-vascular system varies from a mere perivisceral cavity without any heart (Ostraeoda, Cirri- pedia) up to a complete, usually many-chambered heart with well-developed arterial vessels. The venous channels, how- ever, always have the nature of more or less definite lacunse. The blood corpuscles are colourless, nucleated cells.
Special respiratory organs may be absent, or they may take one of the following forms.
1. BranchuB, Externally projecting processes of the body or limbs, supplied with venous blood, which is thus brought into contact with the air dissolved in water.
2. TrackecR. Tubes which traverse the body and gene- rally open upon its exterior by apertures termed stigmata, and thus bring air into contact with the blood and the tissues generally. Saccular reservoirs of air are often formed by dilatations of these tubes.
The so-called Tracheo-hranchicB of some aquatic Insect larv8B are usually laterally projecting processes of more or fewer of the thoracic or abdominal somites, containing abundant trachese, which communicate with those which traverse the body {Ephemenda, Ferlaridas), They are in no sense branch!®, but simply take the place of stigmata. The exchange of constituents between the air contained
* The extinct TrUohiUt possibly fonn en exception to this rale.
THB ABTHBOPODA. 253
in the traclieee of these aTiimalfl and that of the anrrounding medium, is effected indirectly, by diffusion through the waDa of the tracheo-branchise, instead of directly, through the stigmata, as in other cases.
In the aquatic lanrsB of many Dragon-flies {Libelliilid<B)t the function of the tracheo-branchisB is performed by folds of the lining membrane of the rectum, which contain abun- dant trachesB. Water is drawn into, and expelled from, the c&yitj of the rectum by rhythmical contractions of its walls, 80 as to secure the exchange of gaseous constituents between the air which it contains and that which fills the trachese.
3. Pulmonary sacs. These are met with only in some Aradmida. They are involutions of the integument, the walls of which are folded in such a manner as to expose a hirge surface to the air, which is alternately taken into, and expelled from, their apertures. The blood is brought to these sacs by venous channels.
The exact mode by which the separation of the nitro- genous products of the waste of the tissues from the blood is effected in Arthropods requires further elucidation. In many however, such products, notably uric acid, have been found to abound in the corpus adiposum — a cellular mass which lies in the walls of, and more or less fills, the peri- visceral cavity — and in the Malpighian glands. In the latter case, they are conveyed out of the body by the intes- tine.
The nervous system consists primitively of a pair of ganglia for each somite, but the number of ganglia dis- coverable in the adult depends on the extent to which these primitive ganglia coalesce. There is usually, if not always, a well-developed system of gauglionated vis- ceral nerves, connected with the cerebral ganglia and distributed to the gullet and stomach.
Eyes are usually present ; and, when they exist, they are almost always situated in the head and are connected with the cerebral ganglia. Among the Crustacea, however, Euphausia has eyes in some of the thoracic limbs, and in some abdominal somites. The eyes may be simple or compound.
254 THE ANATOMY OV IKYB&TEBSATED ANIMALS.
In the latter case there are, in correspondence with the namber of parts into which the transparent corneal con- tinuation of the chitinous cuticula over the eje is divided, a namber of elongated bodies which lie between the ont^r surface of the ganglionic expansion of the optic nerve and the inner face of the cornea. These bodies consist of two parts : an external transparent crystalline cane and an internal prismatic rod. The broad end of the cone is external, and is applied to the inner surface of the corneal facet ; its narrow end is continuous with the outer extremity of the prismatic rod, which, by its inner end, is connected with the ultimate ramifications of the optic nerve. Each of these crystalline cones and prismatic rods is separated from the rest by a pigmented sheath.*
Distinct auditory organs have been observed in Crus- taceans and Insects. They are not exclusively confined to the head. In the opossum shrimp (Mysis), for example, they are placed in the appendages of the last somite of the abdomen. And in Insects, the only organs to which the auditory function can be certainly assigned are situated in the thorax or in the legs.
There is some reason to think that the antenns of Insects are the seat of the olfactory function, but no certain information on this head has been obtained. The very fine setffi to the bases of which nerves can be traced, which abound on the antennary organs of Insecta and Crustacea, but are found in other regions of the body, are probably partly tactile and partly auditory organs.
As a general rule, all the muscles of the Arthropoda, even those of the alimentary canal, are striated. Those of the body and limbs are often attached by chitinised tendons to the parte which they have to move. As the hard skeleton is hollow and the muscles are inside it, it follows that the body, or a limb, is bent towards that side of its axis, which
* Leydig, ' Das Auee der GUe- eye of the lobster, accompttnied
derthiere,' 1864. Schulxe, * Un- by full references to the liten-
tersoohungen/ 1868. Mr. £. T. tare of the subject, in the *Quar-
Newton ham given a vary eood terly Journal of Microscopical
aeoouat of the stracture of the Bcienee' for 1875.
THE ABTHBOPODA. 255
is opposite to that on whicli a contracting muscle is ntaated.
Sounds are produced bj many Insects; but in most oases, they cannot be properly referred to a voice, in the sense in which that term is applied to the sounds pro- duced in the higher animals, by the vibrations of the atmosphere arising from the impact of a current of air upon the free edges of membranes bounding the aperture of exit of the current. The chirping and humming of Insects often arise from the friction of their hard parts against one another, or from the rapid vibration of their wings : in some instances, however, recent investigations render it probable that they are produced by the action of expiratory currents on tense membranes which bound the stigmata.
Agamogenesis is veiy common among some groups of the Arihropoda, such as the Onuicicea and the Insecta, but has not yet been observed in the Myriapoda or the Arachnida. It may be effected in one of two ways :
1. Either individuaLs which are, by their structure, incap- able of being impregnated and are therefore physiologically sexless, though it may happen that they more or less ap- proximate females moi'phologically, give rise to offspring {Cecidomyia larvse. Aphis) ;
2. Or individuals which are capable of being impregnated, and are thus both morphologically and physiologically true females, give rise to eggs which develope without impreg- nation. (The queen bee, so far as the production of drones is concerned ; many Lepidoptera.)
The cases of Apus, Daphnia and Cypris would belong to the latter category, if it were certain that the very same females which, for a certain period, produce young agamogenetically, at another time undergo fecundation. Multiplication by fission or external gemmation is not known to take place in any Arthropod. Hermaphrodism occurs as a rule in some few Arthropods {e,g. the Cirripedia and Tardigrada), and as an abnormal 'sport' in sundry CruHaeea and in many Irueeta.
256 THE ANATOMY OF INYBSTEBBATBD ANIMALS.
In absolute number of species, the Arihropoda far exceed all the rest of the animal kingdom put together. Thus Grerataecker,* while allowing 60,000 species for the latter, estimates the number of species of Arihropoda as rather above than below 200,000 ; by far the larger proportion of these, probably more than 150,000, being Insects.
The Arihropoda are commonly divided into the Crustojoea^ the Araehnida, the Myriapoda, and the Inseeta ; and though it is impracticable to give a definition which shall absolutely separate the first two groups, it is perhaps not worth while to disturb an arrangement which has much practical con- venience. But, for purely morphological purposes, it may be instructive to regard them from another point of view.
The Arihropoda may, in fact, be divided into two series. One of these consists almost wholly of air-breathing forms, which, if they possess 8x>ecial respiratory organs, have either pulmonary sacs or tracheee, or both combined ; while the other includes a corresponding predominance of water- breathing animals, which, if they possess respiratory organs, have branchiffi. The latter series contains the Crustacea ; the former comprises the Arachnida, Myriapoda, and Inseeta.
In the course of the development of the higher Arihropoda, there is a stage in which the body begins to be segmented, but the appendages are not developed. This is foUowed by a stage in which appendages make their appearance, but the antennary and manducatory appendages (gnathites) are like the other limbs : and, finally, there is a stage in which the gnathites are completely converted into jaws. Now, among the water-breathing Arihropoda, no trace of limbs has yet been certainly discovered among the Trilohita ; in the Merostomata {Eurypterida and Xiphosura) the gnathites are completely pediform; while, in the Entomostraea and Ma^lacoHraea, more or fewer of the gnathites are so modified B8 to subserve manducation and no other function.
• Bronn't ' KlAMen und Ordnungen des Thierreiohs,* vol. ▼. p. S78. 1866.
THX 0&0UP8 OF THE ABTHBOPODA.
267
In the air-breathing series, no completely apodaJ forms are known. The Tardigrctda and the PewUutomida appear to have no jaws ; but the presence of oral stilets in the former, and the position of the hooks which represent the limbs in the latter, throw some doubt upon this point.
In the Arciehnida and the Peripatidea the gnathites are completely pediform. But, in the Myriapoda, and still more in the Iruecta, the gnathites lose the character of legs, and are completely converted into manducatory organs. Thus we arrive at the following arrangement of the Arthro^ poda: —
Abthsopoda.
J. Without Onathiiea.
Tbilobita. Tabdiosada (P) Pentastomida (P)
JJ. With Pediform Onathites,
Mebostomata.
Abachnida.
Pebipatidea.
III. With Maxilliform Gnathites,
Ektomostbaca.
Maulcostbaca.
% ^ '
Water-breathers.
Mtbiapoda. Iksbcta.
Air-breathers.
For the most pait.
Of the four great groups, the Crustacea are those which present the greatest and the most instructive variations upon the fundamental type of structure ; while the modifi- cations of the Insecta^ Arachnida, and Myriapoda, are less extensive, and may be regarded as of secondary morpho- logical importance. The Crustacea will therefore be treated of at some length, while the other groups will be passed over more lightly.
268> THE UTATOMT OF nrVBBTBBBJLTED AHDCALS.
THS OSUSTAOEA.
The* TftiLOBiTA. — These ancHent Arthropods, which have been extmct sibce the latter part of the Palseozoic epoch, occur in the fossil state in great nnmbers, and in conditions yerj favourable for their preservation ; but, up to this time, no certain indications of the existence of appendages, nor even of any hard sternal body-wall, have been dis- covered, though a shield-shaped labmm, which lies in front of the mouth, has been preserved in some specimens. The body consists of a eephoMe shield (Fig.. 57, A) ; of a variable number of movably articulated thoracic «omife« (Fig. 57, B) ; and of a pygidiwm, composed of a variable number of the somites which succeed tne thorax, united together (Fig. 57, 0).
Each thoracic somite presents a median portion, convex from side to side, termed the axis or tergvmi, and two flattened lateral portions, i}iQ pleura. The former overlap one another largely wh^i the body is extended ; the latter, when it is flexed ; and the freedom of' motion permitted by this arrangement is so great, that many Trilobites were able to roll themselves up like woodlice, and are found fossilised in that condition. At the lateral edge of each pleuron, the cuticular substance of which it is composed folds inwards, and can be traced on the ventral or sternal side for some distance. But in the middle of the ventral region no indication of a sternum is discoverable. It may therefore be concluded that the sternal region of the somite was of a soft and perishable nature ; and that the thoracic somite of a Trilobite resembled one of the abdo- minal somites of a crab in this and in some other respects.
The glahellum (Fig. 57, 4), or central raised ridge of the cephalic shield, is a continuation of the thoracic axis, the lobation of its sides perhaps referring to the number of primitive somites it represents. The limb, or lateral area on either side, answers to a thoracic pleuron; its ihickeiMd margin (Fig. 57, i) is produced into two longer
TBM TSIIiOBITA.
259
or shortOT posterior angles (g) ; inferiorlj, the marginal twnd ia reflected inwards tor a short distance, as the tM^fnNUnJ /oM, the remaining eternal area being inooniplete. A median moreable plate answers to the labnun of Apttt Mtd Limubu. On the occipital or lateral margin of the
Fig. sr.
Fig. S7— Diagram of Dalmanita (after Piotet)— A, head ; !, margi- nal band; 2, marglnsl groove, mternal to (he band; 3, ocdpftal •Cfment; 4,'glabe11uni ; 5, great lulure; 6, eves; a, fixed gena; b, ■eparable gcnaj ,o, genal anglej B, Ihoru; T, aiii or tetgiun; P, pleoion ; C, pjgidium ; 9, tei^l ; 10, pl«anl portiooi of the pjrgl-
Umba eature (Fig. 57, 5) commences, and passing between the e^e and the glabellam. meets that of the oppoaite side either in front of the latter, or on the margin of the limb, or on the snbf rontal fold, and is connected with the I&bral
260 THE ANATOMY OV INTBBTBBKATBD AKIHALS.
suture by one or two sutures. The limb is thus divided into two parts — one fixed (the fixed gena, Fig. 57, a), attached to the glabellum ; the other separable (thie moveable gena. Fig. 57, h), on which the eye is placed. The eyes are absent in some genera. In others, they occur as isolated ocelli ; or in groups, their interspaces being occupied by the common integument ; or they may resemble the compoxmd eyes of other Arthropods.
M. Barrande * has succeeded in tracing out the develop- ment of some species of Trilobites. He finds that the smallest, and therefore yoxmgest, forms are discoidal bodies, without any clear evidence of segmentation. The division into somites takes place by degrees, the number increasing up to the adult condition. It is possible that still yoxmger conditions may have escaped f ossilisation, but the analogy of Limulus suggests that these small discoidal forms reaUy represent the condition in which the Trilobite left the egg.
The MBBOSTOMATA.f — ^The only existing representative of this division of the Cmdcuxa is the genus Limuku (the King Grabs or Horse-shoe Grabs), the various species of which are found in America and in the Moluccas. They are usually classed as a distinct order of the Crustacea, termed Xiphosura or PcBcUopoda.
The body of Limulus (Fig. 58) is naturally divided into three parts, which are moveably articulated together. The most anterior is a shield- shaped portion, curiously similar in form to the head of a Trilobite. Its convex dorsal surface is similarly divided into a median and two lateral regions; its edges are thickened, and its posterior and external angles are produced backwards. At the anterior end of the median region two simple eyes are situated, and at its sides are two large compound eyes. The sternal surface presents, anteriorly, a flattened subJrofUal area,
* *Sv8t^e Silorien da centre graph of the Britbh fossil Cms-
deBoheme.'TomeL Trilobites. Uoes belonging to the order
1851 Msrostometa,' 1866.
t H* Woodward, 'A Mono-
THX KIBOBT0IU.TA. ^61
bdund whicli it ta deeply excavated, bo tliat tbe labrom and llie ^ipendageB ore hidden in a deep canty formed bj itc ■ludring waJlB. The middle diviaion of the body of iFtmtiiut exliibita markingB which indicate that it iB composed of, ab teweatiSixcoaleacedBoimtes; its margin* are epiaoae, and its excaT&t«d itemal face lodges the appendages of this region.
Fig. as.
FIb- 58,— a, Limului aolxninta (doraal vi«w). B, L. n.
(Tantnl view), (mfter Milne- Ed wtnli) : a, Mleriat, b, middle diiiiien of tha bod;; c, te1»inj d, gubfroDlal area; e, mntennolMi/, uiten- XB i 9t optrcDlum ; h, bnaohireraui appcndigM.
The temtinai diTieiou is a long, pointed, and latemlly serrated spine, which ia termed the telion.
The month is placed in the centre of the sternal anrface of the anterior division; the anus opens on the same 8nr< face, at the jonction between the middle division and the teleon. A moveable, eacntcheon-shaped, labnun projects backwards in the middle line, immediately behind the snb*
262 THB ANATOMY OV nfYBBTBBBATED ANIMALS.
frontal area (<2) ; and, on each side of it, is a three-jointed appendage, the second joint of which is prolonged in snch a manner as to form with the third, a pincer or chela. The attachment of this appendage is completely in front of the labmm, which separates it from the mouth. . In each of the next five pairs of appendages, the basal joint is enlarged ; and, in the anterior four, its inner edge is beset with numerous moveable spines. The attachment of the basal joint of the foremost of these appendages (the second of the whole series) is in front of the mouth ; but its prolonged, spinose, posterior and internal angle may be made to project a little into the oral cavity. The basal joints of the following three appendages are articulated at the sides of the mouth, and the inner angle of each is pro- vided with a spinose process which projects into the oral cavity. The second, third, fourth, and fifth appendages in the females are chelate; in the males of most species, the second, and sometimes the third, are not chelate. The large basal joint of the sixth appendage is almost devoid of spines, and bears a curved, spatulate process, which is directed backwards between the anterior and middle divisions of the body. The fifth joint of this limb carries four oval lamellae. The appendages which form the seventh pair, very unlike the rest, are short, stout, and single-jointed.
The eighth pair of appendages, again, are of a totally different character fi*om those which precede them. They are united in the middle line into a single broad plate, which forms a sort of cover, or operculum, over the succeeding appendages, when the animal is viewed from the sternal side. On the dorsal face of this plate are seated the two apertures of the reproductive organs.
From the inner face of the anterior, or sternal, wall of each half of the operculum a strong process arises, and passes upwards to be attached to a corresponding process of the tergal wall of the anterior division of the body. By far the greater part of the large levator muscle of the appendage arises from the tergal wall of the anterior divisioii of the body, and the nerve which supplies
THB MBB08TOMATA. 263
the limb is derived directly from the posterior paart of the multiganglionate cord which surroxmds the gaUet and supplies the appendages which lie in front of the operculnm.
The five pairs of apx>endages which remain resemble the opercolnm in their general form, and have ascending processes, which are connected with inward prolongations of the tergal wall of the middle division of the body. Their nerves are derived from the g^anglia which lie in this region of the body.
Thus there are altogether thirteen pairs of appendages^ eight of which are connected with the anterior, and five with the middle division of the body ; and the apx>endages in the region of the mouth are essentially ordinary limbs, the basal joints of some of which are so modified as to subserve manducation.
The determination of the homologies of the parts hither- to spoken of as the anterior and middle divisions of the body, and of their appendages, is a matter of some difficulty ; but, on comparing the disposition of the limbs and their nervous supply with what obtains in the higher Crustacea, it seems hardly doubtful that the first pair of appendages answer to the antennules ; the second, to the antennse ; the third, to the mandibles ; the fourth and fifth, to the maxillsB ; and the sixth, seventh, and eighth, to the mazillipedes of Aataeus or Homarus ; and> in this case, the anterior division is a cephalo-thoraz. If the position of the genital openings marks the hinder boxmdary of the thorax, the middle division of the body represents an abdomen, composed of five somites. But, on the other hand, it may be that the genital organs open in front of the hinder extremity of the thorax, as in female Podophthahnia, and that the five somites which form the middle division correspond with the remaining five somites of the thorax of a PodophthaJ- mian. In this case, the region which corresponds with the abdomen in the higher crustaceans is xmdeveloped.
The alimentary canal of Limuhu is very peculiarly ar- ranged. The gullet passes directly forwards and upwards, and gradually widens into the stomach, the walls of which
264 THE ANATOMY OF INYSBTBBBATBD AKDIALS.
ar6 proTided with mauj longitudinal f olda. The pjloroB is prolonged into a narrow tube which projects into the intes- tine. The two biliary ducts on each side are far apart, and branch out into minute tubules, which form a mass occupying the greater part of the cavity of the body. The rectum, a slender canal with plaited wtdla, and very short, opens into a sort of cloaca situated between the telson and the sternal wall of the abdomen.
The heart, in Limultu polyphemus, is an elongated mus* cular tube, divided into eight chambers, and having as many pairs of lateral valvular apertures. It lies in a large peri- cardial sinus, which, in its abdominal portion, presents on each side five apertures, the terminations of the branchial veins. The branchisB consist of numerous delicate semi- circular lamellse, attached transversely to the posterior faces of the five post-opercular appendages, and superimposed upon one another like the leaves of a book.
The nervous system appears, at first sight, to be very con- centrated, its principal substance being disposed in a ring, embracing the oesophagus ; but, on closer inspection, it is found to consist of an anterior mass, representing the prin- cipal part of the cerebral ganglia in most other Crastaeea, and of two ganglionic cords which proceed from the outer and posterior angles of that mass, and extend as far as the interval between the last and penultimate pairs of appen- dages. These cords are thick, and lie on each side of the Cdsophagpis, around which they converge, so as to come into close union and almost confluence, immediately behind it. In front of this point, however, they are connected by three or four transverse commissures, which curve round the pos- terior wall of the (Bsophagus, and become gradually shorter from before backwards.
The first of these commissures unites the two cords oppo- site the origin of the nerves to the third pair of appendages, which I regard as the homologuee of the mandibles. In front of this point, the cerebral ganglia give off, from their anterior edges, the nerves to the ocelli, eyes, and frontal TQgion; and* from their posterior and xmder surfaces, those
THE MSB08T0MATA. 265
to the antennules. The nerres to the anteniUB arise from the cord close to the oater and posterior angles of the cerebral ganglia, and some distance in front of those to the mandibles. Close behind the latter, arise the large nerves to the fifth and sixth cephalo-thoracic appendages.
The nerves to the rudimentary seventh pair of appendages are slender, and arise rather from the xmder part of the post-casophageal ganglia; those which supply the eighth pair of appendages, constituting the operculum, are also slender, and seem to come off from the two longitudinal commissural cords, which connect the post-cesophageal ganglia with those which are situated in the second division of the body, though they are, in truth, only united in one sheath with them for a short distance, and can be readily traced to the post-oBsophageal ganglia, internal to the nerves of the seventh pair of appendages. The longitu- dinal commissures are very long, and are enclosed in a continuation of the same sheath ; they pass back into the second division of the body, and there present four gan- glionic enlargements, whence the nerves of the post- opercular appendages proceed. The last of these ganglia is much larger than the others, and appears to consist of several confluent masses. The nerves diverge from it in such a manner as to resemble a ccmda equina.
The reproductive organs of both sexes consist of a mass of glandular ceeca, which ramify through the body amidst the hepatic tubules, and eventually open on papill® situated on the posterior face of the operculum. The males are much smaller than the females, and present, in many species, an external sexual distinction in the peculiarity of their second and third appendages already referred to.
The young of Limulus acquires all its chai'acteristio features while still within the eg^. The interesting obser- vations of A. Dohm * have shown that, in an early stage^
* *' Unteraachungen uber Baa tions of Lookwood and Paokard,
und Eatwickelung der Arthropo- ' American Nataraliit,' vol. iv.
den." (<Jenai8die Zeitachrift/ 1871, vol. vii. 1873, and * Memoirs
Bd. vi.) See alao the obaerva- of the Boston Society of Natural
266 THE AjfATomr ov nryxBTiBBATBD ahucals.
the embryo is proyided with the nine anterior pairs of ap- pendages, and is marked out into fourteen somites by trans- yerse grooves upon its sternal face. The body has tiie form of a thick ronnded disk, divided into an anterior shield composed of six somites, and a posterior, likewise shield- shaped region, formed by the union of eight somites. The telson has not made its appearance. In this condition, its resemblance, apart from the limbs, to snch a Trilobite as TriniAcleus is, as Dohm points out, most remarkable.
The Xiphosura were represented in the Carboniferous epoch (BeUinwrus),
The Eurypterida (Fig. 59) are extinct Crustacea of Palseo- zoic (Silurian) age, which sometimes attain a very large size and in many respects resemble LimtUus, while, in others, they present approximations to other Crustacea, especially the Copepoda. An anterior, eye-bearing, shield-shaped divi- sion of the body is succeeded by a number (12 or more) of free somites, and the body is ended by a broad, or narrow and spine-like, telson. IHve pairs, at most, of limbs, pro- vided with toothed basal joints, are attached to the sternal surface of the shield, and the mouth is covered, behind them, by a large oval plate which appears to represent a metastoma (Fig. 59, B g,). Some of the anterior limbs are frequently chelate {Fterygotus) ; the terminal joints of the most posterior pair are generally expanded and paddle-Hke. The integument often presents a peculiar sculpture, simu- lating minute scales. The sternal surface of one or more of the anterior free somites is occupied by a broad plate, with a median lobe, and two laterally-expanded side-lobes (Fig. 59, B h), having a remote resemblance to the oper- culum of Limuhu,
The Entomostbaca. — All the remaining Crustacea have completely specialised jaws ; and as many as six pairs of appendages may be converted into gnathites.
Hisloff7,'187a; with the diteot- Lmmhu by E. van Beneden, tfan or Um wjtUmstin plaoe of < Journal de Zoologies' 1872.
IBM INTOKOBTBACA. 267
In Ute EtUoTMdraea, if the body poarwiep an tbdanuai
(reckoning aa Bnch the eomitea which tie behind the genitftl
apertnre), its Monites are devoid of appendage*. HoreoTer,
Fig. S9.
B, ventnl aapect. ClJi, Oie cephalo-thorwlo ibleld b „ , vjtt, and b, e,d, t,f, the looomotlva limba; T, talton;;, the meta- nomk; k, liieMa'aal pUtaof the uiterioT f^MMmlM*.
the Bomitee, coantiog Uiat which bears the eyes ae the first, are more or fewer than twenty. There are never more than
* 'Der EurypttTHt Ttwtipa, ia» dan abarrilnrlMhaii SoUeUni dir ludOaiaL' 1B».
268 THB AJIATOXY OF UfYESTSBSATED AJIIMALS.
three pairs of gnathites. The embryo almoBt always leaves the egg in the condition of a Navplitis ; that is, an oval body, provided with two or three pairs of appendages, which be« come converted into antennary organs and gnathites in the adult. The division of the Entomostnica comprises the Copepada, the ^istoa, the Branchiapoda, the Ostraeoda, and the Pectostraea,
The CoPEPODA. — In these Ewtomostraca, which come nearest to the Evrypterida, the cephalic shield, which is dis- coidal and not folded longitudinally, is succeeded by a certain number of free thoracic and abdominal somites. The antennules and antennsB are large, and, as in the Eurifpterida, are organs of locomotion and sometimes of prehension. The anterior thoracic members are converted into foot jaws ; the posterior serve as paddles, the limbs of each pair being often united together in the middle line, as in Limulus. The embryo leaves the egg as a Navplius,
The various species of the genus Cyclops, which abound in fresh water, afford excellent illustrations of the struc- ture of the Copepodd,
The minute animal (Fig. 60) is shaped something like a split pear, the larger end corresponding with the head, and the convex side with the dorsal surface. The anterior third of the body is covered by a large carapace, which, at the sides, extends downwards as a free fold over the bases of the appendages, but is hardly at all free posteriorly. Anteriorly, in the middle line, it curves forwards and downwards, and is produced into a short rostrum, on each side of which a considerable excavation lodges the base of the long anten- nule, by the vigorous oar-Uke strokes of which the animal darts through the water. At the anteiior boundary of the head, the double, black, median eye, which, xmless very closely examined, appears single, shines through the cara- pace, and at the sides of the latter, two coiled tubes with dear contents, the so-called shell-glands, are seen.
Pour distinct and moveable somites succeed the carapace, and gndually diminish in diameter. The body then sud<
THX COPKFOD^ 269
denl^ enlarges, and becomes divided, in the fenude, into tonr segments, the last of wMoh girea attachment to two long setose etjlea, vHch possibly represent another somite. There is a well-developed and prominent labrom (or con- joined epistoma and labnun) in front of the month, and behind it is a bilobed metastoma. The first pair of appen> Fig. 60.
Fig. 60.—Cyekpt.—Blia Tlew of ta adult fanile cwninga , OTiaaei, and veotnl ii>w of tbe hosd, (bowing the labnun, niBia- ■toD» ud appendsgra of tbe left dde. V, eja, II', tnteuniile. III', antenna, IV ', mandible, V, flrtt MulUs, Vl', woond muilla (erro- nea(uljmark«l VII'),a,oaUr,A, iauerdlvitioii. I,9^S,4,A,thoi«do limb*. B, roMium ; A, labruia ; aU, mtUitODUu
dages are tbe long and manj-jointed anUnnviei, which ore the chief organs of locomotion. These are encceeded by the short and few-jointed antennn. The third pair of append^es, or first pair of gnathitee, differs from the cor- responding limb in JAimdvt in the redaction of the greater part of the appendage to a ndiment terminated by
270 TH« AJIATOMY OF INTBBTBBRATSD ANIMALS.
•
setfiB, while the strong basal part is the principal gnathite or ma/ndible. The second pair of gnathites are strong and incurved ; following upon these is a third pair of appen- dages, each divided into two portions, an inner and an outer. The latter is by far the larger, and is so constructed, that the three distal articulations can be bent back upon the proximal ones, and opposed to the internal division, constituting a prehensile organ, the "hand" of Jurine.* Thus the gnathites of Cyclops are a pair of mandibles followed by two pairs of maxills. At some distance be- hind the third pair of gnathites the first pair of thoracic apx>endage8 is attached to the hinder part of the cephalo- thorax. Each consists of a two-jointed baeal part {protopo- diie), terminated by two three-jointed divisions {eoBopodUe and endopadUe). Three similar pairs are appended to the three anterior free somites, while a fifth rudimentary pair is connected with the next and smallest of these somites. The suddenly enlarged following segment of the body carries the apertures of the reproductive organs in the female, and supports the ovisacs. It is commonly regarded as the first abdominal somite ; but, according to Glaus, it is composed of two distinct somites, which become united only after the last moult.
The alimentary canal is straight and simple, and without any distinct liver. There is no heart nor any special respiratory organ.
The single ovary, situated in the thorax, is provided with two oviducts, which open on the sides of the coalesced first and second abdominal somites. On the ventral face, between the apertures of the oviducts, is the median aperture of a colleterial gland which secretes the viscid matter which forms the coat of the ovisac. Short lateral ducts connect the gland with the extremities of the oviducts.
• That these are two divisions ppden.' Wunburg Natorwiss.
of the third gnathite, and not two Zeitschrift, 1862.) Under these
separate appendages, has been de- oircomstances 1 do not know why
monstrated by tracing ont their they shoold be termed ** nuudlli-
development. (Glaus, * Organisa- pedes.*' tkm and Verwindfachaft der Cope-
THE OOPBPODA. 271
The male is much smaller than the female, and the two enlarged somites of the abdomen remain distinot. There is a single testis provided with two vasa deferentia. A specially glandular portion of the latter secretes the material of the spermatophores, or cases in which the spermatozoa are enclosed. The antenna are thickened, and provided with a peculiar hinge-joint, by means of which the male firmly seizes the fourth pair of swimming legs of the female during copulation, and then bending up his abdomen, deposits two of the spermatophores on the median opening of the colleterial gland, into which the spermatozoa pass on their way to the oviducts. The gland thus plays the part of a spermatheca. The eggs are carried about in the ovisacs xmtil they are hatched.
The vitellus undergoes complete division, and a morula results, the blastomeres of which soon become differen- tiated into a superficial epiblast, surrounding a deeper- coloured mass, which gives rise to the hypoblast and mesoblast. The whole embryo then becomes divided by two constrictions into three segments, and the hypoblast arises by delamination aroxmd a central cavity, whicl becomes the alimentary canal. There is a large labrum on the ventral side of the first segment in front of the mouth. The eye appears on the tergal aspect of the most anterior segment, as two pigment-spots which soon coalesce into one ; and a pair of jointed setose limbs grows out of each segment. In this Naupliu8'Bta,te the yoxmg Cyclops leaves the egg.
The posterior part of the body elongates and becomes divided into the somites of the thorax and abdomen, from which their respective appendages bud out; and these changes are accompanied by exuviation of the cuticle. The three pairs of appendages of the Nauplitis are converted into the antennules, antennse, and mandibles of the adult.
There are a few other fresh-water and many marine genera of Copepoda. Among the latter, the PonteUidw are remarkable for the separation of that part of the head which bears the antennules and the antennse, from the rest, a peculiarity to which a parallel can be found only among
272 THE ANATOMY OW nr^BBTEBKATBD AKOfALS.
the Stomcdopoda, CoryecBus has two large, more or lees lateral eyes in addition to the median eye, sabchelate antennas, and a rudimentary abdomen. The beautifully iridescent Sctpphirina has an extremely depressed body, short fili- form antennffi, two eyes, and rudimentary gnathites. A short thoracic heart is present in some genera.
The Epizoa. — ^Insensibly connected by such genera as Ergcuilu8 and Caligus with the typical Gopepods, are a great number of very singular Crustacea, which, from their habit of living parasitically upon aquatic animals (whence their vulgar name of "fish-lice"), have received the title of Epizoa. ChondracarUhtu ffibhaafu, commonly found in great abundance on the walls of the branchial chamber of the Fishing-frog {Lophiu8 piscatoriua), may serve very well as an illustration of the most remarkable peculiarities of this aberrant group.
The female (Fig. 61) is not more than half an inch long, but, posteriorly, two long slender cylindrical filaments, (Hke the rest of the animal, of a whitish or yellowish colour,) are attached to its body, which is broad and flattened, and as it were crimped at its edges, so as to present two principal transverse folds. The angles of the folds are elongated into lateral processes (h, t,/), and similar processes {d, e) proceed from the middle line of the body, which by these outgrowths and foldings becomes singularly distorted ; and the grotesqueness of the animal's appearance is not a little enhanced by the bowing motion, accompanied by a flapping backwards and forwards of its gouty limbs, which it exe- cutes when detached from the integument of the Lophiut,
The head is expanded into a sort of hood, the convex anterior margin of which bears the antennules and antenme, the latter being metamorphosed into the strong curved hooks by which the ChondraMifUhtu is securely anchored to the infested animal. A subquadrate labrum overhangs the mouth, but does not enclose the mandibles and form a guotorial apxmratus, as it does in some EpiMoa,
The mandibles and the two pairs of maxillffi resemble
THB IPIEOA.
carved booVe or daws. Two pairs of appendagea (Pig. 61, b e), composed each of a protopodite, terminated by an
Tig.Sl.— Chowlracantkiugitbona.—Ytmtlv: A, lateral view. B, tbd- tim) view, enlu-ged. a, lind ; b, c, tpptndmgea ; d, medlui dsmi pTooen ; i, msdlui ventral proceuM ; /, i, A, lateral proceuee ; t, tenninkl legment, (, male ; g, avIwMa ; ■•, n, medlo-danat ovuian tabes ; p, bteral OTarian lubes ; o, ovidact. 1, 3, anleiinulei ;
endopodite and exopodite and exbibiting haidly any tsuft
274 THE AKATOMY OP INYEBTEBBATBD ANIMALS.
of articulation, are attached to the anterior 'port of the body behind the head.
The body ends in a rounded segment, situated in the deep notch between the hindermost marginal processes, and bearing the two projecting vulvae. Above each of these is a small triangular papillose lobe (Fig. 62, tr), probably a modified appendage, to which, as we shall see, the male attaches himself, while below them are two other rudi- mentary appendages (Fig. 62, y). The alimentary canal is a straight tube running from the mouth to the opposite extremity of the body. No heart is discoverable, and the nervous system and organs of sense (if any) are equally un- distinguishable. The interspace between the alimentary canal and the walls of the body is almost wholly occupied by the ovarium, which consists of four tubes, situated on each side of the intestine, and giving off ramified caeca, in which the ova are developed. Anteriorly, each pair of tubes opens into the oviduct of its side, which passes down along the side of the body to terminate at the vulva. The lower part of the oviduct contains a clear gelatinous substance, and is very similar in aspect to the cement duct of a cirri- pede ; this substance is secreted by the walls of the oviduct, and forms the walls of the ovigerous sac. The latter, as has been stated, has the form of a long cylindrical fila- ment, the upper end of which is firmly held between the prominent lips of the vulva (Fig. 62, «).
The male Chondracanthus does not attain to a twelfth the length of the female, and looks, at first, like a papilla upon her body near the vulva. On close examination, how- ever, he is seen to be firmly fixed by his antennary hooks to one of the two triangular lobes described above. The hooks are doubtless at first attached to the lobe by muscular contraction ; but the connexion once effected seems indis- soluble— at least maceration in caustic soda does not cause the male to become detached. It does not appear that more than one male is attached to a female.
The body of the male (Fig. 62) is pyriform, and exhibits indications of a division into six segments beside the head.
CHOMSKlCAHTHUa QIBBOBUS.
275
The anterior extremity presents a black eje-apot imbedded in its substance, and gives origin to a pair of radimentary antennoles, and to the strong, hooked, prehensile antenUEB. Behind and below them is a large labram and three pairs of hook-like gnathitea. These are sncceeded bj two pairs of enbcjlindrical appendages, which apparently represent ambnlatorj limba.
The caudal extremity ia terminated by two styles, and there are two prominent tubercles on the ventral surface of the penaltimat« somite, in which the genital apertorea are seated. The alimentary canal ia a delicate, irregular tube,
Fig. 63.— C. Male CAonrframntAiu, in tif, m temsle ; v, tri*ngut*r papillose lobrg; q, ■ cje-ipol: (, tt«ti»; «, vu defwena ; b, rudimentUT appeadagei of Ihi feoule ; y, ot
r, tuItib of
inllal aperture; y,
having many brownish granules imbedded in its walls, A wide cBaophagna is connected with its anterior extremity ; but the opposite end appears to be rounded, and to be united with the ventral surface of the integument only by connec- tive tissue. A complex muscular system, composed of atriped fibres, ia visible through the integument, and the eye-spot seems to be connected with a subjacent gan- glionic mass. The body is sufficiently transparent to allow the pulsations of a heart to be seen, but none can be discovered. The testis is a large oval bilobed n
il
276 THE ANATOMY OF IKYEBTEBBATED ANIMALS.
lying like a saddle upon the anteiior part of the intestine. From this body a thick yas deferens runs back upon each side of the intestine, and dilates in the penultimate and antepenultimate somites into a thick walled pyriform sac — a soH of yesicula seminalis. The embryo leaves the egg as a Naupliu8, like that of Cyclops.
There are many genera of these parasites, some of which, such as the almost completely vermiform Lemcece, deviate even more widely than Clwudracanthiis from the ordinary form of CruslcLcea, while others, such as Ergasilus and Noto^ delphys, differ but little from the free Copepoda.
In CaliguSt the labium and metastoma are elongated and united into a tube in which the sharp styliform mandibles are enclosed ; and from the prevalence of this suctorial form of mouth in some of the best known species of parasitic Copepoda, they are frequently termed " suctorial " crus- taceans. Suctorial disks for attachment are developed from the coalesced posterior pairof thoracic members in Achtheres ; and, in this genus, the head, as a distinct part, becomes almost entirely obsolete.
Arguhis, the parasite so common on the Stickleback, is worthy of notice as one of the most curious modifications of the epizoic type.* It is extremely flattened, and is com- posed of an anterior cephalo-thoracic disk, behind which lies a very short and broad, notched, abdomen. A median styliform weapon lies in a sheath in front of the mouth, and the small mandibles and maxillsB are enclosed in a short tube formed by the labrum and the metastoma. Six pairs of appendages lie behind the mouth, the anterior being metamorphosed into suckers, the next pair into strong limbs with a toothed second joint, and the four others constituting biramous swimming feet. There are two pairs of antennary organs, and two compound eyes. Ac- cording to Leydig, the males are provided with cups on their
* Clans (* Ueber die Entwic* of Argultu with the Copepoda,
kelung, Organization nnd syste- bnt proposes to regard it as the
matisdie Stellung der Arguliden,' type of a special group, the Bnm*
1875) has proved the close affinity dUicra.
THB BBANOHIOPODA. 277
penultimate swimming feet ; and, during copulation, these are filled with the senunal fluid, which is thus transferred to the Yulva of the female, and thence to the spermatheca. The eggs are laid, and not carried about in ovisacs. The larva is provided with two pairs of principal 8¥mnming appendages, the future antenuBB and the mandibular palps, the latter eventually entirely disappearing. There is a pair of small antennules, a pair of strong legs in the place of the suckers, and, behind them, the rudiments of the prehensile legs and the first pair of biramous appendages, the others being rudimentary.
Notodelphys, which may be found very commonly in the branchial sac of Asddians, closely resembles an ordinary Copepod, except that it becomes much distorted, and that it carries its ova in a chamber formed by the dorsum of the carapace.
However strangely modified the adult form may be (and it must be remembered that it is always the female which undergoes the greatest amount of change), the larvae of all these epizoic parasites resemble those of the ordinary free Copepoda in possessing only two {Achtheres, Trachelitistea) or three pairs of appendages, (which appertain to the anterior region of the head) ; and they are endowed with considerable powers of locomotion.
The Beanchiopoda. — The genera Nehalia, Apus, Bran- chipus, lAmnetis, Daphnia, and their allies, are usually divided into two orders, the Phyllopoda and the Cladocera ; but these pass into one another so gradually, and have so many structural peculiarities in common, that the subdivi- sion of the group of Branchiopoda appears to me to be a step of doubtful propriety. Closely resembling the lower Pod- ophthalmia^ such as Mysis, in some respects, these Crusta- ceans are invariably distinguished from them by the possession of a greater or less number of somites than twenty; Nehalia, which most nearly approximates the higher Onutacea, having twenty-two somites. Furthermore the thoracic and abdominal appendages of the BraucKvygo^
278 THE ANATOMY OF INYEBTEBBATED ANIMALS.
are, in the majority of cases, more or less foliaceoos, re- sembling, in many respects, the anterior maxillipede of an AstacuSy and being constructed on essentially the same plan.
Aptis glacialis (Fig. 63) presents an elongated vermiform body, terminated by two long, multiarticulate, setose styles, and covered anteriorly by a gi*eat shield-like carapace, deeply excavated behind. The posterior three-fifths of the carapace are free, and merely overlap the segments of the body; the anterior portion, on the contrary, is united with, and forms the tergal surface of the corresponding region of the head ; the free portion of the carapace shelves away laterally from a median ridge, on each side of which a curious concentric marking, indicating the position of the shell gland, (Fig. 63, B, SB,) is visible. This gland is a coiled tube with clear contents, which, according to Clans, opens on the base of the first pair of thoracic appendages, immediately behind the second maxillaa. Where the free joins the fixed portion of the carapace, the ridge is abruptly terminated by a trans- verse depression. A little distance in front of this is another deeper transverse groove, close to which, in the middle line, are the two reniform compound eyes, converging towards one another anteriorly (Fig. 63, B, i).
The ventral siQ-face of the anterior division of the carapace (Fig. 63, C), presents a flattened, semilunar, subfrontal area, as in Limulusy behind which it slopes upwards on all sides into the posterior division, thus forming a wide chamber, in which the anterior thoracico-abdominal seg- ments are lodged. In the middle line, the subfrontal plate sends back a long and wide process, moveably articulated with it, and rounded at its free end — the labrum ; above and behind which the mouth and gnathites are situated. Behind these follow twenty-six spinulose thoracico-abdominal seg- ments; the anterior twenty of which bear the swimming feet, while the twenty-sixth, much larger than the others, is produced into an incurved point posteriorly, and carries the anus and the terminal setae.
The compound eyes, as has been said, are seated upon the npper surface of the anterior division of the carapace. On
THE BSAKCHIOPODA. 279
the under surface, just above and behind tbe posterior boundary of tbe subfrontal area, and on each side of tbe labrum (Fig. 63, 0, lb) yia a delicate jointed filament — tbe antennule (Fig. 63, C, ii'). Bebind tbis, Zaddacb found in some specimens of Apus cancriformia, a second very small filament, tbe rudiment of tbe antenna, wbicb in tbe larva is so large and important an organ; but I bave observed notbing of tbe kind in A, glacialis. On eacb side of tbe labrum is a large, convex, strong, tootbed mandible, and tbe aperture of tbe moutb is bounded posteriorly by a pro- foundly divided plate, tbe metastoma. Succeeding tbis are two pairs of small maxillse, tbe second pair being f oliaceous, and almost rudimentary. Bebind tbese appendages, a cer- vical fold marks off tbe boundary between tbe bead and tbe tborax, and at tbe same time corresponds witb tbe commencement of tbe free portion of tbe carapace. Wbetber tbe carapace is also to a certain extent attacbed to tbe first tboracic somite, as Grube states,* or wbetber it is entirely cepbalic, as Milne-Edwards considers, is a point upon wbicb I bave been able to come to no very clear de- termination ; indeed, it is a question ratber for tbe embryo- logist tban tbe anatomist.
Of tbe twenty pedigerous segments, tbe first eleven bave eacb one pair of appendages; but, bebind tbe eleventb, eacb segment gives attacbment to a gi*adually increasing number of limbs, so tbat tbe twentietb carries five or six pairs. Altogether twenty-eigbt pairs of appendages are attacbed to tbese nine posterior tboracic segments ; tbese, added to tbe eleven preceding, make tbirty-nine appendages in all. Wbile eacb of tbe anterior eleven segments must be regarded as single somites, tbe nature of tbe posterior ones is open to doubt; tbey may be single terga, tbe sterna and ap- pendages of wbicb bave multiplied; or, more probably, tbey eacb represent a number of coalesced terga.
Eacb appendage consists of tbree divisions — an endopo- dite, exopodite, and epipodite, supported on a protopodite or basal division (Fig. 63, D, E, F). Tbe latter consists of tbree
* * Bemerkangen uber die Phyllopoden,' p. 81 «
THI AHATOMT OT tNVBBTSB&l.TEI> AHIKALB. Fig. 61.
THB BIKANOHIOPOD^. 281
Fig. 6^—Apu$ glacialu,—Ky lateral view, with the right half of the carapace cut away. B. Dorsal view. C. Anterior part of the body, ventral aspect. D, one of the anterior, £. one of the (middle, and F, one of the pa«terior limbs, without their ooxopodites. x, con- voluted " shell gland " in the carapace ; y, caudal filament ; &, labrum. 1, 2, 3, 4, Endopodite. 6. Exopodite. 7. Epipodite or branchia. I', eye ; IT, antennule ; IV', labrum ; V, YI', maxillfe.
joints — a coxopodite produced intemallj into a strongly setose prominence (not represented in the figures), a basi- podite, and an ischiopodite, the latter elongated internally into a lanceolate process, and bearing on its outer side two appendages, of which the proximal — the epipodite or branchia (Fig. 63, D, E, 7) — is pyriform and yesioular in specimens preserved in spirit. The distal appendage, which appears to represent the exopodite (6), is a large flat plate, provided with long setsB on its margin.
The endopodite consists of four joints, the two proximal ones berng much the longer, and, like the penultimate, giving off internally a long process. Finally, the terminal joint is claw-like and serrated on its concave edge.
The avei'age form of these appendages is represented by (E) taken from the middle of the series; anteriorly the limbs become more slender and leg-like (D); posteriorly, on the other hand, they ai'e completely foliaceous, as (F) ; but the same elements are recog^nisable throughout.
The eleventh pair of appendages alone depart, in any important respect, from the rest of the series, each of these being modified so as to serve as a receptacle for the ova. To this end the joints of the endopodite are greatly expanded, and converted into a hemispherical bowl; the exopodite, metamorphosed into another such bowl, shuts down over the endopodite; and, into the box thus formed, the ova are conducted by means of the oviduct, which opens into it.
On the dorsal surface of each side of the terminal segment of the body there is a tubercle produced into five spines anteriorly, and carrjdng, posteriorly, a long and delicate setigerous filament (Fig. 63, B, g).
The alimentary canal of Apva is very simple, couKifl^xw^
282 THE ANATOMY OF INVSBTBBRATED ANIMALS.
of a vertically ascending (Bsophagus, which bends back into the small stomach, situated immediately behind the com- pound eyes, in the middle of the region bounded by the two transverse furrows on the dorsum of the carapace ; from the hinder end of the stomach the straight intestine passes back to the anus, which is seated beneath the terminal segment. The liver consists of caeca, which branch off from the stomach and lie, on each side of it, in the head. Zaddach describes a pair of glands which he regards as salivary, placed above, and opening into, the stomach itself, like the salivary glands of the Scorpion.
The heart occupies the tergal region of the eleven anterior thoracic somites, presenting as many chambers, with lateral venous apertures.
The nervous system consists of a quadrate cerebral mass, placed immediately under the compound eyes, and giving off large nerves to them and to the remains of the single eye of the larva, which lies in front of their anterior extremities. Commissures pass downwards and backwards on either side of the oesophagus, and connect the cerebrum with a chain of numerous ganglia placed on the median line of the ventral surface. It is worthy of remark, that the antennary and antennulary nerves are given off from the commissures, far behind the chief cerebral mass.
In the female, the ova are developed in the csecal branches of two long tubes, situated one on each side of the body, and opening, as above described, in the eleventh pair of appendages. Apus usually propagates agamogenetically, and the examination of thousands of individuals, extending over more than thirty years, failed to reveal to Von Siebold the existence of a male fonn. In 1856, however, Kozubow- ski * discovered a small proportion of males (16 in 160), among the specimens taken in the neighbourhood of Cracow ; and near Rouen, in 1863, Sir John Lubbock found the largest proportion of males to females yet known, viz., 33 in 72. On the other hand, between 1857 and 1869, Von
• " Ueber den mannlichen Apus cancriformis.'* (* ArchlF flir Natur- geschichte,' 1857.)
THE BBANCHIOPODA.
283
Siebold examined many thousands of specimens of the Bavarian Apus without finding a single male.*
The testis is similar to the ovary in form, and its duct opens upon the eleventh pair of appendages, as in the case of that of the female organs. The spermatozoa are oval aud without motion.
The young Apus {cancriformis), when just hatched, is a Nauplius, The body is oval, indistinctly divided into a few segments, and entirely destitute of appendages, except a shorter anterior, uniramous, and a longer posterior, biram- ous, pair of oar-like organs, situated at the anterior extremity, on either side of the single median eye. The carapace is rudimentary, and there are no caudal filaments. The little animal soon casts its skin, and the mandibles, which are provided with long palps, make their appearance.f With successive ecdyses, the larva assumes more and more the form of the adult, and acquires the pair of compound eyes ; the anterior pair of appendages being converted into the antennules, the posterior pair disappeai-ing, or remaining as rudimentary antennae, and the mandibular palps also vanishing.
Singular and highly instructive modifications are exhibited by the other genera of the BranchiopocUiy such as Nehcdia, Branchipus (Cheirocephalus), lAmnetis, and Daphnia.
In Daphnia and its allies (Fig. 64), the thoracic members are reduced to six, five, or even four pairs, some or all of which may take the form of ordinary limbs ; the abdomen is rudimentary ; the heart is short ; and the carapace presents a posterior division (omo8tegite)y obviously developed from the anterior thoracic somites, the lateral halves of which are deflexed so as to resemble a bivalve shell, into which the hinder part of the body can be withdrawn. The anterior division of the carapace (cephalogtegite) in Daphnia has, on the contrary, the same structure as the corresponding part
* Beitrage zur Partheno- t According to Claus*8 recent
?;ene8i8 der Arthropoden,* 1871. investigations, this third pair of
t appears that, in Apus, the im- appendages is present from the
pregnated ova alone give rise to time the young Apus leaves the
males. egg.
284 THE ATU.TOMT Or IIITEBTXBK1.TSD AHIHAU.
of the canLpac« of Aptu, but tlie compound ejw, repn- sented by a smgle maBB.are eitnated at tbe anterior extremis of tbe bead, ratber tban on its upper sorface, and tbe BUigle eye is quite diBtinct, and far posterior to tbem (Fig. 64, B, t, u"). Tbe antennnles (Fig. 64, A. ii*) are small, radi- mentary, and placed at the sides of tbe produced frontal
Fig. 64.
CI, cephaloitegitc, or that part of the carap»ce which a heM; Ml, oDiDsteeite, or thoracic portioa of the can, , . llMrt; •(. eeniCBl depretiion; III, labriUD ; 1', compound eye; U', almple eye; t, we ' Bhell-glanil,' wbloh opens Dchipd tbe
rostrum, but tbe antennffl are very large, and congtitute tbe principal locomotive organs. The posterior, or second, maiillEe are obsolete. In Evadne, Polyphemvs, Sida and other genera, sucker-tike organs of adhesion are situated OD the anterior region of the carapace. Tbe eggs are de- veloped in the cavity of tbe carapace, and tbe embijoe pass
THE BSANCHIOPODA. 285
directly into the form of the parent, except in Leptodorut where they are, at first, Natiplius-Mke,
Idmneiia and Estheria present a DaphniaAike carapace, though more completely bivalire, combined with the nmne- roxLB segments of the body and the f oliaceons appendages of the typical Phyllopods (Fig. 65).
Nehalia has a large carapace, provided with a moveable rostrum, like that of Squilla, and arising entirely from the head, which is remarkable for its very slight sternal flexure. In this genus, the eyes are large and peduncu- lated ; there are well-developed antennules, antennse, man- dibles, and two pairs of maxillse, the anterior of which ends in a long palp.
Branehipus, finally, developes no carapace either from the head or the thorax, the segments of the latter being entirely free, while the former is similar in shape to that of an Insect, or Edriophthalmous Crustacean, and carries two large stalked eyes, two antennules (singularly modified in the male), two antennae, a pair of mandibles, and two paira of maxillse.
In Eatheria and Limnetis, the males are met with in full proportion to, and may be even more numerous than the females. No males are known in Limnadia gigas, although thousands have been examined, while, in L, Stanleya/na, more males than females have been found. In Branchipus, males are fewer than females; in Artemia, they occur only at rare intervals. In Daphnia, the males are few, and appear only at certain seasons of the year. But notwith- standing the rarity or absence of the males in many of these genera, reproduction proceeds with great rapidity. The ova are capable of development without fecundation ; and isolated females of the genus Daphnia will thus go on producing broods for generation after generation, with- out any known limit.
Under certain circumstances, however, bodies of a different nature from these " agamic ova," as they have been well
* "Ueber die Gattongen Estheria and Limnadia:* ('Arcbiv fur NAtargeichichte,' 1854.)
286 THE ANATOHT Or IHTIBTEBEATKD AHIIUX8.
termed by Sir Jolm Lubbock,* are dereloped within tl QYKrj, the Bubstance of which acqairee an accninvilation < strongly refracting gnuinlea at one spot, and forms a dai maas, the BO-called " ephippial ovum." When fnlly fonne two of these bodies pass into the dorsal chamber of the oar: pace, the walls of which have, in the meantime, becoa
Fit. ei.~LimiKlit limdymM <ftfI«T Grubc).— The appet lefUku figure ia tlie male, tbe other the femsle ; one tbIvb of the carapai in each caaebda| removed. A'. Antennules. A'. AntaoDte. J Toiing larva. B. The umB further adranccd. c. Head. o. E; d. Carapace, if. Uody. A'. AntennK, M. Maodibles. a*. Gtei plate (labnun ?) which coven the minitb.
altered. The outer and inner layera of the int^omei acquire a peculiar stmctnre, a brown colonr, and a moi firm consistency, over a large saddle-like area. When tl
epblpplni BoyalSo
THE OSTBACODA. 287
next moult takes place, these altered portions of the integu- ment, constituting the " ephippium," are cast off, together with the rest of the carapace, which soon disappears, and then the ephippium is left, as a sort of double- walled spring box (the spring being formed by the original dorsal junction of the two halves of the carapace), in which the ephippial ova are enclosed. The ephippium sinks to the bottom, and, sooner or later, its contents give rise to young DaphnicB.
Jurine*s and Sir J. Lubbock's researches have proved that the development of the ephippial ova may commence without the influence of the male, and they seem to indicate that these ova may even be fully formed and laid without the male influence. On the other hand, there appears, under ordinary circumstances, to be a certain relation between the complete development of ephippial ova and the presence of males ; and, as yet, no ephippial ova produced by virgin females have been directly observed to produce young. The question, therefore, seems to stand thus, at present : the agamic ova may certainly he produced, and give rise to embryos, without impregnation; the ephippial ova may certainly be produced without impregnation; but whether impregnation is or is not absolutely necessary for their further development, there is, at present, no evidence to show.
The great majority of the Branchiopoda inhabit fresh waters. Artemia, however, delights in brine pools. The genus Estheria is of Devonian age, and it seems probable that the Silurian Hymenocaris and its allies were related to Apv^.
The OsTBACODA. — This group contains several genera of both recent and fossil Crustacea, for the most part of very small size, and distinguished by their hard, often calcified, and completely bivalve shell, provided with a distinct hinge. The valves of this shell consist of the lateral moieties of the carapace; they are commonly unequal and unsymmetrical, and present a peculiar orna- mentation. The shell gland is very small. The Ofl^ooodA
288 THE ANATOMY OF INYEBTEBSATED ANIMALS.
are also remarkable for the extremely mdimental condition of their abdomen, and for the paucity of their thoracic appendages, which instead of being f oliaceons, are strong and snbcjlindrical, like the ambulatory legs of the higher Crtutacea.
The cephalic flexure is as well marked as in the highest Crustacea, so that the eye, obscurely diyided, and median in Cypris (Fig. 66, A), but double and lateral in Cyihere (B), is situated in the upper part of the anterior region of the body. The antennules and antennse, attached to their respective somites, the sterna of which constitute the an- terior boundary of the body, arc similar in form and func- tion to ambulatory limbs. The ducts of a peculiar gland open, according to Zenker, at the end of the strong spine with which the antenna of Cythere is provided. Thelabmm is conspicuous, and the mandibles are strong, and possess a well-deyelox>ed palp. The first maxilla is provided with a large foliaceous setose appendage (epipoditeP) The second maxilla in Cythere, is represented by the first of the three pairs of ambulatory limbs (Fig. 66, B, e, e, e), present in this genus. In Cypris, which possesses a second pair of maxillsB, there are only two pairs of ambulatory limbs (Fig. 66, A, p, I. II.). The apertures of the reproductive organs, provided in the male with a wonderfully complex, homy, copulatory apparatus (described with great minuteness by Zenker), are situated between the last pair of thoracic members and the large caudal hooks.
Strong adductor muscular bundles pass from one yalve of the carapace to the other, and leave impressions discernible from without, the form and arrangement of which furnish valuable systematic characters.
The alimentary canal of the Ostracoda is provided anteriorly with an apparatus of hard parts, resembling in many respects the gastric armature of the Isopodti, and gives origin to two hepatic cseca. Cypris and Cythere have no heart; but, in Cypridina, Conchoecia and Halaerypti9 there is, according to Glaus, a short saccular heart with one anterior and two lateral apertures. The nervous
THE OSTBACODA.
289
system is difficnlt to make ont; but, in Cythere lutea, the same observer found a large cerebral ganglion in front of the mouth, whence filaments passed to an ophthalmic ganglionic mass, and to the antennaxy organs. A double ganglion, behind the mouth, supplies the gnathites ; three ganglia, situated in the thorax, send filaments to its append- ages, and a terminal ganglion supplies the caudal appendage and genitalia. In the female, the ovaries lie in the valves of the carapace, and terminate in oviducts which open by distinct apertures in front of the caudal appendage. Immediately anterior to them are the openings of two
«
Fig. 66.
Fig. 66.— A. Cypris.—A. i. ii. Antennules and Anteimee. M. i. ii.
ni. Mandibles and Maxillas. P. i. n. Thoracic members, c.
Caudal extremity, b. Mandibular palp. o. Eye. B. Maxillary
appendage. B. Cythere. — o. Eye. a, Antennule. b. Antenna, c. Mandible, d.
First maxilla, e. e. e. Second maxilla and two thoracic members.
/*. Caudal extremity. (After Zenker.)*
homy canals, called vaginsB by Zenker, each of which is continued into a long convoluted transparent tube, and eventually terminates in a large vesicle, the spermatheca, into which the spermatozoa of the male are received.
In the males, the antennae, the second maxillse, or some of the thoracic limbs, are modified in such a manner as to enable them to seize and hold the females. The testes are elon- gated csBca in Cypris, globular vesicles in Cythere, and communicate with a long vas deferens, which opens into
• M Monographie der Ostracoden." (* Archiv fur Naturgesehichte, 1854.J
290 THE AKATOMT OV IHTEBTBBBATED ANIICALS.
the copnlatoiy i^paratos. In Cypris, a very singalar cylin- drical mncotis gland is connected with the Yas deferens ; but perhaps the most remarkable peculiarity abont the genital apparatus in the male consists in the size of the spenna- toeoa, which in Chfpris ovum are, according to Zenker, more than three times as long as the body. Thej possess a spirally-wound coat, and are totally deprired of mobility.
The Odracoda either attach their eggs to aquatic plants, or carry them about between the yalves of the carax>ace.
Glaus * has worked out the development of Cyprxs, which passes through nine successive stages, dlsting^aiBhed from one another, not merely by the shape of the carapace, but by the number and form of the limbs. An ecdysis of the chitinous cuticle of the body and carapace terminates each stage of development. When the CjfprU leaves the egg, it resembles a ^oicpZttM, in possessing a single median eye and only three pairs of limbs (the future antennules, antenn®, and mandibles); but none of these are divided into two branches. The body is laterally compressed and has a bivalve carapace.
The changes undergone by the marine (htracoda after they leave the egg are much less marked.
Fossil Ottracoda abound in strata of all ages, from the older paJsBozoic formations onwards; and, so far as the characters of the carapace furnish evidence, the most ancient forms differed very little from those which now exist.
The PSCTOSTBACA (Bhizoeephala and Cirripedia) leave the egg as a Nauplitu, provided with three pairs of limb-like appendages, of which the anterior pair are simple, while the two posterior pairs are bifurcated (Fig. 68, A). An additional pair of filiform appendages subsequently makes
* * Eiitwiok«lii]ig8geeehicht« von Cjpris' (IS68); sad 'Qrond- svlge,'p.487.
THB PBOTOSTSACA. 291
its appearance in front of the nndiyided pair of members, in most caaes ; and there is a disooidal carapace, the antero- lateral angles of which usually become greatly produced. Subsequently, the carax>ace becomes biTalve (as in many PhyUopoda, and in the Cladoeera and (htreieoda), and the anterior undivided pair of limbs are converted into re- latively large, jointed appendages, provided with a sucker- like organ. The thorax grows and usually developes six pairs of appendages.
Finally, the bivalve-shelled larva fixing itself by the suckers of its anterior limbs, the prsB-oral region of the head becomes enlarged, and is converted into the base, or peduncle, in ordinary Cirripedes; while it gives off the root-like processes which grow into the tissues of the animals on which the Bhi%oceplvala are parasitic. The Peefof^rooa are almost all hermaphrodite, a condition which is very exceptional among Arthropods. They possess no heart.
The CiBSiPEDiA. — It can hardly be a matter of reproach to the older naturalists if they failed to discover the aflSnity connecting the sedentary " Acom-ehells " of a rocky coast with the active Shore-crab which runs amongst them; or if they classed the Barnacles with MciOusea, instead of ad- mitting them to that place amidst the Crustacea which is now assigned to them by every naturalist of competent judgment. Nothing, in fact, at first sight, is less suggestive of a Orustacean than a Balanus, or a Lepas; the former firmly fixed by the base of its multivalve conical shell, the latter by its fleshy and contractile peduncle ; the only sign of life in either being the alternate protrusion and retraction from the valvular opening of the animal's case of a bundle of curved filamentous cirri, which sweep with a brushing motion through the water, and scoop the floating nutritive matters towards the mouth.
The valves through which the cirri make their egress are strengthened, in both BalawM and Lepa$, by four cal- cified pieces, two on each side ; those of each. ba2kl \)fisai%
292 THE ANATOMY OF INYEBTEBBATSD AKIKAIiflL
united together by an oblique suture, or by a regular articu- lation ; while the two pieces of opposite sides are connected only along one margin, either immediately (Balanus)^ or by means of an intermediate piece {Lepas).
The upper, or distal, pieces are termed the tergcL, the lower, or proximal, pieces the scuta, the intermediate piece is the carina. In Lepas, there are no other hard external pieces ; but, in Balanus, the conical shell, into which the valves can be more or less completely retracted, is com- posed of six portions or compartments. Of these, one is situated on the same side as the opening between the valves and another at the precisely opposite point, or on the same side as the line of union of the valves. The latter is the homologue of the intermediate piece, or carina, in Xepof ; the former, in Balanus, consists of three pieces united together, the median rostrum and the two rostro-lateral com- partments. On each side of the carina is a compartment termed carino-lateral, and between them and the complex rostrum lies a lateral compartment.
If the shell consisted of its eight typical pieces (as it does in the genus Octameris), it would be found that each presented a triangular free middle portion and two lateral wings. The former is always termed the paries, but the latter receive different names, according as they overlap or are overlapped by others. In the former case, they are termed radii, in the latter, alee. Thus, typically, the carina! and the rostral compartments are overlapped on both sides, and their wings are consequently both alse ; the lateral and carino-lateral compartments are overlapped on one side, and overlap on the other, hence they have an ala on one side, a radius on the other ; while the rostro-lateral com- partment overlaps on both sides, and hence its wings are both radii. In Balanus, however, the rostrum and rostro- lateral compartments, being replaced by a single compart- ment formed by their confluence, this piece has radii on both sides.
Different as is the appearance of Lepas from that of Balanus, they closely resemble one another in essential
THX OIBBIPBDIA* 293
structure. Thus, to commence with Lepas, On cutting away the scutum and tergum of one side (Fig. 67, B), the hinder part of the body of the animal is seen within the sac of the eapUndum, formed by the valves of the shell, to which it is attached only on the rostral side and inf eriorly by a comparatively narrow isthmus. Immediately behind iJiis point the body widens, to constitute what Mr. Darwin* has termed the prosoma, but the thoracic segments, which succeed the prosoma, gradually taper posteriorly. Six pairs of appendages (a) are attached to the thorax, each limb consisting of a basal joint (protopodite), terminated by two long multi-articulate cirri, the representatives of the endopodite and exopodite ; and a rudimentary abdominal segment, terminated by two short caudal appendages, succeeds the thorax, and is produced in a long setose an- nulated penis (/). Filamentous appendages depend from some of the thoracic somites, and, projecting from the inner wall of the sac on each side, is a triangular process, the ovigerous frcBniMn (m).
The mouth is situated at the posterior part of a pro* tuberant mass, seated on the rostral face of the prosoma. This is principally composed of a large, bullate labrum, behind which are a pair of mandibles with large and setose palps, and two pairs of maxillse. Anteriorly, the prosoma passes by a narrow isthmus into the rostral part of the peduncle, into which it, as it were, expands; while the posterior margins of the peduncle become continuous with the walls of the sac.
The extremity of the peduncle is fixed by a peculiar ce- menting substance to the body to which the Lepas adheres ; but, if it be carefully detached, there will be found connected with the rostral portion of the surface a pair of very minute, singular-looking, organs, consisting of two proxi- mal joints, succeeded by an articulation which is dilated into a sucker, and terminated by an elongated setose joint (Fig. 67, A, B, I). These are the remains of the anterior appendages of the larva.
• * Monograph of the Clrripedia,' 185l« 1854.
294 THB ANATOKT Ot IBTERTBBBATBD AITHCALB.
Froro what has been said, it foUows that the fixed end i the peduncle is, in fact, the anterior eitremity of the boe ef the Lepat, and that a B&macle ma; be said to be Crustacean fixed bj its head, and kicking the food into i month with ite l^a.
Fig. 07.
.{ LqMU.—a
Fig. 67.— A. Diigmnmatic section of Bate , plKsd In the cavity of the nc. and lieiavei um iBuium; a, C carint; c, I, eariDO-lmtenl compartnieDt ; I, lateral menl ; r, nntruiD ; i, ■culiun ; (, tergum ; /, pmii, ,. . fnrmed glind ; h, duct cODneelinff (Ms willi i, i, cement duU gt*nda ; f, anleiiDK; i, peduneulmr or anriu tabulei garoul fnenum ; if, aniu.
The mouth in Lepat looks towards the posterior eztremit of the bodj, and leads into a tubular awopha^s, whic passes forwards, and opens bj a wide superior eitremit into the globular stomach. From this point, the alimentai
TBI CIBItlPBBIA.. 295
canal bends back upon itself, and gradually narrows into the intestine, which terminates in the anus, situated at the extremity of the abdomen, on the tergal side of the penis. Two considerable branched coca, probably hepatic, proceed as diverticula from the stomach, corresponding very closely in position with those of Dapknia. No heart or other circulatory organs are known to exist ; and it may be doubted if the ovigerous frssna of Lepas exert, as they have been supposed to do, a branchial function.
The nervous system consists of a pair of cerebral ganglia situated in front of the oesophagus, and connected by long commissures with the anterior of five pairs of thoracic ganglia, whence nerves are given off to the limbs. In the middle line, the cerebral ganglion gives off two slender nerves, which run parallel with one another in front of the stomach and enlarge into two ganglia, whence they are continued to a double mass of pigment, representing the eyes. From the outer angles of the cerebral ganglion arise the large nerves which proceed into the peduncle and supply the sac. These appear to correspond with the antennary and frontal nerves of other Crustacea ; and Mr. Darwin describes an extensive system of splanchnic nerves.
Lepas, like the minority of the Cirripedia, is hermaphrodite. The vesiculsB seminales are readily seen in fresh specimens, as white cords distended with spermatozoa, which run from the canal of the penis, into which they open, forwards, on each side of the body, to the prosoma, where they end in dilated extremities, which are connected with a multitude of ramified cseca forming the proper testis.
The ovaries are ramified tubes provided with cseeal dilatations, and lodged in the peduncle. The oviducts pass into the body, and, according to Krohn, terminate in aper- tures situated on the basal joint of the first pair of cirri.* Two ' gut-f oimed ' glands, as they are termed by Darwin, lie, one on each side of the stomach, and are probably
* The position of these aper- to the thell-glands in Umnadia tnict eoiresponds with that of the snd Aput, openiiigty fuppoMd to appertain
296 THE AKATOMT OV DnTBBTBBBATBD AHIMALS.
accessory glands of the reprodnctiye organs, analogous to those which secrete the walls of the ovisac in the Ccpepoda.
The mode of exit of the ova from the ovary is not oeitainl j known, nor is the place of their impregnation ascertained; bat they are eventually f oilnd cemented together by chitin into large lamellse, which adhere to the ovigerous frsBna, and, ordinarily, at once strike the eye when the capitolnm of a Girripede is opened.
Yelk division is complete, and the embryo attains to its earliest larval condition within the egg. li a series of the fresh ovigerous lamelbe be taken and pulled to pieces with
Fig. 68.
Fiff. 68,— A. Larva of Balantu bafanoidet on leaving the esg (after Spence-Bate). B. Attached pupa of Lepas amtrali* (after Darwin) : », antennarv apodemes; f, gut-formed gland, with cement duct running to the antenna.
needles in a watch-glass full of sea-water, one is pretty sure to be found whence a number of active little NauplwM' like animalcules are set free (Fig. 68, A). Each presents a somewhat triangular body, produced in the middle line posteriorly and at its anterior lateral angles. The mouth is situated on a proboscidiform projection placed nearly in the centre of the body, and in the midst of three pairs of natatory limbs, of which the two posterior pairs have bifid extremities. In front of the mouth, either in this stage, or after one or two moultings, two filaments are often deve* loped. A single eye-spot is situated in front of the bases
THB DBTBLOPMBKT OF THB OIBBIPBDLA.* 297
of the anterior appendages. After moulting several times^ tlie larva assumes a new form, passing into its second stage. The carapace is now oval and compressed, so as more nearly to resemble that of a Daphnia or Cypris. There are two eyes. The first pair of swimming appendages of the Nauplitu are converted into antennif orm organs, each provided with a sucker, and the rudiments of the six pairs of cirri make their appearance behind the mouth.*
In the third stage, the larva is, as Mr. Darwin states, "much compressed, nearly of the shape of a Cypria or mussel-shell, with the anterior end the thickest, the sternal surface nearly or quite straight, and the dorsal arched. Almost the whole of what is externally visible consists of the carapace; for the thorax and limbs are hidden and enclosed by its backward prolongation; and, even at the anterior end of the animal, the narrow sternal surface can be drawn up, so as to be likewise enclosed." The larva, in this stage, is provided with two large compound lateral eyes, while the median eye is arrested in its development. The oral tubercle exhibits all the gnathites of a Oirripede, but they are covered by an imperforate integument, so that this "locomotive pupa," as Mr. Darwin terms it, is unable to feed. There are six pairs of legs, and the thorax ends in an abdomen, consisting of three somites terminated by two caudal appendages. There is no penis. The most remarkable structures in the pupa, however, are the ** gut- formed glands," which are already well developed, and from which the cement ducts can be traced to the disks of the antenniform organs, on the faces of which they open. The pupa, after swimming about for awhile, at length selects its permanent resting-place, to which it adheres, at first, only by the action of the suctorial disks. The temporary attachment, however, is speedily converted into a persistent one, the cement pouring out from its excre-
* According to Clans (* Grand- case the antennaiy organs repre-
ziigeder Zoologie,' 3te Auflage, p. sent antennules, and Uie limbs of
460), the second pair of appen- the Cirripede Nauplhu corre-
dtkfpea disappears, and the third spond with those of the Gopepod
gives rise to the mandibles* In this and Branchiopod Aowpfiai.
298 THE ANATOMY OV Iir7BBTSBRATSD AITIMALS.
toiy apertures on the disks, and firmly gluing them and the anterior end of the body down to the surface on which they rest.
Goincidently with these changes, several other important alterations take place, during the passage of the locomotive pupa into the fixed young Cirripede. The compound eyes are moulted, and with them the antennary apodemes, fur- nished by the integument of the deep fold which separates that part of the body of the pupa which corresponds with the beak of a Daphma^ or of a LimnetU, from the prosoma. The fold is thus enabled to straighten itself; and, as a oonse- quence, the carapace of the Cirripede, instead of remaining more or less parallel with the surface of attachment, be- comes perpendicular to it. Again, in the pupa, the axis of the carapace and that of the body are identical in direction > but, during the last moult, the chamber of the carapace extends forwards far more on the tergal than on the sternal side, separating the tergal part of the prosoma from the ** beak,'* with which it was at first continuous, and thus allowing the body of the Cirripede to take its final position, which is nearly transverse to the axis of the carapace.
The terga and scuta now appear as homy thickenings, and, afterwards, as calcifications in the wall of the capi- tulum. The frssna and the penis make their appearance, and the genitalia become developed in the prosoma and in the peduncle, which is produced by the gradual elongation of the " beak " of the pupa.
With the assumption of its perfect form, the Cirripede ceases to moult its carapace, ecdy sis being hereafter confined to the inner lining of the sac, and to the integument of the contained body.
Such is the structure and development of- a typical pe- dunculate Cirripede. In other genera, such as PoUieipe$, calcareous plates are developed on the peduncle, f oreahadow- ing the compartments of the sessile forms. The latter, of which Bakmus may be regarded as the type, differ in struc- ture from Lepas in no very essential particular. The peduncle, yery short and broad, instead of slender and elon-
THB CIBBIFBDIA. 299
gated, is encased by its compartments, and is sometimes fixed by a shelly basis. The arrangement of the layers of cement is often extremely complicated ; the scnta and terga are articnlated together ; the frsna are mnch larger organs, and possibly subserve the respiratory function; the thoracic ganglia are concentrated into a single mass ; and the cementing apparatus is much more complicated.
The pedunculate and sessile Cirri^ecUa, taken together, constitute by far the largest of the three great gp*oups which Mr. Darwin recognises ; namely, the Thoraeiea, cha- racterised by having limbs attached to the thoracic somites, while the abdomen is rudimentary.
The second group, the Ahdominalia, contains only one genus, Cryptophialus, (Fig. 69, 5, 6) which has no thoracic limbs, but is provided with three pairs of abdominal ap- pendages. The larva is very imperfect in its first and second changes, which are undergone within the sac of the parent.
The third group, Apoda, likewise contains only one genus, the remarkable Proteolepas (Fig. 69, 7), which is devoid of either thoracic or abdominal limbs; it has a vermiform body, and a rudimentary peduncle, represented by two threads terminated by the characteristic antenniform organs.
In the great majority of the Cirripedia the sexual appa- ratus is disposed as in Lepas, but Cryptophialus and Alcippe are unisexual, the male differing veiy widely in form and size from the female (Fig. 69, 3,6).
The BdUmidcB, or sessile Cirripedes, all present the normal serual relations ; but the other division of the Thoraeica, the LqMididcB, contains two genera, Ibla and Scalpellum, which not only possess species having the sexes in distinct indi- yiduaJs, but others presenting the unique combination of males with hermaphrodites. Thus, ScalpeUwm vulgare is hermaphrodite, possessing well-developed male and female organs. Nevertheless, on the inner side of the occludent margin of its scutum there is a fold, over which and im- bedded in the spinose chitinous border of the scutum, a
300 THK AHATOXT OF nnnEBTBBRi.TXD ABUtAIA.
minato, otbI, aac-Uke creature is commonly fonnd, firmly
Fig. 65.
Fig. 69.— 1. Alcif^e lampai ; female. 3. The H H. Horny dl»k of BtMchment; in 1, the male iprcki on either ride of the upper part of the s )b1t of cirri, i, I. n. Three leginentg uf the thorax « the atber three legmenM, bearing the three pain of terminal ol e very short. 3. Male Alclppr. a. Anlennaty appendage*.
«IT. A. 1
withoDt d
Ve.icii Penli.
.. Eje. . oF Akippi in a portioi
. i. Orifice of the aao. of a "
. aboTB IdIo the rim of the apeit a, b, I. n. Abdominal cirri, i. Appendsgea of anknown natn 6. Male CrypltfAiahu. 7. J'rotfolrpat birfiirta. m. Mouth, g. Peduncie and antenna, i. i. Veelcula uminillt and penii. (Af Darwin.)
tached by cement which covera the characteriatic anteimiL
THX OntBIPBDIA.. 901
of a Oirripede. Witliin the sac is a thorax, with f onr pairs of rudimentary appendages terminated by a short abdomen. There is neither mouth, alimentary canal, nor gnathites, the cavity of the body being pidncipally occupied by a great seminal yesicle ; and no trace of female organs exists. This is, therefore, an accessory, or " complemental " male. In Scalpellum omatwn, the individuals are males and females, two of the former being lodged in cavities of the scuta of one of the latter, as in the preceding species, and in 8. rwtilwn. The males have no mouth. 8. rostratum has complemental males, provided with alimentary organs at- tached to the interior of the sac of the hermaphrodite, while 8. Peronii and vUlosum have still more perfect complemental males fixed in a like position. In Ibla Cwningii, the female has a vermiform male, provided with well-developed ali- mentary organs, attached within her sac ; but, in the only other species of this genus, I, quadrivaUns, a similarly constructed, but here only complemental male, is lodged in a relatively large hermaphrodite form.
With regard to the habits of the Cirripediay the ma- jority are merely cemented to foreign bodies. Anelasma and Ihtbicinella, however, partially bury themselves in the integuments of the shark and whale, and thus prepare us for the completely boring habit of Cryptophialtis, Lithoirya, and Alcipp€f the latter of which (Fig. 69, i, 2, 3) burrows in dead shells on our own coasts.
Proteolepas lives within the sac of AUpas comtUa, and appears to be truly parasitic upon it, sucking the nutritive juices from the soft prosoma of the animal which it infests.
The Cirripedia are almost exclusively marine, only a few species tolerating even brackish water. The Thoracica alone have yet been found in the fossil state. The oldest known genus, Pollicipes, occurs in the lower oolite ; there 18 a single cretaceous species of Verruca, but the sessile Oirripedes become numerous only in the tertiary epoch.
The Bhizocephala {Peliogader, SacctUina) are small and parasitic; usually upon the abdomen of other Ctuk.
302 TEX AXUOXT or BTSKmSAXBD AMXKUJL
(lOM 'PodopdUfculaia^. ^e body is Eke • «e «r di^ nd dcToid of KgineiiUiion aad at limb*. Tbr ^ertm» of As ^ ifl fiiiinel-ahap«d and mpported hj k nng of cUUb. nie cmTamteraice of the tunnel give* off k nimibtr «f Toot-like rmrrnim. vhich bnach vMt thrao^ tfae boctf rf tke infested »«i»n»l The alimeatarj aaal ia ofcaoleta^ ud tbere mie no Manent glkndi. Tltey :
Fig. 70.— A. A'mrp/Hu-itiiBe nf Sacixliiui imi pinia : ai, nnfuet.
B. QTiTM-stdgc of Lenutoditcmi paretOons. C. AdiUt wmdlrtmi «f
Pthogailer paguri : a, anlcrlor ead of tbe body ; b, Kpertoie i e, nwt-
lik> proc««M. (After F. Hilllcr.)
the joong, like those of the other Peeie^raca, pus throngh a Savpliv* and a C^prit stage.*
The Halacostbaca. — The groops of Otulacea known as the Fod^hlhahnia, the Cuntaeea, the SdnopMhoImia, and the Btomatopoda are here inclnded under this head.
*TlwtanDQ7n-ii-iUeg, nnull; Imply any (peolftt ■flfadty with
ap]di«d to that condition of tho the Oitracitda. On Ui* oontrmiy,
Uttb of the AEtoMrona Id wblel) the lena In the CWiMtueli
" * '-k * kCI-. ^ I. . _i It ffT- ^^^^" ^j
THB PODOPHTHALMIA. 903
The body consists of twenty somites (counting that which bears the eyes as one), and, of these, six (bearing the eyes, antennnles, antennsB, mandibles, and two pairs of mazillffi) oonstitate the head ; eight enter into the thorax, and bear the foot-jaws and ambulatory limbs; and six form the abdomen and swimming limbs. In some few instances the number of somites is reduced, but they never exceed twenty.
The Na/uplius-iorm of the free embryo is rare, bat occurs in some cases {Peneus). In others (Myns) it is represented only by a temporary condition of the embryo, during which, however, a chitinous cuticula is formed and subsequently shed ; and what appear to be remains of such a transitory record of an original Ncnipliue state, are seen in many Amphijpoda and laopoda, which nearly attain their adult form within the egg. In most PodophthainUa, the embryo leaves the egg, not as a Natiplitu, but as a Zoma, which has thoracic, but no abdominal, appendages, and in many respects resembles a Copepod.
The Cvmacea take an intermediate position between the jfodopMhalmia and the Edriophthahnia on the one hand, and the PhyUopoda {Nebalia) on the other. They thus serve to connect the MalacosUraca with the Entomottraea,
The PoDOPHTHALMiA. — It wiU be convenient to com- mence the study of the Malacostraea with the Podoph' ihoUmia, and, as excellent examples of this division of convenient size are readily obtainable in the fresh-water Crayfish {Astacua fiuviatilU) and the Lobster (^onutriM vvlgafrig\ and as they furnish a very intelligible guide to the general plan of structure of the higher Arihropod>ay the organisation of Adobcus will be described at length. With some unimportant modifications, what is said about it will be found to apply to the Lobster.
The upper and anterior portion of the dense and more or less calcified exoskeleton, which covers the body of AdaeiUt has the form of a large expanded, shield-like plate, tlfte oan^aoe, produced into a strong frontal sgiiift \»^*
304 THE ANATOMY OF IKYBBTBBRATBD ASTMAXA
tween the eyes, and bent down at the sides, so as to reach the bases of the legs. The posterior division of the body, on the other hand, presents a very different aspect, being divided into a series of distinct moveable somites. This is called the 'abdomen, while the anterior division, covered by the carapace, corresponds with the head and thorax of other Arthropoda, and receives the name of cephalo- thorax.
On taming to the ventral surface of the Crayfish, a great number of limbs or appendages, twenty pairs in all, are seen to be attached to the cephalo-thorax and abdomen, six pairs belonging to the latter, and fourteen pairs to the former region of the body.
The six pairs of abdominal appendages are commonly known as the "false" or "swimming" feet; and it will be observed that they are attached to the six anterior segments of the abdomen only, the seventh being unpro- vided with any such organs. Of the fourteen pairs of cephalo-thoracic appendages, the five posterior are called the *' ambulatory " legs, being the organs by which the Crayfish is enabled to walk. Strictly speaking, however, the anterior of the five pairs is not more ambulatory than prehensile, being so modified as to constitute the great claws or " chelie."
Of the six next pairs of appendages, passing from behind forwards, five are not at first sight apparent, the posterior pair, which are applied over the mouth and cover the others, being alone visible. These and the two pairs which lie im- mediately under, or in front of them, are called mcuBiUu pedes, or '* foot-jaws." The next two pairs, delicate and foliaceous, are the maxillsB; while beneath or rather in front of them, are two strong, toothed organs, the mandi- bles. These, the maxillae and the maxillipedes, thus con- stitute six pairs of gnathites.
The remaining three pairs of appendages occupy the sides of the fore-part of the cephalo-thorax, in front of the mouth. The most posterior pair, or the long feelers, are the antennsB; the next, or the short feelers^ are the anten*
nuls ; while the most anterior pair are the moveable stalks, which support the eyes upon their extremities ; the " oph- thalmic peduncles," or " ophthalmites.**
To arrive at an understanding of the composition of this complex body, with its multiform appendages, we must first detach and study carefully one of the abdominal segments — say the third. Such a segment is nearly semi- circular in vertical section, the dorsal wall, or tergum, being very convex, and where it reaches the level of the almost straight ventral wall, or sternum, sending down a flattened lobe, which is reflected at its free edges into a corresponding prolongation of the ventral wall, so that each inf ero-lateral angle of the s^pnent is prolonged into a hollow process, the pleuron. Near the outer extremities of the straight ventral portion of the segment two rounded articular cavities, which receive the basal joints of the ap- pendages, are situated. A transverse groove will be seen on the tergum, separating rather more than the anterior third of its surface, as a smooth, convex, lenticular facet, which is completely overlapped by the posterior margin of the preceding segment, when the abdomen is extended, and is left uncovered only in complete flexion. This is the iergai facet, A corre8x>onding flattened and rather excavated surface upon the anterior half of the pleuron, which is similarly overlapped by the preceding pleuron, and is left imcovered only in complete extension, may be termed the|?2ettraZ/acei. It will be observed that there is a close correspondence between the skeleton of an abdominal somite of a Oray-fish, and that of a thoracic somite of a Trilobite; except that, in the latter, the sternal region is not calcified.
The appendages of the segment (Fig. 71, E) are very simple, consisting of a cylindrical basal portion, divided into two joints, a shorter proximal, and a longer distal, to the latter of which two terminal many-jointed filaments are articulated. The inner of these is distinguished from the outer by possessing a more elongated and wider basal joini. The whole basal division of the appendages is tjx^
THX liri.TOICT OF nmSTIBKATID FIr. 71.
t*"^
ASTACUB VLITYIATILIS. 807
Fig. 71.— Astactu fluvkaUii. A. Mandible, a, (, endopodite ; o. its tenninal joints constituting the palpus of the mandible. B. first maxilla. C. Second maxilla. D. First maxillipede. £. Second maxilllpede. F. Third maxillipede. All the preceding, except B. are left limbs. 6. Ambulatory leg. H. Appendage of first, and
1, of second abdominal somite in the male. K. Appendage of third abdominal somite. L. Sixth abdominal somite, with its appen- dages and telson. a, b. Endopodite. e. Exopodite. d. Epinodite. e. Setaceous filaments attached to coxopodite. x. Teigum m sixth abdominal somite, y, z. The two divisions of the tebon. In 6 1
2, basipodite ; 3, ischiopodite ; 4, meropodite ; .'), carpopodite ; 6, pro- podite ; 7, dactylopodite. In A. d marks the tendon of the adductor muscle, and, in K, the joints of a fr, and e are not sufliciently numerous. M. Transverse section of half a thoracic somite (a). b, Coxopodite. c. Basipodite. d. Ischiopodite. A. Brandiiferous epipodite. /*, p. Branchiae, e. Filiform appendage. N. One of the branchi^rous epipodites. a. Its point of attachment. 6. Basal enlargement, c. Branchial filaments, d. Terminal lobes.
proiopocUte ; while the internal and external terminal fila- ments are the endopodite (a h) and exopodite (e).
An abdominal segment, or somite, then, is composed of a tergum, two pleura, and a sternum ; but it must be remembered that these terms rather indicate regions than anatomical elements, the whole segment being continuously calcified, and no sutures or other absolute demarcations separating one portion from another. Furthermore, the somite carries two appendages, each divided into a proximal portion or protopodite, tei*minated bj two branches, the endopodite and exopodite.
The whole exoskeleton of the Adacus, however Tariou» may be the appearance of its different parts, consists of somites and appendages essentially similar to those which have just been described, but which are more or lesa masked by the connation, the coalescence, the abortion, or the extreme modification of their primitive elements.
If, in the first place, we follow out these modifications in the posterior somites, we find the fourth, fifth, and sixth abdominal somites to be, in all essential respects,. «iTwiln.r to the third; but the appendages of the sixth (Fig. 71, L) are singularly changed, the protopodite being represented by a single strong, short joint, and the exopodite and endopodite having the form of wide oval setose plates. The exopodite is again divided into two poxtiona V^*^ ^
308 THE ANATOMY OV INYXBTBB&ATBD AJriMAI.8.
transverBe joint. The seventh division of the abdomen (Fig. 71, L, y^ z) is the teUan. This telson bears no appen- dages ; dorsallj it is completely calcified, bnt is divided by a transverse suture into two portions, the posterior of which is moveable upon the other ; ventrallj, on the contrarj, it is onlj the posterior part which is f ullj calcified, the middle of the anterior portion, in which the anus is situated, being completely membranous, and the sides only being strength- ened by calcareous plates extending inwards from the dorsal hard skeletal element, or aelerodemUte.
The x>owerful tail-fin of the Astacus is formed by the telson, combined with the two distal divisions of the sixth abdominal appendages on each side. The other abdominal appendages can have very little influence on locomotion. In the female, however, they play an important part, as the carriers of the eggs; and, in this sex, there is nothing worthy of special notice about the first and second ab- dominal somites or their appendages, except that those of the first are rudimentary. In the male, the appendages of these two somites have undergone a very interesting metamorphosis, whereby they are fitted to subserve copula- tion. Those of the second somite (Fig. 71, 1) are enlarged, and the protopodite and basal joint of the endopodite are much elongated ; the latter being produced internally into a plate rolled upon itself, and thence concave outwards and forwards. It is as long as the rest of the endopodite (which like the exopodite is many-jointed) and serves aa a sort of sheath for the reception of the appendage of the first abdominal somite (Fig. 71, H) which consists of a single plate rolled upon itself in a similar manner, so as to resemble a grooved style. These organs, doubtless, help to convey the spermatophores from the male g^enital aper- tures to the body of the female.
The compact and firm cephalo-thorax seems at first to difEer widely from the flexible, many-jointed abdomen; bat the most posterior of its somites offers an interesting transi- tion from the one to the other. This somite is, in fact, only vnited bymiembruia to that whioh precedes it, and is henoe.
▲STACirS VLUniTILIS. d09
to a certain extent, moveable. Ite sternal portion is com- pletely calcified, bat the epimera^ are only partially cal- cified.
The appendages of this somite differ widely from those of the abdomen, representing (as their development shows) only the protopodite and endopodite of the latter. Each is a long firm leg, composed of seven joints, the proximal one being thicker than any of the rest, while the terminal joint is narrow, curved and pointed. To these seven joints, Milne-Edwards has applied the following terms (Fig, 71, G). The proximal one which articulates with the somite, is the oomopodUe (1), the next, small and conical, is the haai- podiie (2), the third cylindrical, short, and marked by an annular constriction, is the ischiopodUe (3). Next comes a long joint, the meropodUe (4), then the earpopodUe (5) and propodUe (6), and, finally, the terminal daetylopodite (7). f
The next four somites, proceeding anteriorly, have a similar general character to that which has just been de- scribed, but they cease to be moveable upon one another, partly by reason of the calcification of the interepimeral and interstemal membranes, partly on account of the development of these membranes by a folding inwards, or involution, into processes, the apodemea, which project inwards and unite with one another in the cavity of the thorax. In an Aiiacua which has been macerated, or, better, boiled in caustic alkali, the floor of the thoracic cavity is seen to be divided into a number of incomplete cells, or chambers, by these apodemal partitions, which will be observed, on careful examination, to arise partly from the interstemal, partly from the interepimeral membrane connecting every pair of somites. The former portion of each apodeme is the endostemite, the latter the endoplewite of Milne-Edwards. As a general inile, each endostemite is
* The tenn epimeron is here sppendage and the plearon.
•mployed In a more special sense f Probably the coxo- and basi-
thaa that oomnioDly used, to de- podite together answer to the
note that part of the lateral wall protopodite of the abdominal
of a somite which is situated appendages, the remaining j<tott
between the articulation of the representing the endo^g'Qidl^A*
310 THE ANATOMY 09 DrYEBTKBRATBD ANIMAL8.
distingoisliable into three apopTiyses — tlie aHhrodial^ which passes outwards and unites with the descending diyiaion of the endopleurite to form one boundary of an artioiilar cavity for a limb ; the mesophragmal, which is directed in- wards, uniting with its fellow, and forming an arch over the passage left in the middle line between each pair of endostemites — the so-called sternal canal ; lastly, the para* phragmal division is a small process, which passes forwards, upwards, and outwards, and unites with the anterior division of its own endopleurite, and with the posterior division of the endopleurite in front of it.
The endopleurite, likewise, divides into three apophyses, one descending or arthrodial, and two which pass nearly hori- zontally inwards : the anterior horizontal ax>ophyBis Tiniting with its own paraphragmal apophysis, the posterior with the paraphragmal of the antecedent endostemite. The posterior horizontal apophysis, therefore, crosses the space between every pair of apodemes diagonally, whence the appearance of a double row of longitudinal cells opening above, on each side of the sternal canal. It will be understood, however, that these cells are veiy incomplete, communicating with one another anteriorly and posteriorly, by the largo apertures left between the endostemites and endopleurites; and, laterally, by the spaces between the endostemites, through which each series opens into the sternal canal ; while above, they are in free communication with the thoracic cavity. The apodemes give attachment to the muscles of the appendages, while the chain of ganglia and the sternal artery lie in the sternal canal.
The appendages of the penultimate, resemble those of the last, thoracic somite, but the three preceding pairs difEler from them by being chelate, that is, by having the posterior distal angle of the propodite produced so as to equal the dactylopodite in length, and thus constitute a sort of opposable finger for it (Fig. 71, G, 6, 7). The first ambulatory or prehensile limb, again, is remarkable for its greai, size and strength, and for the ankylosis of its badpodite with the ischiopodite.
▲8TACV8 VLUTIATILIB. 811
The four anterior pairs of ambtilatory limbs differ from the last pair in possessing a long curved appendage (Fig. 71, N), which ascends from the coxopodite, with which it is articulated, and passes into the branchial <^iamber, in which it lies. This is the epipodUe ; its relation to the function of respiration will be adverted to presently.
The sterna, which are wide in the three hindmost thoracic somites, become verj narrow and almost linear in the anterior ones. They and their apodemes, however, remain perfectly recognisable.
The sternal regions of the three maxillipedary somites have the same characters, their appendages and articular cavities becoming smaller ; while, by the contemporaneons excessive narrowing of the interarticular regions of the sterna, these cavities are closely approximated.
The stemxun of the next anterior somite (bearing the second pair of maxillsB), on the other hand, though very narrow from before backwards, has a considerable width, and its articular cavities, already much larger than those of the anterior maxillipedary somites, are consequently thrown outwards. Hence results a sudden widening of the second maxillary, as compared with the first maxilli- pedary somite; and, as a consequence, we find a deep fold or depression on the sides of the body, where these two somites join. This fold is directed upwards and backwards on the flanks of the body, parallel with an important im- pression on the carapace, the eervieal groove. Not only on this ground, but because the fold really represents a true neck, or separation between the head and thorax, it may approximately be termed the eervieal fold. The scapho- gnathite, (Fig. 71, C, e, d,) an important appendage of the second maxilla, lies in this cervical fold.
The appendages of the three maxillipedary somites (Fig. 71, D, £, F) are highly interesting, inasmuch as they afford transitional forms between the ambulatory limbs and the gnathites. Each maxillipede is composed of three diTirions, articulated with a stout protopodite. The outermost of these divisions is a curved, elongated lamina
812 THX AKATOXT OF U f IMaBKlMID AKDCALS.
(<!), preciselj resembling the epipoctite of tbe pooterior thoracic limbs in the two hinder maxillipedes (£« F) ; bnt^ in the anterior (D) not modified so as to nerve as a branehiat and rather approaching the scaphognathite in form.
The middle division of each maxillipede (c), answering to the exopodite, is long, slender, manj-jotnted, andpalpiform» while the inner division, or endopodite, (a, h,) not ontf corresponds with one of the ambulatory limbs, bnt in the posterior maxillipede (Fig. 71, F) very closely resembles one, and contains the same number of joints. In the next maxillipede, however (Fig. 71, E), the endopodite is pro- portionally shorter, and in texture and form rather approaches the foliaceous endopodite of the anterior maxillipede (Fig. 71, D), in which a flat plate is applied to the posterior surface of the slender exopodite. A perfect transition is thus produced between the corresponding divisions of the second maxillipede and of the second maxiUa.
The intermaxillary apodeme, or that developed from the connecting membrane of the two maxillary somites, is very remarkable for its stoutness and for the great size and expanded form of the mesophragmal processes, which unite into a broad plate, whence prolongations are sent forwards and outwards, in front of the tendon of the great adductor mandtbuUE muscle on each side. These prolongations appear to be the calcified posterior horizontal apophyses of the mandibulo-maxillaiy apodeme, which elsewhere remains membranous.
The second maxilla (Fig. 71, C) much resemUes the an- terior maxillipede, but the epipodite (d) and exopodite (c) appear to be combined into a wide oval plate, the seajpfto- gnathite, of which mention has already be^ made.* In the first maxilla (Fig. 71, B) the epipodite and exopodite appear to be undeveloped, and the joints of the endopodite are completely foliaceous. The somite which supports the
* Until the development of of the homologies of their psrCs ihete Appendages has been must be reganted as provisional. woilBad oat, the detemiaatioa
ASTACmi WLUTUSUM, 813
mandibles is, to a greafc eztent» memlHranoiis in its sternal region; it is united with the corresponding region of the first maYJllary somite, itself represented merslj bj a narrow, distinctly calcified, band, in front of the second maYJllary sternum, by monbrane only. In this mem- branoos space the elongated apeitnre of the mouth is sitoated.
On each side of, and behind, the mouth are two little elongated oval calcified plates, between which, an oval process, setose at its extr^nity, proceeds downwards and forwards, and lies in close apposition with the x>osterior face of the mandible of its side. This is one^ialf of what is termed by most authors the lahiwnf but, to avoid con- fusion with the labium of Inseeta, from whioh it is wholly different, it may be called the metcuioma (Fig. 72, f). It obviously answers to the structure so named in the Cope* poda.
The mandibles fill up a large space in the sternal mem- brane, with which their edges are continuous on each side of the oral aperture ; externally, the sternal membrane bends suddenly downwards into the pleural ridge, continuous with the branchiostegite of the carapace, and becomes calcified ; while, anteriorly, it is very difficult to say where the mandi- bular sternum terminates. In front of the mouth the sternal membrane becomes developed into a large median lobe, containing three small calcified plates on each side of the middle line. This is the labrum (Fig. 72, e).
Hie mandible itself (Fig. 71, A) is thick and strong at its inner end, where it is divided by a deep excavation into an upper and a lower portion, (a, h,) the edge of each being toothed. The outer division of the mandible extends along the whole width of the somite, and tapers to its ex^mity, which presents an articular head, the outer condyle. Attached to its anterior margin is the palp(o), which represents the terminal joints of the mandibular endo- podite. The exopodite and the epipodite have no re- preaentatives in this appendage. Superiorly, the outer portion of the mandible is concave, and its posterior ed%<^
314 THZ AKATOn OT UTTE&TIBKUID JUntAi^,
gives attadunent to the calcified tendon of the miHaatot lu&ndibuls {S).
In &ont of the labrom and mandibles ia m wide, Bome- what pentt^onal area, prolonged into a point in tlie »»iAlLi line fornards, and presenting a small spine on encli n^; thia IB the epittoma (Fig. 72, B, Q, and it is chiefly, if not entirely, formed hy the sternnm of ibe antennaiy ■^m't^, On each side of ite triangular anterior eitremitj it pre- Fig. 72.
. „. . -A. AnMrior eilreniity of tfae o«p _ .
with A portion of the cuvpace lemoved. B. Vertical Ketionorilta tnttriat part of ths cephftlu-tbom. a, rostrum; A, ophthalmle pedoDoln ; iT,Bnt<Dnulae; d, snteQiuB ; c, Ubnim ; /, mgtwtonk ; ff, 0 aI ftpertnn ; A, procephalic proc««es ; f, ophthunic •tanun ; il,'auteDualu7 ucrDum ; I, aateiiDary itenium or sfdatoma.
■enta a wide articular cavity for the articulation of tlie antenns. Is these organs (Fig. 72. B, d) the same parts can be recognised as in the other appendages, viz., an imperfect basal joint, produced into a prominent cone, perforated behind and internal to, its apex; and here called eoxoceriU, Next, a haticeriU, to the outer portion of which a flattened plat«, the representative of tlie exopodite, and here called the icaphocmHe, is articnlated; while to its inner portion an itchioeeritt is connected, bearing a m^oeeriU arid earpoeerite, while the last segment, or proceriie, coneista ot a long multi-articnlato filament.
The stoma of the next two somites are narrow and elon- gated; that of the antennary somite is well calcified, but that of the ophthalmic somito is almost entirely mem-
A8TACU8 FLUYIATIIJ8. 315
The anteimiiles (Fig. 72, B, e) present an enlarged trigonal basal joint, succeeded b j two others. These represent the protopodite, and carry at their extremities two many-jointed filaments, which probably represent the exo* and endopo- dites.
Thepednnclesof the eyes (Pig. 72, h), lastly, are composed of two joints, a small proximal b<uu)phOialmUe, and a larger terminal podopMhalmUe.
Such are the stmctore and arrangement of the sternal portions of the several cephalo-thoracic somites, and the nature of their appendages. On regarding the sternal region as a whole, there are yet some very important points (the morphological value of which has been fully pointed out by Milne-Edwards) to be noticed. A longi- tudinal median section, in fact, shows, that while a line drawn through the sterna of the somites behind the mouth is nearly straight and parallel with the axis of the body, a similar line drawn through the sterna of the somites, in front of the mouth, ascends as it passes through the antennary, antennulary and ophthalmic sterna, and thus takes a position at right angles to the former line (Fig. 72, B). The sterna of the somites, in front of the mouth, are, therefore, bent up so as to look forwards instead of down- wards ; and it is of essential importance to bear in mind this cephalie flexure, in considering the structure of the head in these and other ArthropocUi.
Just as the lateral regions of the abdominal somites are produced into the plewra, so are the lateral regions of the cephalo-thorax similarly prolonged. Thus the membranous lateral walls of the posterior cephalo-thoracic somite are reflected superiorly, and bent down again to the level of the bases of the legs, where they become continuous with a calcified layer corresponding with the tergal half of the pleura, and forming the posterior part of the carapace. In like manner, the more or less calcified epimera of all the other Bomites are reflected superiorly into a membrane which passes downwards, and the free lower edge of which is con- tinuona with the edg^ of the carapace. The caxOk^^fi^^
316 THE ANATOMY 09 ZVYSBTXBBATBD ANIKAX8.
therefore, correspondB in poeitiosi with the terga and tergtl halves of the pleura of all the Bomitea which are thm reflected into it, and these somites include all, without exception, from the last thoracic to the ophthahnie. Fot- tei-iorly, the edges of the carapace are a little prolonged beyond the last thoracic somite, and take the form of a fold, with an imder layer distinct from the upper. An* teriorly, in the middle Une, the carapace is prolonged in a similar manner, but to a much greaJter extent; it thus gives rise to the long rogtrumj which overhangs the sterna of the ophthalmic and antennulary somites. At the sides of the antennulary and antennary somites the rostral prolongation of the carapace is the direct con- tinuation outwards of the epimera of those somites, and there is nothing to be compared to an apodeme ; but the sternum of the ophthalmic somite, after giving off the lamella which forms the inf eromedian region of the roetmnit is prolonged on each side of the middle line backwards and outwards into a free, expanded, thin, calcified process, which applies itself against the carapace by its upper surface, and by its under surface gives attachment to the anterior gastric muscles. Corresponding processes are developed from the carapace itself, in some Podophthalmia (e. g. Oalatkea^ Car* cinus), for the attachment of the posterior gastric muscles. From the last thoracic to the maxillipedaiy somites, the pleural, or free part of the carapace, termed, from its function, the branchiogtegUef or cover of the gills, encloses a wide space, bounded internally by the epimera of the somites. This is the branchial ohamher. In front of the maxiUipedes and cervical fold, however, the chamber snd* denly becomes narrowed by the rapid widening of the sterna of the maxillary and mandibular somites, and by the lowering of the point at which the reflection of their epimera into their pleura takes place. Finally, on the antennary somite, and in front of it, the pleuron becomes a mere fold separated by a shallow groove, the repreeen> tative of the branchial chamber, from the epimera. On the dorsal surface, there is no indication of any division
ABTACxrs WLjmATiua, 317
of the carapace into terga oorrespomding with the sterna of the Bomit^ bnt it is marked by a well-defined, curved groove, the posterior convexity of which extends across the carapace, rather behind its middle, and the lateral portion of which runs downwards and forwards, towards the anterior part of the antennary stemnm. This is the eerviaU groove ; that part of the carapace which lies in front of it is the o^halotUgite, while that which is behind is the omoHegiie.
The omostegite, again, is divided into three portions by a groove on each side of the middle line — the brone^to- cardicus grooves. The branohiocardiac groove, and the lateral portion of the cervical gpx>ove, on the dorsmn of the carapace, correspond very closely with the line at which the epimeral is reflected into the pleural membrane, on its ventral surface. The transverse portion of the oervical groove, on the other hand, corresponds with the posterior boundary of the stomach, and the anterior extremity of the heart, and continues inwards the line of the cervical fold; so that, in a longitudinal section of an AMkusus, the direction of the cervical fold, if followed upwards and backwards, strikes against the inner surface of the cara- pace, at a point corresponding with the summit of the cervical groove, on its outer surface. By cutting through the cervical fold, therefore ; through the membrane joining the second maxillary with the first maxillipedaiy sternum ; and through the carapace in the transverse part of the cervical groove, it is possible to separate an anterior portion of the cephalo-thorax, containing the whole of the cephslo- stegite, and the first six somites, with their appendages, from a posterior portion, consisting of the omostegite, and the last eight cephalo-thoracic somites. And, in making this artificial separation, we should be merely carrying out a distinction between these two sets of somites, already very clearly indicated by the cervical fold and groove.
It is for this reason that I differ from Milne-Edwards in regarding the somite which bears the first maxillipedes as the first of the thorax, and not as the last of the head. And tlie acceptance of this natural delimitatioa ol ^<^
31^ THE ANATOMY OF INVERTEBRATED ANIMALS.
head in the higher Crustacea, has the advantage of bringing its structure into accordance with that of the same region in the Eniomagtraca, in which it is the role that the hatd possesses ejes, antennnles, antennfle, mandibles, and two pairs of maxillse.
Another mark upon the carapace is a large and ronnded convexity, occupying nearly a third of the whole width ol the x>osterior half of the cephaloetegite. This impression is bounded internally by a line drawn from the outer angle of the base of the rostrum, directly backwards ; and, exter- nally by a curved depression, deepening into a pit ante- riorly ; it corresponds with the attachment of the base of the adductor muscle of the mandible.
The mouth of the Crayfish is a wide aperture, situated between the labrum in front, the metastoma behind, and the mandibles on each side. It serves as the entrance to an equally wide oesophagus, a short tube with plaited walls, which takes a slightly curved direction upwardJs and a little backwards, to open into the large stomach, which is not only situated directly over, but extends forwards in front of, the gullet. The stomach, in fact, occupies almost the whole cavity of the body in front of the cervical suture, and is divided by a constriction into a large anterior moiety, the cardiac division, and a small posterior, pyloric portion. The anterior half of the cardiac division has the form of a large membranous bag, the inner surface of which is closely set with minute hairs ; but, in the posterior half of this, and on the whole of the pyloric division, the walls of the stomach are strengthened by a very peculiar arrangement of uncalcified and calcified plates and bars articulated together, which are thickenings of the chitinoos cuticula of the epithelium of the alimentaiy canal, and constitute the gastric skeleton. The most important part of this apparatus is that which is developed in the posterior cardiac region.
It consists, in the first place, of a transverse, slightly arooated emrdiao plate (Fig. 73, ea\ calcified posteriorly, which extends across the whole width of the stomach, and
▲BTACUS VLUYIATILIS. 319
articulates at each extremity by an oblique suture with a small curved triangular* an^o-2a(eraZ or pierocardi4ie {pt) ossicle. On each side, a large, elongated postero-laieral or vygocardiac ossicle {se) wider posteriorly than anteriorly, is connected with the lower end of theantero-lateral ossicle, and, passing upwards and backwards, becomes continuous with a transYcrse arcuated plate, calcified in its anterior moiety, and situated in the roof of the anterior dilatation of the pyloric portion; this is the pyloric ossicle (Fig. 73, py).
These pieces, it will be observed, form a sort of six-sided frame, the anterior and lateral angles of which are formed by moveable joints, while the posterior angles are united by the elastic pyloric plate.
From the middle of the cardiac piece a strong calcified wroeardiae process {ca') extends backwards and downwards, and, immediately under the anterior half of the pyloric ossicle, terminates in a broad, thickened extremity, which presents inferiorly two strong rounded tuberosities, or cardiac teeth. With this process is articulated, posteriorly, a broad pre- pyloric ossicle, which passes obliquely upwards and forwards, in the front wall of the anterior dilatation of the pyloric portion, and articulates with the anterior edge of the pyloric ossicle, thus forming a kind of elastic diagonal brace between the urocardiac process {ca')t and the pyloric ossicle. The inferior end of this pre-pyloric ossicle is pro- duced downwards into a strong bifid urocardiac tooth (oe). Finally, the inner edges of the postero-lateral ossicles are flanged inwards horizontally, and, becoming greatly thick- ened and ridged, form the large lateral cardiac teeth (cc). The membrane of the stomach is continued from the edges of the pre-pyloric to those of the postero-lateral ossicle in such a manner as to form a kind of pouch with elastic sides, which act, to a certain extent, as a spring, tending to approximate the inferior face of the pre-pyloric oande to the superior face of the median process of the cardiao ossicle.
The result is that there is a certain position of equilibriuxoi
rVEKT&BRATED AN III A LS.
of the whole apparatus, when the urocardiac process and the pre-pfloric oedcle make a small angle wiUi one uu>thw.
Flf . 73.— .^riacw.— Upper FIgur« ; LongltndliiKl & A. Aotnlornitrio muMle. B. Poaterior gutrlo miwcle. (B. sa>- phuni. P.PjloTiu. en. CtrdioctiHicla. < a'. lu nioeudiao proMM. a e. llrMtrdiM tooth. /))i. PjlorieosglFle. The oblique bar, eztend- Inf ttma tbe aid ef the ourdlae Ui th« pjlorie, li the prepylotlfl iiliil«
B^ptarooardlao. it. PoMero-Uteral cudUo, vith it« grtat tooth, ac
i tbn*ll Inftriot tooth, t. C»rdlo-pjloric vaWe. 6. lufercMBedln
vrlorlo tidp. a, Lktanl pyloric lidn. d. Superior pjlorla ridge.
^ Uio-pjlorio odialc. zy. Line of Mctlon ; tbe eotertor &>• of
the poetorior eegment being ihown in the lovar Sgiu«.
and the antero-lateral oisicleH form an almost nnbrokon tmursrse curve with the cardiac. When nudiBtarbed, the l^puatna totdi to UKune this poaition.
▲8TACU8 rLUTIATILia. 821
Two pairs of powerful muaoles are afctacHed to thia gas* trie skeleton. The anterior pair arise from the prooephalio processes, and are inserted into the roof of the stomach, somewhat in front of the cardiac ossicle ; the posterior have their origin in the carapace immediately aboYS and behind the pyloric end of the stomach, and their insertion into the pyloric ossicle and the wide posterior part of the postero- lateral pieces.
From the attachment of these muscles it is dear that their action mnst, in a general way, resemble that produced by pulling ux>on the cardiac and pyloric pieces when the stomach is removed from the body. Now the result of doing this is that, the cardiac and pyloric pieces being diyaricated, the pre-pyloric ossicle assumes a vertical position, and the uro-cardiac tooth turns downwards and forwards. At the same time the antero-lateral or pterocardiac pieces are pulled backwards, and, owing to their oblique articulation with the cardiac piece, their inferior ends moye down- wards, backwards, and inwards, carrying with them the anterior ends of the postero-lateral pieces,, the teeth of which (lateral cardiac) come into contact with the uro- cardiac and cardiaq teeth, with a force proportional to that exerted in traction. On ceasing to pull, the apparatus returns to its former position, its backward moyement being facilitated by the reaction of the elastic pouch men- tioned above, and being doubtless also assisted, in the living state, by a pair of small cardio-pylorie muscles, which pass, one on each side, between the cardiac and pyloric ossicles, beneath the membrane of the stomach, the looseness of which, in this region, where it unites the various ossicles of the gastric mill, greatly assists the free move- ment of the whole apparatus.
Nothing can be more easy than to perform the ex- periment, and to convince oneself that these structures do really constitute a most efficient masticatory apparatus; and it is surprising that Oesterlen, in his elaborate essay on the stomach of AHacus, should have questioned the crushing action of the teeth.
rtl'l THE ANATOMY OF INVERTEBKATED ANIMALS.
A ^reat bilulx'd valvular process i.Fig. 73, c) rises up from the sternal region of the stomach, opposite the cardio-pyloric constriction, and apparently prerente the food from piifwiTig inte the pyloric division until it is properly comminuted. And, in front of this valve, the infero-lateral parietes of the stemach are strengthened by a number of other plates and bars; one of which on each side bears a small tooth {infero-lateral cardiac, I), and is continued into a brotd uncalcified plate, lying in the hinder and lower part of the side walls of the stomach, and covered with hain internally. There are, therefore, altogether seven gastric teeth, three median, the cardiac, and the uro-cardiac ; and two lateral on each side, the latend cardiac, and the infeio- lateral cardiac.
In the pyloric diWsion of the stomach the food has to undergo a further series of comminutions and strainings. A ridge covered with long hairs projects in the median line above; other hairy ridges extend inwards from the sides to meet it, and nearly close the passage laterally. These ridges are very convex inferiorly, and their con- vexities abut against the concavities of an inferior median ridge, which rises up to meet them, and is prolonged posteriorly into a sort of valvular process, covered at its termination with long hairs, which bar the space left between the upper parts of the lateral ridges. The concave faces of this median process are covered by close-set parallel ridges, which only become free hair-like processes at the posterior margin of the plate, each ridge giving attachment to a regular series of minute hairs. These are directed inwards nearly parallel with the surface, which looks at first as if it were merely ruled with close-set transverse lines, connected by still finer and closer longi- tudinal ones.
This apparatus constitutes the " ampoule cartilagineux" of Milne-Edwards. Behind it there is yet another laf era* median, and two lateral setose, valyular prominences, which form the last barrier between the food and the intestine.
Mr. T. J. Parker, who has recently carefully examined the
ASTACV8 FLUTIATILIS. d2S
Btmctnre of tbe stomacli of tLe Crajfisli,* finds that, be- sides the anteriorand posterior gastric and the cardio-pjlorio muscles, there are intrinsic fibres in the walls of the stomach, some encircling the posterior pyloric region, others passing between the hindermost accessory ossicle and the postero- lateral and pyloric pieces; these must tend to diminish the cavity of the stomach, and the last-named fibres possibly assist in mastication by bringing the lateral cardiac into contact with the infero-lateral cardiac tooth. Moreover there are nine pairs of minor extrinsic muscles, of which two pairs pass from the anterior wall of the stomach and gullet to the antennary sternum, passing between the (esophageal commissures and on either side of the azygos nerve of the visceral system ; three pairs pass between the side walls of the stomach and oesophagus and the mandibular sterna ; a sixth pair arises from the forward processes of the intermaxillary apodeme and is inserted into the oesophagus ; two more pairs arise, one from the internal thickened edge of the mandible, the other from the intermaxillary apodeme, and are inserted into the inferior surface of the pyloric region ; and a ninth pair arises from the carapace just behind the posterior gastric muscles, and goes to be inserted into the posterior pyloric dilatation. There are also a few more inconspicuous fibres passing between the oesophagus and the neighbouring hard parts. All these, at least when acting together, must antagonise the intrinsic muscles, and dilate the stomach.
The pyloric portion of the stomach passes into the an- tenor portion of the intestine, which is smooth internally, and presents superiorly a csecal process, the remains, according to Bathke, of one lobe of the vitellaiy sac of the embryo.
This anterior portion of the intestine is, however, very short, and almost immediately becomes dilated into the wider posterior division, which extends to the anus. The inner surface of the dilatation is produced into six ridges, jwhioh are continued into a corresponcUng number of series of papillffi along the rest of the intestine.
The only glandular apparatus of any kind which opens * * Journal of Anatomy and Phytiology,' Oei. l^l^.
^4 THE ANATOMY OF INYBRTSBBATED ANHCAUB.
into the alimentary canal is the liver, and the apertures of the wide hepatic ducts are seen on each side of the pylorus. Each duct conveys the secretion from the mul- titudinous csecal tubes, which constitute the principal mast of the corresponding bilobed half of the liver. The two halves lie on each side of the stomach, and, though thej remain perfectly distinct from one another, come into dose contact below.
Asicicua possesses neither salivary glands nor any csBcal appendages to the intestine, such as exist in the Brach^ura and some Macrura, unless the short csBcum just now described is the homologue of the longer caeca of Maia and Hatnaru8,
In the spring and summer, two very curious diacoidal calcareous plates, the so-called " eyes *' of the Crayfish, are found imbedded in the walls of the dilated anterior portion of the cardiac division of the stomach, the middle of the lateral surface of which they occupy. These bodies com- mence as calcareous deposits underneath the chitinous gas- tric lining, and increase in size until the period arrives at which the Crayfish casts its skin. They are then cast, together with this lining membrane and the gastric arma- ture ; and it would appear that, like the latter, they become broken up and destroyed within the new stomach. The purpose of these concretions is not understood ; the ordi- nary theoiy, that they are stores of calcareous matter, ready to be distributed through the young integument after ecdysis, appearing to be negatived by their small size. Oesterlen states that they rarely weigh more than two grains, and judiciously suggests, that if it be admitted that the Crayfish can derive all the calcareous matter it requires, except two grains, from other sources, it is hardly necessary to look on those two grains as a special supply.
The circulatory apparatus of Askieua is well developed. The heart (Fig. 74, C) has the shape of an irregular poly- gon, and lies immediately behind the stomach and beneath 4he cardiac region of the carapace, in a chamber which is oommonly tenned the ** pericazdium," to the walls of which
ASTAOHB IXUTIATILIB.
Fig. :4.
Fig. 14.—Aitaau, Longiludiiial SeaUon.— 1. II. III. Slenw of fint, MOond, and third Hmltei. a. (Etopbagu*. A. I^hruiD. L Mcla- ■toiDS. G. M«mbnnouipartof theilomwh. e.CardlacoMiole. pt. Urourdiae. cL LuanI eardlaa. p. Cwdio- JnfBTior Djiorle tkItuIu appantiu. ■>. Antariol I*. iDimfaD of pa«t«ikiT gaatrlo moaclM. pt. UpaniBg of bapaiia dMot •. V^teiSK
Planwardlao. Iijlorie Tal*e. p natrio muacle. PiaocphaUo
326 THE ANATOMY OF INYBRTEBRATSP ANIMALS.
cecum. I, k. Intestine, p n. Testis, p n\g n". Vas deferens. C. Heart. a o. Ophthalmic artery, a a. Anfennary. a A. Hepatic. a«. Sternal. a p. Superior abdominal artery, b. Cerebral ganglia. 9g, Aiygot vigceral nerve.
it is attached bj six ligaments, corresponding with the alsB of the heart in insects, but not, like them, muscular. Except by these ligaments, and by the arteries, which pass through it, the walls of the pericardial cavity, or blood ainua (for such it really is), are wholly unconnected with the heart» >diieh thus is, in a manner, suspended freely in the blood.
Six apertures, two of which are superior, two inferior, and two lateral, provided with yalyes, which open inwards, allow the blood to enter the cavity of the heart during the diastole, and preyent its egress, except by the arteries, during the systole. The arterial trunks are six in number, fiye being given off anteriorly, and the other from the posterior portion of the heart.
Of the five anterior arteries one, the ophthalmic, is single, and situated in the middle line ; it passes forwards on the stomach to the head, where it supplies the eyes and ant^n- nules. The other arteries are in pairs ; two pass on the stomach forwards and outwards, giving off branches to the carapace, and eventually supplying the antennae ; the other two pass downwards, between the anterior lobes of the genitalia, and divide into a multitude of branches upon the hepatic caeca.
The posterior trunk, or sternal artery, is the largest of all, and presents a sort of bulbus arteriosus at its com- mencement. It turns almost directly downwards, usually (m the right side of the intestine, to the sternal canal, which it enters, passing between the antepenultimate and pen- ultimate thoracic ganglia to the lower surface of the ganglionic cord ; it gives off two abdominal branches, one superior, close to its origin from the heart, which tra- verses the middle of the tergal region above the intestine, the other inferior, which takes a corresponding course along its sternal region beneath the nervous system. The arterial trunks are provided with valves at their com- menoement, so arranged as to prevent the regurgitation
ASTACU8 PLUVIATILIS. 327
of the blood. Thej ramify minutelyi but How far a capillary ajstem can be said to exist, is a question requir- ing farther investigation. In transparent Zocb<b, I have plainly observed the abrupt termination of the arterial trunks by open mouths, through which the blood was poured into wall-less lacume, and into the general cavity of the body ; nor can there be the least doubt that a similarly lacunar condition of the circulation exists in those lower adult CruBtiicea, the transparency of which allows of their examination with the requisite powers of the microscope. The probability is that a similar state of things obtains in the vascular system of all other Crustacea, and that, after undergoing a greater or less amount of subdivision, the arterial vessels, or their capillary continuations,, cease to exist, the blood then making its way into lacunn between the organs, and into the general perivisceral cavity; and, as in most MoUttsca, ceasing to be contained in vessels with distinct walls.
The blood thus poured out, eventually makes its way into irregular sinuses or reservoirs, the chief of which, lodged in the sternal canal, communicates by lateral channels with others which lie above the bases of the thoracic appen- dages, and from which the afferent branchial canals pass into the stems of the branchisB, on the exterior faces of which they ascend, giving off branches to the lateral filaments. Corresponding canals return the blood from these filaments to the efferent branchial canals, which run down the inner side of the branchial stems, and unite above the bases of the limbs into six trunks, which ascend beneath the epimera and open into the sides of the pericardial sinus. The floor of this sinus is formed by a continuous membrane, which appears to shut it off completely from the general visceral cavity (at least it retains air or fluid thrown into it), and, if this be really the case, it may be said to be functionally a branchial auricle, containing pure unmixed aerated blood.
The branchise are eighteen in number upon each side, and are attached from the eighth to the fourteenth somites inclusively. Six of these branchise are attached to thft
328 THE ANATOMY OF imTEBTEBRATBD AVIMALS.
epipodites of the eighth to the thirteenth somites, and difier very considerablj in appearance from the other twelve. Each epipodite ia, in fact, expanded at its npper extremitj into a broad, bilobed membrane, which is folded upon itself, so that the two lobes are directed posteriorly, and receive the epipodite of the next limb (Fig. 71, K). The membrane of the lobes is obliquely plaited, so that, doubtless, they subserve respiration to a certain extent ; but, in addition, the anterior edg^ of the epipodite is beset with a number of branchial filaments, similar to those on the other branchis.
The latter (Fig. 71, M,/, g) are simple plumes, consisting of a stem, to which are attached many delicate, cylindrical filaments. Two of these plumes are attached to the epimera and coxo-epimeral articular membranes of the ninth, tenth, eleventh, twelfth and thirteenth somites. They increase in size posteriorly. The eighth and fourteenth somites, on the contrary, only carry one plume. A tuft of long byasus- like filaments is attached to the coxopodite of each of the last six thoracic appendages (Fig. 71, F, M).
The respiratory organs of the Crayfish, not being pro- vided with cilia, require some special arrangement for the renewal of the water with which they are in contact. This object is attained principally by the action of the scaphognathite, which lies immediately behind the anterior opening of the branchial chamber ; and, during life, is inces- santly in motion, baling out, as it were, the water which has become impure through the anterior opening, and thus compelling the flow of fresh fluid into the branchial chamber through its posterior and inferior opening, constituted by the space left between the lower edge of thebranchiostegite and the bases of the limbs.
The nervous system of Antaeus^ is composed of thirteen principal ganglionic masses, of which one, cerebral, lies in the head, in front of the mouth ,* six, thoracic, are situated
* For the histology of the stnicrure of the tissuee of the nervoiit mtem, tee an elaborate Crayfish, in the * Arcliiv fur Ans- hj Hacekel, on the mlnate tomie,' 1857.
ASTACUS FLUVIATILIS.
329
in the sternal canal ; and six, abdominal, lie in the median sternal region of the six anterior somites of the abdomen.
The oerebral ganglia (Fig. 74, b; Fig. 75, a) give o£f nerves to the eyes and to the muscles of the ophthalmic appendages; to the antennnles and the auditory org^ans which they contain ; to the antenn» and the sae of the antennary gland ; to the carapace in front of the cerrical suture; and finally they send posteriorly two long and stout commissural cords to the anterior thoracic ganglionic mass. These commissures are connected by a transrerse
Fig. 75.
Fig. 75. — Visceral nerves of Attacu»,~-a, Cerebral ganglia, b. Com- missures, that of the right side is cut and turned back. c. Trans- 1^.^ verse oord uniting them behind the oesophagus, (£. «U d, d» ^*r Aj^gos nerve. h. Ganglion. t. Lateral branch of azygos, anmng with postero-lateral nerve g, e. Antero-lateral nerve. /. ICedio-lateraf nerve, k. Hepatic nerve. P. Pyloric. C. Cardiac portion of stomach.
cord immediately behind the oesophagus (Fig. 75, c). The size and form of the anterior thoracic ganglion would lead to a suspicion of the complex nature which development shows it to possess. It supplies the somites and their appendages from the fourth to the ninth inclusively, and sends forwards delicate filaments to the cesophagus.
Posteriorly it is connected with the ganglionic mass of the tenth somite by two commissures, and the other tho- racic ganglia are similarly brought into communication, the commissures of the ultimate and penultimate on^.^ >^vci'^
330 THE ANATOMY OF INYBRTXBBATSD ANIMALS.
remarkable for their brevity. The abdominal, which are much smaller than the thoracic ganglia, are, with the exception of the last two, united hj single cords, which represent coalesced double commissures. Each of these ganglia supplies the muscles and the appendages of the somite to which it belongs, and the posterior abdominal ganglion sends brunches into the telson.
The Crayfish possesses a remarkably well-developed system of visceral or stanuitogastric nerves, which has been the subject of special study by Brandt, Milne-Edwards, Krohn and Schlemm, each of whom has described a larger or smaller portion of the system with accuracy, but has omitted to mention, or has denied, the existence of some other part. Each of the great commissures (Fig. 75. b), as it passes over the sides of the oesophagus, becomes slightly swollen, and from the enlargement four nerves arise ; one, external, passes towards the mandibular muscles ; a second postero-lateral branch (Fig. 75, g) runs upwards and backwards to the inforo-lateral regions of the stomach, and eventually enters into the composition of the hepatic nerve [k); a third branch (/) turns directly inwards and upwards, and unites upon the oesophagus with its feUow and with an azygos nerve (d) which passes up in the middle line of the anterior face of the oesophagus and stomach, and enters a ganglion placed between the anterior gastric muscles (i^), from whence a lateral branch is given off on each side, while a posterior median branch (d) continues the direction of the azygos nerve. Having reached the cardiac ossicle, this nen-e divides into two branches ({), each of which passes downwards and out- wards, unites with the postero-lateral nerve of its side, and thus forms the hepatic nerve (k). The fourth and last, or an tero- lateral branch (e) descends at first to near the mouth, and then cui*ving forwards, ascends to unite on the anterior face of the oesophagus with the anterior con- tinuation of the azygos nerve, which passes forwards and upwards and enters the cerebral mass. I am inclined to think that this part of the azygos nerve forms a portion of a
ASTACirS PLirVIATILIg, 331
of neiTOus filaments which pass from the cerebral ckwards to the lining membrane of the carapace, flection of these fine filaments and the demonstra- ir continuity is a matter of no ordinary difficulty, estine is supplied by two nerres which arise last abdominal ganglion, and unite into a single •m which small branches are given off back- [ two principal ones forwards, which supply the rt of the intestine. According to Brandt, the ■eceiye branches of the fourth, fifth, and sixth mglia.
J certainly known organs of sense in AeAacnis « and the auditory organs. The eyes are seated emities of the ophthahnic peduncles, the integu- e outer extremity of which becomes translucent 'orm space, and constitutes the corneal membrane. >rane is divided into a great number of minute •al facets, each of which corresponds with the rystalline cone.*
»er face of the trihedral, proximal, and largest e antennule presents an oval space, covered by rush of complex hairs having their points all iwards. On cutting these hairs away close >ases, however, it is seen that they cover an rider above than .below, and about one-sixteenth long. The hairs are attached to the outer lip of ire, and some are directed so as to lie within lip, but the majority cover it. A good-sized ses with great ease into this aperture, and if and outer walls of the basal joint of the be now removed, and the soft parts carefully way, the end of the bristle will be seen to have > a wide delicate sac about one-twelfth of an which is attached by a narrower neck round the
r. Newton's careful 1875, to which I have referred
of the eye of the above, may be taken as a guide
The Quarterly Jour- to the study of the minute struo-
icopical Science ' for ture of the eye in the Cnb')6sA^« >
332 THE ANATOMY OV IirTXRTXBBATSD ASTDCALS.
aperture, the lipe of which are continuooB with its walli. The sac is filled with minute sandy particleB, sospeiided in a mucous, dirtj-looking fluid, and when emptied of then contents a band, consisting of several lines of Yerj fine hairs, like those which guard the mouth of the sao, but more delicate, is seen to skirt its inner contour. The hairs, projecting inwards, come into dose contact with the solid particles suspended in the mucous fluid.
A nerve may be traced accompanying the antennnlarj nerve to the sac, and appears to be distributed principally along the setigerous band, so that the extremities of the nerve fibrils come into close relation with the bases of the hairs. Some, if not all, of the sandy particles are insoluble in strong acetic acid, and would appear to be silicious.*
Two glandular sacs commonly known as the green ^lancEf, which were formerly regarded as the auditory organs, lie in the cavity of ^the head. An aperture is visible on the inner or oral side of a conical prominence, upon the inferior portion of the coxal joint of the antenna. A bristle passed into this aperture enters a larg^ but very delicate and transparent sac, filled with a clear fluid, which is usually conspicuous on each side of the anterior end of the stomach, when the carapace is carefully removed. A nerve which comes off from the cerebral mass close to the antennary nerve, passes to the neck of this vesicle, and is distributed over its surface between the outer and inner membranes, of which it is composed. Inferiorly the vesicle rests upon a large greenish ap- parently glandular mass, but is directly connected with the latter only at two points, firstly by a vascular cord, which passes to the central, and usually more yellow portion of the gland, and secondly by a short neck-like continuation of the sac itself, which is attached over a small circular space, midway between the centre and the periphery of the gland, and opens into the circular
* See for a fall account of the Hcnsen's ' Studien uber dai Bniaate structure of the auditory Gehororgan der Decapoden,* oigaiif in ,the higher Cruiiacea, 1863.
A8TAOI78 rLITYIATILIS. S33
principal duot of the gland. There is, therefore, a free commnnication between the cayitj of the gland and the exterior by means of the sao, which is, in this respect, simply a dilated duct. A section of the gland shows it to be composed of two substances, a central and a cortical. The latter is composed of minute csca, fiUed with a homogeneous gelatinous matter, containing many large nadei ; the former is trayersed in all directions by large canals, so as to have a spongy appearance. The ccBca open into the ultimate ramifications of the canals, and the spongy, lung-like texture of the central mass seems to arise merely from the very free anastomosis of their larger brandies, which eyentually enter the circular canal which communicates with the yesicle.
There is little in these structural features to suggest an organ of special sensation, but much to show that the green mass is a secreting organ, and that the vesicle acts (whatever other purposes it nmy subserve) as its duct. In all probability the green gland is an organ of the same nature as the shell gland of the Entomodraoa.
Leydig has attributed an olfactory function to certain groups of delicate eetsB which occur on the joints of the outer division of the antennule of the Crayfish.
The most remarkable part of the muscular system of the Crayfish is the great extensor muscle of the abdomen, a complex mass of fibres which is attached in part to the endophragms of the thorax in front, and, behind, to the sterna of the abdominal somites, a large part of the cavity of which it occupies.*
The essential parts of the reproductive organs in the male and female Astaeus are very similar to one another in f orm^ both ovarium and testis having the figure of a trilobed gland, situated immediately behind the stomach, and below the heart. Two of the lobes are applied together, and pass
* For details, see Suckow, Lobster at length In the *Hif-
« AnatomlBch-Pl^siologische Un- toire natuielle des Cnulaces,'
tersacbongen.' Milne-£dwards torn. i. hat deseribed the mnsclet of the
334 THE ANATOMY OF IKYSBTEBIIATSD ANIMALS.
forwards; the other lobe is directed in the middle line backwards. The ducts take their origin, one on each side, at the junction of each antero-lateral with the posterior lobe.
In minute structure, however, the two organs differ widely. Each lobe of the testis is composed of a number of small cteca, in which the spermatozoa are developed, and which open into a central duct. The ovarium, on the other hand, is essentially a wide sac, produced into three large OBca, each of which corresponds with a lobe; and the ova are developed in the epithelial lining of the sac. The efferent ducts, again, have little resemblance, the oviducts being short, wide tubes which open on the coxopodites of the antepenultimate thoracic appendages, while the vasa defe- rentia are canals as long as the body, at first veiy narrow, but afterwards widening, which lie coiled up on either side of the posterior part of the thoracic cavity, where their white contents make them very conspicuous (Fig. 74, 4^'). Eventually, they open on the coxopodites of the posterior thoracic appendages.
The spermatozoa, like those of many other Cru&Uteea, are motionless, and have the form of cells, provided with a nucleus and produced into several delicate radiating pro- cesses. They are united in their course down the vas deferens into cylindrical masses, which becoming invested by a fine membranous coat, probably secreted by the walls of that duct, constitute the spermatophoree, which maj not unfrequently be found adhering to different parts of the body, not only of female but of male Crayfish.
The ova are fecundated while still within the parent; they become surrounded in their passage down the oviduct, by a coat, corresponding with that of the spermatophore, which is produced into a pedicle, the extremity of which becomes attached to one or other of the abdominal appen- dages. Great numbers of ova, attached in this way, may be observed during the breeding-season, within the incuba- tory chamber formed by the flexure of the abdomen upon itBdf; and it is in this cavity that the •embryos pan through the whole of ih&x f ostal existence.
THB DEYELOPMBNT OF A8TACUS. 835
The deyelopment of the Orayfish has been the subject of one of the most beautiful of tiie many admirable memoirs on development, for which we are indebted to the genius and patience of Bathke.* After fecundation a blastoderm arises upon the surface of the yelk, and gradually extending over the whole yelk, becomes thickened at one part, so as to form an oval germinal disk, with a central depres- sion.
This disk next becomes widened and bilobed at its anterior extremity, the lobes being identical with the prooephalic lobes, to be hereafter described in the embryo of Mysis, The edges of the disk are raised into a fold, and within the fold a papilla, the rudiment of the abdomen, and of the greater part, if not of the whole of the thorax, makes its appearance, while, anteriorly, three pairs of transverse elevations constitute the rudiments of the antennules, the antennae, and the mandibles. The labrum arises as a median papilla, situated at first between the antennules. The ocular peduncles are next developed in front of the antennules as ridges, which only subsequently become free processes.
The thoracico-abdominal process lengthens, and the anal aperture makes its appearance. It is to be remarked, that the anus is at first situated on the dorsal side of the ex- tremity of the abdomen, and that there is no telson. This is developed only at a much later period from the dorsum of the end of the abdomen ; and, by its outgrowth, forces the anus to the ventral side of the body.
In the meanwhile, the oral aperture is developed behind the labrum, which moves backwards; while the maxiUsB, maxiUipedes and ambulatory feet appear in succession as elevations or ridges of the substance of the embryo, which are, at first, all alike, and gradually become speciidised into their ultimate forms.
* *Ueber die Bildang and Brochet de la Perche et de
Entwiokelimg des Flosskrebses,' I'EcrevieM,* 1862 ; and the ao-
Bd. 29. See also LerebouUet, countof Bobretaky'sretearchesin
* Beeherehei d'enibr>ologie com- Hofmann and Sohwalbe, ' Jahret-
pai^ for le d^veloppement da bericht' for 1873 0^7&)*
336 THB AHATOMT OF INYEBTBBBATBD AHIICALS.
When these appendages first appear, the maTillm and first pair of maxillipedes are attached to the embiyo in front of the thoracico-abdominal process, the seoond maxillipedes lie in the angle between them, and the third maxillipedes and following appendages are attached to the sternal surface of the thoracico-abdominal proceBi itself ; and, as this process is at first bent forwards upon the rest of the germ, it follows that the appendages attached to it look upwards, while those attached to the anterior part of the embryo look downwards. As develop- ment proceeds, however, the embryo gradually straightens itself; more and more of the anterior part of tiie thoracico- abdominal process becoming continuous in direction with the anterior part of the embryo ; until, at length, the whole of the cephalo-thoracic portion forms a convex surface, parallel with the vitellary membrane, only the abdomen remaining bent upon the ccphalo-thorax. The middle portion of the carapace is formed by the continuous calci- fication of the dorsal walls of the cephalo-thorax of the embryo. Its pleura are developed as two distinct folds, one of which, the rudiment of the branchiostegite, encircles the embryo posterioriy, and extends forwards on each side as far as the mandibles; while the other, the rudiment of the rostrum, and anterior cephalic pleura, is developed in front of the eyes, and extends on each side to meet the former. Rathke^s clear account of this matter is in perfect accordance with what I have observed in Mysis, and shows conclusively that the carapace is not developed from any one or two somites in particular^ but that its teif^ portion corresponds with, and is formed by, the terga of all the cephalo-thoracic somites, while the branchiostegites and rostrum are developments of the lateral portions of all these somites; in fact, represent their pleura, which, like the terga, are connate and continuously calcified.
The appendages are thus, at first, similar to one another, and each consists of a ridge which eventually takes the form of a plate^ free at the outer end. This plate, in ^all the
THB DBTBLOPMBHT OV A8TA0U8. 337
members, except the ophthalmic peduncles and the man* dibles, then becomes bilobed externally, the inner lobe re< presenting the endopodite, while the outer is the represent tative of the exopodite and epipodite. The two latter, when they are independently developed, become separated by the division of the outer lobe. The gills arise partly as ontg^wths from the epipodites, partly as distinct processes from the parts to which they are eyentually attached. The division of the limbs into articulations takes place from their distal towards their proximal ends. The heart appears late, at the posterior extremity of the cephalo-thorax, and therefore behind the yelk sac.
The nervous system of the post-oral portion of the ce- phalo-thorax consists at first of eleven pairs of ganglia, corresponding with the mandibles, maxilla, maxillipedes, and ambulatory legs. The six anterior post-oral ganglia of each side soon coalesce in pairs, so as to form as many single ganglia ; and of these the four anterior, namely, the mandibular, the two maxillary, and the first maxillipedary ganglia, unite into a single mass ; the two hinder ganglia, that is to say, those of the second maxillipedary somite, next coalesce in the same way, and it is only subsequently that the two masses thus formed become fused into the single anterior post-oral ganglion of the adult. The other ganglia not only remain separate, but become wider apart with advancing age. A ridge on each side of the cesophagus at first represents the cerebral ganglion and the com- missural cords, the latter being developed out of the pos- terior part of the ridge, and the former from its anterior portion. The cerebral g^anglia are at first two on each side, but the posterior, whence the nerves to the antennary organs proceed, is much larger than the other, and would appear to represent two ganglia. The endostemites arise as processes from each of the eight posterior cephalo- thoracic sterna, which eventually arch over the ganglionic cord, and unite with one another.
The alimentary canal is produced by the gpradual dif- ferentiation and demarcation of the sternal part of the
338 THE ANATOMY OT nTTSBTBBItATKD AVDCALS.
hypoblast, which invests the whole yelk, from the tergal part, which becomes the yelk sac.*
After the liyer, genitalia, and antennary glands axe deyeloped, the yelk sac eventnally becomes rednced to a small c»cal diverticalum, situated at the pyloric end of the stomach. The genital dncts in both males and females are originally diyerticnla from the corresponding regions of the genital glands; their external apertures and the copnlatory appendages of the first abdominal somites in the male are not deYel<^>ed nntil some time after birth.
The modifications of stmcture observable within the limits of the PodopMhalmia are exceedingly interesting.
Excluding, for the present, the SquiUidcR, the gronp ii divisible on clear morphological gronnds into the following subdivisions: — 1. The Bfuchyvra ; 2. The Anomwra; 3. The Mctcrura ; 4. The Schizopoda.
The morphological relations of the Macrwra are nearly such as are indicated by their position in this series ; and Astaeus, as a central genus of the central group, thus becomes a sort of natural centre for the whole of the Podophihalmiia, whence we may trace a gradual series of modifications, lead- ing on the one hand to the SchiMopoda, with their large abdomen and small cephalo-thorax ; and on the other to the Brctchyura, with their rudimentary abdomen and comjMura- tively enormous cephalo-thorax.
In all the Mticrura, the branchiae are numerous, and are covered by the branchiostegites. The abdomen is large, and is used as a locomotive organ, the appendages of its sixth somite being well developed. The thoracic ganglia usually form an elongated chain, and the external maxilli- pedes never form broad opennilar plates over the other jaws.
* A ccording to Bobretsky (/.c.) encrottches upon the vitellos, nntH
there \s no proper velk-sac, the the latter is all absorbed. The
Btmctnre lo termed by Bathke hypoblattic sao is converted into
beins the saccular hypoblast, the liver and the intestine. The
whion Is formed by invagination stomach arises independently by
of tbs primitive bkttodenn and invagination of the epiblast.
■■4
THB PODOPHTHALMIA. 339
In some of the lower Macrura {Peneus, PctatpJuBo), the exo- I>odite persists as an appendage at the base of the thoracic limbs ; and in two genera, Sergeties and Acetes, the posterior thoracic members become rudimentary or even entirely abortiye, though the abdominal appendages remain.
In the higher Maerura, snch as Palinurus, the nervous system exhibits a greater degree of concentration, the thoracic ganglia constituting an elongated oral mass ; and it is in this genus and its allies that the head and its appen* dages exhibit modifications, which prepare ua for those which are presented by the BraehyiMra. In this respect the PaUnwnu vulgaris (Bock Lobster, Sea Crayfish, or Spiny Lobster) is particularly worthy of attention. The rostrum is rudimentary and represented by a mere spine, leaving the anterior cephalic somites uncoyered. The cephalic flexure is so strong as to throw the ophthalmic sternum, which is yery wide, completely to the top of the head. The basal joints of the antennsB, or coxocerites, are enormous, fixed to the surrounding parts, and united by their anterior extremities in the middle line below. Supe- riorly, they seemed to have coalesced with the antennulary sternum, so as to form a projecting wedge-shaped mass, which separates the antennules from the ophthalmic ster- num, and causes them to appear, at first, as if. they were inferior to the antennsB. In this genus, the basicerite, ischiocerite, and merocerite are much thicker and stronger than the corresponding joints of any of the other appen- dages ; and in the closely allied ScyUarus, the facial iiegion of which is, on the whole, similarly constructed, these joints become extremely expanded and flattened, and are succeeded by no procerite. In these genera the scaphocerite, or squame, usually attached to the base of the antenna, is absent; and, in ScyUaruSf there is another approximation to Brachy- uran structure in the existence of distinct orbits, formed by a lobe of the carapace, which descends on the inner Bide of the ocular peduncle, to meet t^e base of the antenna. No median septum is formed by the rostrum, however, nor are the antennules capable of beinf^ lc&dkfidL
Li
340 THE ANATOMY OV INYBBTBBItATBD AKIMkLS.
back into distinct chambers in any Macmran at present known.
The Anomura are so completely intermediate in stractare between the Macrura and the Brciehywra, that they need not be specially noticed, except to draw attention to the singular deviation from the ordinary habits and form of the higher Crustaceans, presented by the Pciguridct, or Hermit Crabs, so common on all coasts. Essentially Ma- cmran in their organisation, these Crtutacea are distin- guished from all true Mcierura by the uncalcified and soft condition of the integ^ument of their unsymmetrical abdo- men, the appendages of which are for the most part abortiye, those of the sixth somite being modified so as to serve as claspers. It is by means of these that the Hermit Crab retains firm hold of the columella of the empty gasteropod shell into which it is his habit to thrust his unprotected abdomen, and covering over his retracted body with the enlarged chela, which takes the place of an operculum, resists all attempts at forcible extraction.
The internal structure of the Brcuihywa is, on the whole, similar to that of the McLcrwra ; but the thoracic ganglia have coalesced to a much greater extent than in Aataeug, forming a single rounded mass. The branchis are few, never exceeding nine on each side, and sometimes not more than seven. The branchiostegite fits closely down upon the bases of the four posterior pairs of thoracic limbs, and sometimes encloses a space which is very large in proportion to the branchiae. This is particularly the case in the Land Crabs (Oecareintut), where the spacious branchial chamber is lined by a thick and vascular membrane, which, in these almost wholly terrestrial CrugUicea, either takes on to some extent the respiratory function, or serves to keep the air within the branchial chamber saturated with mois- ture.
The abdomen in the Braehyura is comparatively small ; its sixth somite possesses no appendages ; and the othefs, if they exist at all, subserve only a sexual purpose, the two Miterior pairs commonly forming accessory copnlatovy
k> :
THB BSACHYUIU.. 841
organs in the male ; while, in the female, so many of these appendages as remain giye attachment to the ova, which are carried about nntil hatched, between the thorax and the abdomen, which is bent up against it. The female Bra4:hyura also possess a spermatheca attached to each oviduct, which is absent in the Macrura ; and, in this sex, the abdomen is larger and broader than in the males. In accordance with the rudimentary condition of this part of the body, the abdominal ganglia are represented only by a cord, which proceeds from the posterior part of the great thoracic mass. It is in the construction of their skeleton, however, that the BfuchyiMra present the most interesting deviations from the Macrura, Thus, if we select the common Shore-crab, Carcintts moetuu (Fig. 76), as a typical example of a Brachy- uran, we find that the carapace is a wide shield, broader than long, having a somewhat pentagonal shape, and bent sharply inwards at the sides, instead of taking an even sweep down to the base of the l^s. It is in such close contact with the four posterior pairs of thoracic limbs, as to leave no passage or aperture such as exists in Astacua, the only inlet for the water required for respiration being placed above the basal joints of the chelate anterior ambu- latory limbs. The edges of the carapace pass completely in front of the bases of the limbs, and then turn suddenly forwards, parallel with one another and with the axis of the body, as the pterygostomial plates of Milne-Edwards, to join the antennary sternum, which is very wide, but short from before backwards. The space included be- tween the edges of the pterygostomial plates and the antennaiy sternum is the " cadre buccal," or peristome ; the antennary sternum itself receives, as in the Astacus, the specific appellation of epistoma ; and the plate which stretches backwards and supports the labrum, within its posterior forked boundary, is the endostoma.
The middle of the dorsal surface of the carapace is marked somewhat nearer its posterior than its anterior boundary by a short transverse depression, which is continued on each side forwards and outwards, and then curves directly ouV
342 THE ANATOMY OT IirTSBTBBItATBD AHIXAIiS.
irards to the edge of the carapace (Fig. 76, e t). Further than this I cannot ti'ace this homologae of the cervical
Fig. 76.
^
Fig. 76.— Of the two upper figures the left represents the doml lurliMe of the cftrapAoeof ComniMniflnia*. /. Rostmm. o. OrUt. c. «. Ger- Tical groove. ^. Epigastric lobe. g\ Protogastric. ^. MesogM- tric g*, Hypogmstric. g^, Urogastric. c c^ Anterior and posterior cardiac, h. hepatic, ft*, 6*, 6*. Epibranchial, mesobranchial, md metabranchiftl iobes. The lower figure represents a Tentrai Tiew of the anterior half of the same carapace, a. Rostral septom. h. Antennary sternum, c. Suture between these, d, SunrmcaUaiy lobe. €. Internal suborbitar lobe. /. Antenna, g. Articular cavi^ for the ophthalmic peduncle, h. The same for the antennule. o. Or- bit. jA. Subhepatic region, ep. Anterior pleural region. The rigfat- haad upper ^gure gives a tide view of the carapace of fifaao- rAyadka* />4atoiyt»w, the common ** spider-crab.*' o. Orbit. /.>| /^Boatrom. aJU Antennule. of. Antenna. 9. Epistoma.
groove of Adacm: Elevations and depreanons upon the of the oara>paoe in front of the oorvioel groove,
THB BBAOHTCnUL 343
which, as in Agtaeui, is oomposed of the ooxmate terga of the six cephalic somites, mark it out into certain definite regions of considerable systematic importance. An irregu- lar transrerse depression, crossing the carapace near the anterior margin, bounds an anterior or facial region, divided into a middle fironkd lobe (/), and lateral erbiUU lobes (o), from a posterior, much larger, gastro-hepatic area, diyided into small lateral hepatic lobes (h), and a large complex gadrie lobe {g\ g*, etc.)* The latter is again subdiyided into two ^pigcutne lobes (g^), two pratoguuirie lobes (9*), a median meeogasbrio lobe (^), two meUigatstrio lobes (^), and two wrogagtrie lobes (^), making altogether nine subordinate diviaions. The gastric lobes correspond in a general way to the stomach; the hepatic lobes, to a portion of the Urer. The region behind the cervical suture consists of the connate terga of the eight thoracic somites ; it is divided by two strong longitudinal grooves, the branchio' cardiac grooves, into a middle region, corresponding with the heart, and two lateral regions, forming the roof of the branchial chamber. A transverse depression divides the middle region into an anterior and a posterior cardiac lobe, while the branchial region is subdivided into epu branchial (b^), meiobranchicU (6"), and metdbranchial (l^) lobes.
On turning to the inflected inferior portion of the carapace, a sutural line or groove is seen running from the epistoma, outwards and backwards, very nearly reaching the outer edge of the carapace, opposite its external angle, and then sweeping backwards parallel with, and but little distant from, its postero-lateral boundary, until it cuts its posterior edge. The portion of the carapace internal to this sutural line, is called by Milne-Edwards the inferior branchio- gtegiU, and is considered by him to be composed of an anterior {e p) and posterior ^pimeral piece, corresponding with the tub-hepatic (« h) and sub-branchial regions of the surface of the carapace between the suture and the line of inflection. I cannot regard these paxts, however, as having any relation with the true epimera. The sutnxe, or t^^Jbeit
Mi thj: anatomy of invertebrated animals.
groove, seems rather to correspond with that which markB off the pleuron from the rest of the somite in AtiaeuB,
The anterior cephalic somitee in Careinus have undergone some singular modifications, whereby their true relations are g^reatly obscured. The broad trilobed plate (Fig. 76,/) correq)onds with the elongated rostrom of AttaoiM; is< f eriorly it is produced in the median line into a strong ridge or septum, the lower and posterior edge of which is convex, and fits closely into the concavity formed by the antennulary and ophthalmic stema» as they bend back from the sternal flexure. This rostral aeptnm, therefore, abuts below and behind on the epistoma, and constitutes a sort of partition (Fig. 76, a), by which the cavities in which the antennules and eyes of the two sides are lodged, are completely separated from one another. The lateral portions of the rostrum form a flattened roof over the inner portions of these cavitieB, which contain the bases of the ophthalmic peduncles and the antennules ; but the outer angles of the rostrum are produced downwards (d), to form the w/pracUiary lobe* The outer half of the lateral cavities or chambers is more excavated, and is bounded by a strong pointed process, the external orbitar lobe, which is divisible into a eupra-orbiial and avh-orbUal portion. The latter passes gpradually into a strong process of the sub-hepatic region, called the ifUemal eub-orbiiar lobe (Fig. 76, e); this turns forwards and upwards towards the supraciliary lobe, which it ap- proaches, but does not meet, the base of the antenna being, as it were, wedged between the two.
The- supraciliary, external orbitar, and internal sab-oir- bitar lobes, and the ant^insB, thus together circumscribe a cavity widely open in front, which is called the orbit, inasmuch as it lodges the terminal portion of the opbthal- mic peduncles, with the eyes which they support. The proximal portions of the peduncles pass through the oomparatively narrow opening by which the inner and outer chambers communicate, between the antenna and Hie sapmciliary process, jand are inserted as usual into th^
THB BRLOHYJJRk. 345
articular oayities on each side of the opbihahnic stemuin, which is narrow, and hardly wider than the septam. It thus comes to pass that the eyes, lodged in their orbits, appear to be altogether external to the antennules, the enlarged bases of which hide the ophthahnic pedoncles, and appear to be the sole contents of the inner division of the sub-frontal chamber; but the trae position of the eyes is precisely the same as in Astaeua, that is to say, anterior and superior to the antennules. Another interesting peculiarity about the facial region of the carapace is that the basal joints of the antenna have coalesced with the sternum of the antennary somite, and, consequently, that the bases of the antennse are immoTcable. There is no yestige of a scaphocerite, and the aperture of the organ which answers to the green gland of AMtaeua, is proTided with a peculiar moreable plate, provided with a projecting internal stem, to which delicate muscles are attached in Carciwus, It is this structure which has been compared to an auditory ossicle; but, as in Astaeua, the auditory sacs are, in fact, lodged in the dilated basal joint of the antennule.
A cervical fold, lodging the scaphognathite, occupies the same relative position as in Astticus, and marks off the cephalic from the thoracic region, on the sides of the body. The thoi*acic sterna gradually increase in breadth, and the posterior ones are marked externally by a strong median, longitudinal depression, answering to a corre- sponding fold on the inner surface. The apodemal cells are well formed, but the sternal canal, so largely developed in the Macrura, is absent in this, as in all other BrcLchi/wra.
The structure of the appendages is essentially the same as in AHcums, but the third thoracic appendage, or external maxillipede, has its ischiopodite and meropodite greatly en- larged, 80 as to form a broad plate, which, with its f^ow, covers over the other organs, and hence receives the name of the ffntUhostegi^. The three terminal joints of the limb remain small, and constitute a palpiform appendage — ^the endognaikal piUp.
346 THB ANATOMY OS* DfTS&TBBItATSD AHIKALS.
In some of the lower Maerura, the thoracic limbs are pro- vided with a short exopodite, and the posterior maxillipedeB become indistinguishable from the ordinary thoracic limbs. Such forms lead us naturally to the SchiMpadOy a group the name of which is deriyed from the apparent splitting at the limbs produced by the great deyelopment of the exopodite, which, in these Crustacea, is as large as the endopodite. In this group, again, a line can hardly be drawn, in many cases, between any of the maxiUipedes and the thoracic limbs, the anterior pair only being somewhat smaller than the rest. Hence Thysawjpoda is admitted, by Milne-Edwards, to have eight pairs of thoracic limbs (' Crustac^,' ii. 464). The branchiae in the SehiMopoda are frequently absent; when well developed, as in 2%- aanopoda, they are not included under the branchiostegite, but hang down freely from the bases of the thoracic limbs. In Mysis, the only representative of a branchia (if it be one in reality) is a process attached to the first thoracic ap- pendage. CfffUhia has its branchial appendages attached to the abdominal members.
In Thyganopoda, Mysia, and Cynthia, the general struc- ture of the body is similar to that of the Maerwra, except that, in Mysis, the greater number of the abdominal appen- dages are rudimentary.
In Leueifer, the antennary somite is produced into a very long and narrow peduncle, which supports the eyes, on their great stalks, the antennules, and the antennse, at its extremity, separating them from the rest of the cephalo- thorax, which is covered by a delicate carapace, bent down at the sides. The anterior thoracic members are rudi- mentary, and the posterior pair is absent. The heart is short and rounded, and situated, as usual, in the thorax.
It has been seen that in Asta4>iut fluviaHUs^ as in Limulus and Daphnia, the embryo slowly and gradually passes into the form of the adult ,* to which it is so iniwilM* when it leaves the egg, that the changes of the young present nothing comparable to the well-known metamor- phoses of Butterflies and Beetles.
THB BBACHTimA.
S47
But most Podophthahnia rather resemble the Copepoda and the majority of the Eniomo$iraca, in the fact that the joung, when they leave the egg, have a totally diBHimilar form to that of the parent, and only acquire the adnlt condition after a seriee of eodyses.
The obserrationB of Fritz Miiller * have shown that the yonng of a species of Prawn (Peneua) undergo a metamor- phosis which mns parallel with that of the Copepoda,
Fig. 77.
Fig. 77. — Peneus.^Ay Nauplivs-atAge, B, Zocta or Copepod stage. C, Sehizopod-BtBifiB. (After Muller.)
When it leaves the egg (Fig. 77, A), the young Penetu has an oval, unsegmented body with a single frontal eye, a large labrum and three pairs of natatorial appendages — ^it is in fact, to all intents and purposes, a Navpliua. The NiaupUuS'torm next developes a rounded tergal shield, or carapace ; the first and second pairs of appendages remain- ing long, become the antennules and the antennse ; while those of the third pair, their bases enlarging at the expense
* •Fiir Darwin/ 1864.
348 THE AHATOMT OF INYXBTBB1U.TBD ▲VOCALS.
of the rest of the appendage, become the mandiUeB. Four pairs of appendages subsequently appear behind the man- dibles. The hinder three pairs are bifurcated and beoome the two pairs of maxilltB and the first and second mazilli- pedes. Behind these again are five pairs of short lamellar processes, which eyentoaUy are conyerted into the rest of the thoracic appendages. The six somites of the abdomen an long and distinct, and the last ends in two setose prooesaei. Thej are at first without appendages. In this stage (Fig. 77, B), which answers to the so-called ZocBO-torm of other Podophthalmia, the principal locomotiye organs are the antennsB and antennules, and the resemblance to an adult Copepod is so striking that it may be termed the Copepod stage. Next, the antennsB, diminishing in relation to the rest of the body, cease to be the principal organs of locomotion, and the rapidly elongating abdomen assumes that function. The stalked double eyes which made their appearance in the Copepod stage, become more fully dere- loped. The jointed exopodite of the antenna is replaced by a single plate. The greatly enlarged thoracic limbs are provided with an endopodite and an exopodite, as in the Sehizopoda, the branchisB are deyeloped from them, and the abdominal appendages make their appearance. This may be termed the Schizopod stage (Fig. 77, C). Lastly, the median eye yanishes, the exopodite of the locomotiye thoracic limbs disappears, and the larva assumes all the characters of the adult Feneus.
In the great majority of the Podophihalmia the embrjo undergoes as remarkable a metamorphosis after it leaves the egg. This fact was first indicated by Siebold, after- wards demonstrated by Yaughan Thompson, whose obser* vations have been confirmed and extended by many more recent observers, notably by Spence Bate* and Glau8.t But the stages of this metamorphosis differ from thoae
* <*On the Development of stracenlarven.** (W&rzbiirg * |fs>
Decapod Crustacea." (Philoao- tumisHenBchafUiohe Zeittohrifli'
phical Transactions, 1857.) 1861.)
t ** Zur Kenntniss der Malako-
TRI DXTSLOPMBm OT TBI PODOPHTHAUfU. 34d
obaerred in Peneiu in the apparent absence c^ the first or JVawpIitu ooodition. Fosoibly, however, this is represented 1^ a delicate cnticulor iavestmeiit which the larva throws off soon after leaving the e^. It then corresponds with the later form of the Copepod stage of P«n«Ht, and is termed a Zomi. The Zoom has a short carapace, of t^i provided in the median frontal and dorsal regions with long spine-like prolongations. There is a median simple ej'e between the lateral sessile faceted eyes, a pair -of antennnles, a pair
Fig. 78.
of ant«iuiB9, a pair of mandibles, and two pairs of masilln ; in short, all the appendages of the head. Of the append^es of the thorax, the first two pairs are well developed, and terminate in an eiopodite and an endopodite. But behind those, which become the first and the second pair of maiil- Upedea, onlj short radimente of the six remaining pairs at thoracic appendages are to be foond, and the somite* of the long abdomen have no appendages at all. Subse- qocstl; these make their appearance, the posterior thorsciu
S^ix' ZSM AJAXomr K'j
■jsemixn :^-:r«r;iae in hzc. t'-e ejes becose raiaed npon j42^r: p*i:22--*le5. ±r I :"-■* Lin-A r«-eiiible« one of the lower Jf-j«rvi. Tz-i canr^iir =.er: c^x-mea broader, and its iffL^e* si'.r^cr. -^-lilr :"i.r A:r:b-ili:orT thoracic limbs take .'S tj-e .:I:-irA.r:crs -.f :h:5^ ■:■£ the a:i:ili, the bifurcated first i2.i i*^.vai pairs r^:'.min^ metamorphc^ed into the fint Aa«i w-.x^ni ziAxillir^i-r*. The ac-iomen becomes relatively jj^. r: ani slenier. a.ni the Lirva takes on the characters :t .'ce of the -lF.:"i'ir.i. In :"..:s sta^ it has been named .V V'l* y*!- Bv fv.r:b.er ■. haa^r^T? in the same direction, the AaoE'^ran c».nli::.n passes in:-:* that of the young Bra- /jTuraa. Ail ihes-? m-.-iinoations of form are accompanied ?T exuviations of the ohitinou? oiitioula.
The «uoi.'os*iive sta^e.* are well exemplified bj the jonng of the Shure-orab. Oir:-'t u^ }y\'iu'vt .Fig. 78, A, B, C). The larva, on leavinj the ej;^. has sessile eyes, a long ix'inred rostrum, an- 1 a sT-ine projecting from the middle of the oarapavo : ruilimentary antenna?, and two pairs of kvomotive appondaires — the ruiliments of the anterior aiixillipedfs. The ali<li>minal somites are without appen* Jftare^. and the telson is l-roa*! and Inlobed iFig. 78, A).
This, the Ztxea stairo. after repeated ecdyses, assumes the yi^ilopa form represented in Fig. 7s. B. Finally, the oarui^ioe l^eoomes broailer. the abdomen loses its appendages, And is bent up under the thorax : the peculiarities of the ta^'ial region, charaoteristio of the Brachyura, are developed ; the antennules and ambulatory mem]>er8 acquire their cha- •^neristic proportions ; and the little Brachyuran by degrees jitumes the special peculiarities of Carcinus [Tig. 78, C).
The development of the Opossum Shrimp (My»is)* is par- f^iilarly interesting, as it appears to indicate the rela- li,90B between the two modes of development, that with and j|yt without metamorphosis, which obtain in the Crustacea
ri«.79).
Ifce ova consist of a vitelline mass, enclosed within t jilieate chorion. The blastoderm apx>ears as an oval patch
Beneden, " IXcveloppcmcnt des Mysif.** (* Bolletln Bnizellei,' 1869.)
THS DITBLOPMXHT 07 MT8I8. S51
Rirfaoe of tHe yelk (Fig. 79, A, e), thickeflt in the nd here presentiiig a more or leas marked de- Fig. 79, A, B, e). It is sharply defined from the yelk (h), and consistB of a finely granular mass, moltitades of nuclei, about yg^ to j^^ of an ■meter, are imbedded.
Btoderm next becomes larger at one end than at
and a median sinuation gradually diyides this
into two lobes, which will eventually form the
Moietes of the head, and may be called the pro-
lian depression becomes more decided, and, at the ite theprocepbalic lobes, theblastoderm is produced b of papilla, directed forwards. This is the rudi- le caudal extremity. From the anterior part of the Q there arise, on each side, two papiUsB, the points ore directed backwards, and which will become the IS and antennae. The whole of these parts are >y a delicate cnticular membnvne, which gradually rer and invests the whole yelk beneath the vitellary 3. At the end of the caudal papilla it forms a cess, produced into eeta), which sometimes appear sometimes so deeply bifid as to resemble two
bryo has now reached what we may term its larval ; in this condition, it leaves the vitellary membrane lich it was enclosed, and lies free in the ovigerous iie parent. At the same time, the caudal extremity and straightens itself out, so that no indication rious inflexion against the thoracic portion of the n remains. The larva thus much resembles a pear
ceeediDgly interesting the vertebrate embryo; and the
the correspondence cephalic flexure of the Cmsta-
e embryonic structure cean or Insect has its analogue,
I of Myitis (and I may if not its homologue, in the angle
other Arthropoda)^ And which the trabecular region of
head of a vertebrate the base of the skull at first makes
The procephalic pro- with the parachordal region in
mble in a remarkable almost all Vertebrata, e trabecule cranii of
352 THI AVATOMT OT UITIBTXBBATU> *»Tif*iJi,
(Fig. 79, D, !E), with four processes (2, 3). the anteuiinki and aiit«jitue, which have now become much elongated, on i surface.
The joung Xygit next growi lapidl; and Trndergoes great changes in form : but it is a Terj remarkable fact, Uiat tbe primitive integnment remains tmaltered; gradnaUj ea- larging, to accommodate itaelf to the increased aiae of the ftetua, indeed, bat otherwiae taking no share whaterer in its changes. The young Myrit might, therefore, in t condition be joetlj termed a papa, for Hie relation of tiw primitive integnment to tbe animal which it encloses ia preciselj that of the pupa skin to the imago of an insect
Fig. 79.
THE DXTBLOPHBHT OF 1 Fig. 73-,
Pig. 79.— Th« development of AJyiii.— A. Side
which the blutodenn bM jrut appeued. B. Side view further *d- Ttneed. C. Front view of embrro at the nme age, thowlng the proccphalle lobe*, here marked b. D. Larre, TOotral Tiew. " Side Tiew. (Theie two %urea have heen Inverted b; the graver.) F. Toung pupa. G. Further advanced, U. Young ^' «,iihicb hag lettltapupaakin. I. Aatvrloi portion of the ume, ' h the carapaoe ttuown hacic. a. Vitelline meni- I. Tellt. c. Central depresaloQ of the blaelodenn. <f . Pro- cephalic lobe*. /. Larval integument j. Iti caudal eol ' — '
3fViu, which enlarged, and
The anteniMilea and antemue renuun intact within the ■heaths afforded bj the primidre integnment, bat, becoming immenBel; elongated and divided at their extremitiee, aamune more and more their proper adult confonnataon.
In front of the antennulei. a large rounded protsbeisncc mokea ite appearanoe upon each procepbalic lobe, and tnentaaHy beoomea the ophthalmic peduncle. At flivt, the sternal portione of the eomitee, coireepondii^ witti Otmt three pairs of appendage*, occupy the nme plane with tmt % k
354 THE ANATOMY OF nrVEBTEBRATBD ANIMALS.
another and the posterior sterna (Fig. 79, F, G) ; but, by de- grees, they become bent up (Fig. 79, H), and at length the ophthalmic sternum occupies the upper and front part of the head (Fig. 79, 1). In this way the " cephalic flexure " is produced. The mouth is indicated behind the anten- nary sternum, which projects backwards in the middle line to form the labrom. On each side of it the radiments of the mandibles appear, and behind these are the papil- lary commencements of the two pairs of maxillsB. Behind the second pair of maxillse, a distinct constriction in- dicates the commencement of the thorax, the appen- dages of which appear, at first, as tubercular elevations, all of precisely the same character, and all directed back- wards parallel with one another. The abdomen is at first very small, and the appendages of its sixth somite early acquire a far larger size than the others. The telson is developed from the middle line above the anus. While all these changes are going on, the blastoderm gradually extends over the tergal surface of the embryo and closes it in. When the carapace is first distinguishable it appears as a ridge arising from the sides of the posterior thoracic Idomites, beginning at the last but one, and gradually extending forwards as far as the antennary somites. The ridge increases and becomes a fold, which overhangs the bases of the thoracic appendages (Fig. 79, G) ; and if this fold be turned back (Fig. 79, I), its actual attachments may be readily demonstrated.
Having advanced thus far in its development, the foetal Mysis, with all its organs fully formed, though somewhat different in appearance from those of the adult, casts its pupa-skin and straightens its body, which from having its posterior portion bent on the anterior, as in the embryo (Fig. 79, B) had gradually in the pupa (Fig. 79, F, G) assumed the opposite curvature. Its dimensions are three- fold those of the embryo, and it exhibits yivacious moye- inents when extracted from the pouch of the parent. It is not improbable it may yet undergo another change of in* .tegamfint before acquiring the full form of the adult.
THX OLl^S-CBABS OB PEYLLOSOIUTA.
355
Thna it appears that in Myei», tlie Naupliw-atage (Fig. 79, D, E) ia passed over eo rapidly that the embrjo has gone through it at a verj earlj period, and nothing but the cnticnlar eheath of the bodj appertaining to this stage remains to prove its existence. A step further, in the ab- breviation of the ^aupJttM-stage, and there would be nothing
le body; C,
oXIQ'n,
le nenodi ajBtem; I>,'thc
to distingoiah the general coarse of the development of Mynt from that of Aitactu. On the other hand, anothei- Schizopod, Euphangia, has been shown by Uetscbnikoff * to leave the egg as a true NaupUus.
• ' ZdUchrift fur WiM. Zool.,' 1371.
1 Kl
356 THB iLHATOMT OF IVYEBTIIBRATBD AVDCALS.
The Glass-crabs, or PhyUosomaia (Fig. 80), are mngnlar marine pelagic cmstacea, in which the body connsti almost wholly of two large, extremely flat and transparent disks, devoid of any segmentation. The anterior of these bears the pedunculated eyes, the antennnles and the antenna on its anterior margin ; while the labrom, with the mandibles and anterior pair of mazillsB, form a small projection posteriorly on its yentral surface. The second pair of mazillae is situated a little more backwards and outwards, and bears a scaphognathite ; and just behind these appen- dages is the fold of a cervical groove which separates the anterior disk from the posterior. The anterior disk con- tains the stomach and the liver, and in this respect, as in its appendages, corresponds exactly with the cephaloetegite of the carapace of an ordinary Crustacean, and its six cephalic sterna. The posterior disk, on the other hand, contains the short and almost roimd heart, with the intestine, and bears the eight pairs of thoracic appendages, the anterior and posterior of which are not uncommonly rudimentary. The abdomen is usually very small, and situated in a notch at the posterior edge of the thoracic disk. It is provided with six pairs of appendages. No generative organs have been found in the PhyUosomatat and there is reason to believe that they are merely larv» of the Mocruran genera PalinuruSf Scyllarus, Thenua, and their allies.
The CirMA.CE A.. — These are very remarkable forms, allied to the Schizopoda and Nehaliaf on the one hand, and on the other to the Edriophthalmia and d^pepoda ; while th^ appear, in many respects, to represent persistent larvs of the higher Crustacea,
Ownia Bathkii might, at first, be readily mistaken for a Copepod. It possesses a comparatively small thick cara- pace, apparently produced into a rostrum anteriorly, and succeeded by a series of twelve gradually narrowing free segments, the appendages of which are in great part ob- jBolete. The last of these segments is a pointed telson ; the
k* .
THB CUMACBA. 357
anterior five, belonging to the thorax, bear thoracic limbe, while the eleventh, the last tme somite of the body, carries its characteristic stjliform appendages. The appendages of the preceding abdominal somites may be either absent or very small and rudimentary. Dohm has proved that this is tme only of the females among the Cwmaeea, The males, which were formerly referred to the genera Bodatria and Alauna, often have well-developed abdominal limbs, though they appear late. It is interesting to find that the females, in this respect, retain more of the larval character than the males.
On examining the apparent rostrom with care, it is f onnd to be divided along the middle line by a fissure which runs in front of the eye (which is here single and sessile) divides into two branches, which run backwards and outwards, and terminate, before traversing half the length of the carapace ; they thus cut off a median lobe, bearing the eye at its apex, from two lateral processes. The lateral processes are simply prolongations of the antero-lateral regions of the posterior division of the cara- pace (as it were the antero-lateral angles of the carapace of Mysie, excessively produced and meeting in the middle line) ; while the middle lobe corresponds, I believe, with the cephalostegite of the carapace in ordinary PodophthaJmia, the insertions of the mandibular muscles occupying their normal position, towards its posterior boundary. The hinder part of the carapace will therefore correspond with the terga of the three anterior thoracic somites, the five posterior ones being, as has been seen, free and moveable.
The five anterior pairs of thoracic appendages are con- structed much on the same plan as those of the Schuopoda ; the three posterior have no exopodite. In the female, the sixth abdominal somite alone has appendages, but in the male the two anterior abdominal somites are provided with styles. O vigorous plates are attached to the fourth, fifth, and sixth thoracic appendages in the female. The structure of the head is peculiar. No ophthalmic sternum nor oph- thalmic peduncles are discernible, the single, or closely
358 THE ANATOMY OF INYBBTEBBATED AITIMALS,
approximated two, eyes being sessile on the median line on the superior surface of the head. The coxopodites and basipodites of the antennules and antenns are bent down almost at right angles with the axis of the body, and appear to be connate, or confluent, with their sterna. The succeeding joints are free and pass forwards, the anten- nules being much longer and stronger than the antennai in the females, while, in the males, the antennso are yerj long; the labrum is large; the mandibles strong and unprovided with a palp. There is a distinct metastoma, and the maxillffi are delicate and foliaceous. A papillose branchial plate is attached to the base of the first thoracic appendage. The surface of many parts of the body in some species exhibits a very peculiar sculpturing, singu- larly like that exhibited by the Eurypterida,
As in the FodopMhalmiay the heart is short or moderately elongated, and situated in the posterior part of the thorax.
Dohm* has shown that the development of the Cumtuea takes place without metamorphosis. In most respects the embryo resembles that of Mysis ; but, instead of the cuti- cular investment of the transitory Nauplius-staige with its two pairs of appendages, there is only a sort of cuti- cular sac with a thickening in the middle line of the tergal aspect, which the embryo bursts as it acquires a larger size. In this respect, the resemblance of the embryonic development of the Onmacea to that of the EdHophihalmia is, as Dohrn points out, very striking, and no doubt they form a connecting link between the Podophthalmia and the Edriophthalmia. Having regard to their whole organisa- tion, on the other hand, they stand at the bottom of the Malacostracan group, and are comparable to a PeneMB- larva in the Copepod stage, the limbs and body of which are modified in the direction of the Schizopoda, while the fore part of the head has remained Copepodous.
Fossil Brachyura are abundant in tertiary deposits, but are rare in formations of earlier date. J&focrura of a pecii«
* « Ueber den Bau imd die Entwickelung der Camaoeen." (' Unter- saohangen uber Bau und Entwickelong der Arthropodeo,' 1870.)
THB EDBIOPHTHALMTA. 359
liar type {Bryon) occur in the mesozoic rocks, and perhaps the carboniferous Chimpaonyx should be referred to the Podophthahnia.
The Edbiophthalmia. — ^These resemble the Podophthal- mia in never possessing a greater than the typical number (20) of somites, though, in some members of the group, the body is composed of fewer somites, in consequence of the abortiTC or rudimentary condition of the abdomen. Eyes may be absent ; when present, they are usually simple, and are either sessile or seated upon immoveable peduncles (Mtmna), The antennules almost disappear in the terres- trial Isopoda, while the antennss become rudimentary or vanish in some Amphipoda. The mandibles lose their palps in the WoodHce ; which thus, as in the presence of only one pair of well-developed antennary organs, approach Insects. Ordinarily, the posterior seven, and, at fewest, the poste- rior four, thoracic somites are perfectly distinct from, and freely moveable upon, one another. The ophthalmic and antennary somites have coalesced with the rest of the head; the branchise depend from the thoracic limbs, or are modifications of the abdominal appendages; and the heart is elongated and iminy-chambered. But the salient characters of the group will be best understood by the study of such a genus as Amphiihoe, the principal details of the organisation of which are reptesented in Fig. 81.
The body of this animal is compressed, bent upon itself, and divided into fifteen very distinct segments, reckoning the head as the first and the telson as the last.
The head presents a rounded tergal surface ; the anterior face is disposed perpendicularly to the axis of the body, and is produced anteriorly into a strong, curved, and pointed rostrum ; on each side, it bears an aggregation of simple eyes, and, in front, immediately beneath the rostrum, this face gives attachment to two long, many-jointed anten- nules. Below these, two antenns, shorter, and fewer-jointed than the antennules, are inserted, and the inferior part of the face is completed by a large moveable labrum. Behind
B come the ationg. toothed palpigaoiiB .n^nJiMM (ITO. Fig »I.
Fig. 81— ^n/iAitAoF.— Ths 1ett«n Mil flguras have the Mnt algnUo^ lion ■■ Id other Sguret of Cru>t>c» except «, ooategite. A'. bnuchin. C. LatenI Tiaw of itonwcb (D) opeaed (rwa abot*. a,i,e. DifTerent pwUof thearnuturc.
and two pain of more or less foUaceoiu mftrillff, Insa-
TRB BDBIOPHTHAUOA. 861
le ejea are sessile, these five pairs of appendages .t belong to tlie head proper ; but, just as in the ^/mia, certain of the anterior thoracic appendages ted into accessory gnathites, so, in AmphUhoe, »air of these members are applied against the 1 form a large lower lip (VII'). )ad " of Amphithoe, therefore, is formed by the 3 of the seven anterior somites of the body ; but lat the tergum of the seyenth (or first thoracic) bsolete, as in a Stomatopod, and hence that the face of the head of the Edriophthahma corre- ictly with the cephalostegite (or that part of the yhich lies in front of the oerrical grooTe) in InUa. Mr. Spence Bate has shown, in his valu- »rt on the EdriophihcUmict,* that, in the Cnutaoea nnder discussion, a strong apodeme arises on Erom the posterior part of the sternal region of ind passing inwards and forwards meets with its orm an endophragmal arch, which supports the I and stomach, and protects the nervous com- etween the first and second sub-oesophageal hich runs under it.
overer of this structure conceives that it repre- erga of the three somites immediately succeeding I ; but I cannot see that it is other than the re- re of the precisely similar mesophragm formed berior apodemes in Asiaeus, In fact, the corre- in structure between the head of an AmphUhoe )phalic portion of the cephalo-thorax of Astcunis tie striking. There is the same sternal flexure, elative position of the stomach, and of the inser- he mandibular muscles. The great difference 3 abortive condition of the ophthalmic appen-
; endophragmal arch ent connexions (Fig. 83). A very
sub-cesophageal gan- similar endophragmal arch is found
mmiBBures m>m the in the Insect nead. Seethedescrip-
uUia^ but has differ- tion of the head of BhUa {hrfru).
362 THE ANATOMY OF INYEBTSBRATBD AHOCALB.
The Beven free somites of the thorax each give attachment to a pair of limbs. It is characteristic of Amphiihoe, as of the Amphipoda in general, to haye the five anterior pairs of thoracic members directed for- wards. Each limb consists of an expanded coxopodite, succeeded by the other six joints of the typical cmstaoeaii limb.
In the male, a single vesicular lamella, the branchia, is attached to the inner side of the coxopodite of the appen- dages of the ninth to the fourteenth somites inclusiTelj ; but, in the female, an additional plate, convex extemallj and concave intemallj, is attached above and internal to, the branchia of the 9th to the I2th somite. These oods^ gUeSf as they may be called, enclose a cavity in which the incubation of the eggs takes place.
The abdomen consists of six somites and a veiy small terminal telson. The appendages of the three anterior somites are terminated by two multiarticulate setose fila- ments (Fig. 81, XV) while in the three posterior, the corre- sponding parts are siylif orm, and serve as a fulcrum for the abdomen when the animal leaps, by the sudden extension of that region of the body.
The Edriophthalmia are ordinarily divided into three groups. The Amphipoda, which resemble AmphWioet are characterised by their compressed form and their ordinarily saltatory habits ; by having thoracic branchise ; by the for- ward direction of their four anterior locomotive limbs (2nd to 5th pairs of thoracic appendages), and by the contrast between the three anterior and the three posterior pairs of abdominal appendages. The common Sand-hopper is the most familiar example of this division. The second group is that of the Lcemodipodaf distinguished by the rudimen- tary state of the abdomen, which is reduced to a mere papilla, and by the coalescence of the second, as well as the first, thoracic somite with the head, so that the anterior limbs appear to be as it were suspended under the necL The strangely formed genera Cyamus, the parasite of whales, and CapreUa, which is very common upon our own
THX KDSIOPHTHAUfU. 363
eoMt, adhering to coralliaeH, sea-weeds aud etarfisli, beioi^ to this group.
The Iiopoda, which couHtitute the third group of the Edricphthalmia, are UHaally depressed instead of com- preMed, and rtm or crawl instead of leaping. Hanj, like
Fig. ea.
1 Woodlonse (OnUci rolling themselvea into a ball i the last-named genns, are terrMtrial; others, like the A»eUv», inhabit fresh waters, bnt the great majority are marine; and among them are many pecoliarlj modified
364 THB AITATOMT 07 IHTBBTSBRATXD AHDCALS.
parasitio forms (Fig. 82, Cymoihoa ; Bopyrus). The ooiii* position of the head and mouth in the laopoda is essentiAllj the same as in the Amphipoda, though differing oonaider- ablj in details. The branchiie of the thoracic members are absent, their functions being performed bj the endo- podites of some of the abdominal members, which are soft and yascular. The three anterior pairs of thoracic members are usually directed forwards — the four posterior pairs back- wards. In some Jgopoda the abdominal somites, partly or wholly, coalesce with one another.
In all the EdriopMkdImia the alimentary canal is straight and simple, and its anterior gastric dilatation, frequently strongly armed, is situated in the head. The liver is repre- sented by a variable number of straight csBca.
Occasionally there are on6 or two cseca which open into the posterior part of the intestine, and appear to be mrinaiy organs analogous to the Malpighian caeca of insects.
The respiratory organs vary greatly in structure. In most Edriophikalmia, they are simple plates or sacs, the delicacy of the integument of which permits of the free exposure of the blood circulating in them to the air. In the amphi- pod genus Phrosina, however, the branchiae are composed of rudimentary lamellae, attached to an expanded stem, and resemble not a little the epipoditic branchiae of Astacut. In some SplueromidcB, Duvemoy and LerebouUet found the branchial endopodites transversely folded, so as to approach those of the Xiphosura,
The exopodites of the abdominal members of the Inpoia frequently cover the modified endopodites, forming opercu]a» and the first pair of abdominal limbs is, in many genera* altered in such a manner as to form one such large oper- culum for the four pairs which succeed it. In the IdatMa it is, on the other hand, the sixth pair of abdominal limbs which are so modified as to form the curious door-like oper- cula which cover the gills.
In certain of the terrestrial Isopoda, (PoreelUo^ Arm^ diUidifimj) some of the opercular plates of the branchiiB, usually the two anterior pairs, contain curiously ramified
THB KPBIOPHTH ALlf I A . 365
cavities, wldoh open extemallj, and contain air. The genus Tyhs possesses respiratory organs, which present a still more interesting approximation to those of the pnrely air- breathing Articulata. Thej are thus described by Milne- Edwards: —
"The abdomen presents inferiorly a deep cavity, very similar to that of the SphceromcB, in which the five anterior pairs of appendages are lodged; but this cavity, instead of being completely open below, is imperfectly closed, in its posterior half, by two series of lamellar prolongations, which arise from the sides of the inferior faces of the third, fourth, and fifth abdominal segments, and pass horizontally in- wards ; the first pair of these plates is small, those of the third pair are, on the other hand, very wide, and almost meet in the median line. The four anterior pairs of abdo- minal appendages, lodged in this cavity, each carry a wide and short quadrilateral appendage, the surface of which is raised into a transverse series of large longitudinal eleva- tions, and each of these elevations presents inferiorly a linear aperture leading to a respiratory vesicle, the parietes of which are covered with a multitude of little arborescent csBca. These vesicles when extracted from the interior of the limb closely resemble a brush-like branchia, having its longitudinal canal in communication with the atmosphere by a longitudinal stigma. The fifth pair of abdominal members are rudimentary, while the sixth constitute the door-like triangular valves covering the anus, and all the inferior face of the last abdominal segment." *
The nervous system in the Amphipoda consists of supra- oBSophageal or cerebral ganglia, united by commissures with an infra-OBSophageal mass, whence commissural cords pass under the endophragm to the anterior of the thoracic ganglia, of which there are commonly seven pairs, succeeded by five or six pairs of abdominal ganglia. In some laopoda {Cymothoa, Idotea) the abdominal ganglia are also distinct ; bat in others, such as JEga hicarinata (according to Bathke), thej are fused into a single mass placed in the anterior
* 'Histoire Naturelle des Crnitacet,' vol. iU. p. 187.
366 THE ANATOMY OF INYBBTXBSATBD AVDfAIA
part of the abdomen, presenting onlj traces of a divisioit into five portions. In the CymathoadoB and terrestrial laopoda, again, the abdominal ganglia appear to have oom- pletelj coalesced with the last thoracic ganglia and form a mass, whence the abdominal nerves radiate. Finally, in the short-bodied Lcemodipodat snch as Oyomtw, there are not more than eight pairs of post-(Bsophageal ganglia* the posterior commissures of which are so ahortcoied that the nervous system ends in the antepennltiinate somite.
Brandt describes splanchnic ganglia like the lateral pair of Insects in the Oniscidce, It is one of the manj respects in which the Isopoda simulate Insecta.
No other organs of sense than eyes have, as jet, been cer- tainly demonstrated to exist in the Edriophihodmia^ though the fine setse which beset the antennary appendages, haTe been supposed to be organs of the olfactory sense. The eyes vary in their structure, from the simple, more or less closely aggregated ocelli of Lcsmodipoda, and of many Isopoda and Amphipoda, to the strictly compound eyes, as complex as those of the highest Articulata, which exist in .^a and in Fhrosina,
The female genitalia of the EdriophlhcUmia consist of two simple sacs, the ducts of which usually open on the ventral surface of the antepenultimate thoracic somite, or on the bases of the limbs of this somite. In the male, one or moxe cseca on each side constitute the testis, which ordinarily opens on the last thoracic or first abdominal somitei, in connexion with one or two pairs of copulatory organs de- veloped from the anterior abdominal somites.
The eggs of the ordinary Edriophthalmia usually undergo their development in the chamber beneath the tiiorax en- closed by the oostegites of the thoracic appendages. In most cases, the young differ so little from the adults that no metamorphosis can be said to take place. They frequently* however, want the last thoracic somite. The young of the parasitic Ed/riophthalmia, such as Bopyrus, Phryxus, Cfwuh thoa, Cyamus and the Hyperinae, on the other liand, are
THB 8TOMATOPODA. 367
widely different from the adults; and not onlj in their metamorphosis, but in the small proportional size and less aberrant form of the male, Bopyrua and Phrymu recall the parasitic Cop^oda.
In certain Amphipods {(xammarus loetista and Deamo- phUus) the yiteUus undergoes complete division ; while, in closelj allied forms {Chmmarus fitmatUis and ptUex), and still more completely in those Isopoda which have been studied, the part of the yitellus which divides into blasto- meres, becomes more or less completely separated from the rest immediately after fecundation, and the so-called partial yelk division, take place.*
In all Edriophthalmia, the development of which has been examined, before any other organs appear, a cuticular invest- ment or sac is formed, which is eventually burst and thrown off. This appears to represent the Nauplius cuticle of MysUf and, in close relation with it, are peculiar tergal structures, such as the bifid lamellm of AseUus and the unfortunately named " micropjle apparatus " of other Edri- ophthalmia.
The Edriophthalmia are not abundant in the fossil state ; but they may be traced back as far as the later Palaeozoic strata {Prosoponiscua, AmphipeUis),
The Stomatopoda. — Of the Stomaiopoda of Milne- Edwards, two of the three divisions, the Carid&ides and the Bieuirasses have since found a place among the Schizo- podous Podophthahnia, or among the larvsB of certain Macrura; but the third, the Stomaiopodea unicuiraasea, comprising Squilla, Oonodactylus, and Coronis, appear to me to differ so widely and in such important structural peculiarities, not only from the Podophthalmia proper, but from all other Crustacea, as to require arrangement in a separate group, for which the title of Stomatopoda may well be retained.
* E. van Beneden,* Recherches sur la Composition et la Signification de rCEnf/ 1870.
THE AJrA.TOHT OF IHYBSTZBSATED AirilUI.8.
The genera named, in feet, stand alone among the ~ ' n that the opbtlialinio and antennnlu; tomitei [X>mplct« rings, moveable apou one another and the
TRB 8TO1IAT0PDDA. 369
that of the bodj, so that there is no sternal flexure. Nnmerons pairs of hepatic cseca open into the elongated alimentary canaL The heart, again is not short and broad, with at most three pairs of apertures and confined to the thoracic region, as in the proper PodophUudmia ; but it is gpreatly elongated, mnltilocolar, and extends into the abdomen. The branchisB are plumes attached to the abdominal members (Fig. 88, A, br)y and, iso far as I have been able to ascertain, the carapace is, in all, connected exdnsively with the cephalic somites. This is particnlarlj well seen to be the case in SquUla seahricauda (Fig. 83), where five completely developed posterior thoracic terga can be counted, un- covered by the short carapace, beneath which the terga of the three anterior thoracic somites are represented by a membrane which passes forwards to be reflected into the carapace.
The free somites of the thorax, and those of the abdomen, in this species and in the Stomatopoda generally, are so large relatively to the carapace, that the latter is not larger in proportion to the body than the tergal covering of the head in many EdriopMhalmia, with which order the Stoma- topoda present many marked affinities. Indeed, if we leave the eyes out of consideration, the organisation of the Sto- maiopoda is more Edriophthalmian (and especially Amphi- podan) than Podophthalmian. The five anterior pairs of thoracic members are turned forwards, and are subchelate. The first pair are small and slender. The second pair are the largest of all, and have the characters of powerful prehensile limbs, the terminal curved and spinose joint of which shuts down mto a groove in the penultimate joint, as the blade of a x>ocket knife does into its handle. The three posterior thoracic limbs, on the other hand, are turned outwards, and terminated by an endopodite and an exopo- dite.
SquiUa lays its eggs in burrows in the bottom of the sea, which the w.Tiim«.la inhabit. The earliest condition of the free larva is not fully known, but the young larva have a single eye, 'and the hinder thoracic and the abdominal
370 THE AKATOMY OF IVYBSTBBRATBD AITIKALS.
appendages are not developed.* The larvn paas into f onus whicli under the names of Alima, EriehikyM, and SqmUler- iehihys, were formerly considered to be independent geneim. Claos's investigations, however, have rendered it probable that the two latter genera are simplj larval stages of Chnodactylus, and that Alima is a larval stage of SqtUUa.
• Frits MuUer, < FUr Dtrwin.' See also Claus, «Die Metamorphose der Squillideis* 1872.
THB ▲SA.OHmDA. 871
CHAPTEB Vn.
THE AIS-BBBATHINO ABTHBOPODA.
Among these Arihropoda, no forms absolutely devoid of limbs are at present known, though the appendages are reduced to two pairs of minute hooks in theTermiform parasite LinffucUula,
The Abachkida have pediform gnathites, and the least modified forms of this group (the Arthrogadra or Scorpions and Pseudo-scorpions) exhibit, in manj respects, extra- ordinarily close resemblances to the Merostomaia among the Crustacea,
The Abthbooastba. — ^The anterior port of the body of a Scorpion (Fig. 84) presents a broad shield-like tergal plate, resembling that of Ewrypterua in form. Two large eyes are situated one on each side of the middle line of the shield, while smaller eyes, which vary in number according to the species, are ranged along its antero-lateral margins.
Six wide plates, representing the terga of as many somites, follow the anterior shield, and are connected only by the soft integument of the sides of the body with their sterna. The seventh is united with its sternum (XY) posteriorly, while the five following terga and sterna form continuous rings, which constitute the joints of the so-called "tail." The anus is situated behind the last sternum. A moveable terminal piece, answering to the telson of a Crustacean, which is swollen at its base, and then rapidly narrows to a curved and pointed free end, overhangs the anuB, and constitutes the characteristic weapon of offence
872 THx i.KjL,Toinr or nmsTiBKATiD utisAxs.
of the Scorpion. This sting, in fact, contaiiiB two glimda which secrete a poisonotiB fluid, and their dacts convej it to the minute apertore sitoated at the sharp point of the organ. On the sternal enrface of the body, there are fonr
Fig. 8*.
Fig> B4.— AamTwa/ir.^A, teitnland BiSteraalTiewof thabodf ; At, oiialiosra ; iv', pedi|iBlpi ; v', vi', puUrior pair of ocphKllo ^^en- <Um; Tii', vu^iftuurior thoneie^mb*; /■(,. peetinM ; St,sWMi; CM, oepbalD-Uwnx. (After UilnB-EdmrdiAsdDagii.*)
wide and long sternal plat«B (xi-w), which correspond with the third, fourth, fifth and sixth of the free terga. £acb of th«ee bean a pair of oblique sUta, which are Ute op^iingB
* 'Bignt Animd,' Ifiwtn(«d Edition.
TBI ASTHSOOABTKL 373
of tlie reepiratorj organs [Fig. 85, e). The atema of Uie Fig. 8J.
Fig. 85 — A diagnm of
VII-XX, the Kventh to the t-enlieth Kimite. fv, V VI, tha UhI Joiato of tha pedlpalpl, and two tbllowing pain of litnba.
fint and second free eomitea (ix, x) an ver; noalli thaJu (A
374 THE AKATOMT OF IHTBBTSBILLTSD AVI1CAIJ3.
the first carries the valyes which cover the genital apertore ; that of the second bears a pair of verj curious appendageSt somewhat like combs, which are termed the peeHnet, The nerrous trunks which enter the pectines, are distributed to the numerous papillse which cover them, and are probably tactile in function. Thus there are twelve somites behind the eye-bearing shield, and none of these are provided with appendages, unless the pectines be such.
The truncated anterior extremity of the body, beneath the shield, is formed by a very large setose labrum, behind and below which, in the middle line, is the extraordinarily minute aperture of the mouth (Fig. 86, M). On each side of it is attached a three-jointed, pincer-ended, appendage, the ehelieera. Behind these follow the pedipalpi, large che- late limbs, the stout basal joints (iy') of which lie on each side of the mouth.
The following four pairs of appendages are seven-jointed ambulatory limbs, each terminated by three claws. The basal joints of the first two [y*, yi') lie behind the mouth, the posterior and inferior boundary of which they form, and are directed forwards. The basal joints of the last two (yii', yiii'), on the other hand, directed inwards, are firmly united together, and are altogether excluded from the mouth.
Thus the mouth is situated between the labrum in front, the bases of the pedipalpi and those of the first two pairs of ambulatory limbs, at the sides and behind; just as in Limulus, the mouth lies between the labrum and the basal joints of the third, fourth, and fifth limbs, which answer to the mandibles and first and second maxillffi of the higher Crustacea, If this comparison is just, there is one pair of pr»-oral appendages, which exist in Limulus, wanting in the Scorpion ; and the difference between the two may be represented thus :
Lmndnt, Antennule. Antenna. Mandible. Maxilla 1. MazillaS. Scorpio, Chebcera. Pedipalpus. Leg 1. Leg S.
Again, if the eye-bearing part of the head may be r»*
TBI ABTHBOOABniA. ' ' ' ' 375
^rded as a Bomito, then the body of the Scorpion, like that of a nuJaeofltfaooQS crastocean, will ooneist of twAity Bomitea and a teleon. We may regard the six posterior Bomites (ZT-xx) as the homolo^ea of those which, con< Btitate the abdotDOD in the cmetoceent whOe the eight middle aoiniteB (vii-xit) will answer to thoie which enter into the thorax of the,latter ; and the head will rwamble tliatof an Edriophthalmian with one pair of autennary o^ans com- pletely sapprewed. Upon this view, the eye-bearing shield is a campace oorering a oephalo-thorax, into whioh Hm two Pig. M.
Fig. 86. — ScnrfUD.— Vcrtioal lectioD of the oepbalo.thorki ; .if, cheli- oon ; lb, libmni ; M, mouth ; a, pbujageal Bae ; N, N ', inpra. uid laftvHzMphage&l ganglia ; b, mophagiu ; d, opeoing of the i^Tary duett; e,m(««tiDe; U, heart.
anterior thoracic somites only enter. These are followed by six free thoisoio somites, the four posterior of which are pulmoniferoos. Bnt no trace of the sapposed misginff antennary appendage has been met with in the embryonic conditioii, so that the alternative poedbilitj that the month is situated one lomitfi further forward in the Scorpion than in the Omstacean most be borne in mind. It is a very interesting fact that Meldchuikoff * has fotind mdiments of • " Etnbiyologle dcs SooiploDi." (' Zcitwihiift fSr Wiu. Zoologle,' 1871.)
376 THB ANATOKT OF INYBBTSBR^TSD AKOfAIiS.
limbs on those somites of the embryo Scorpion on the stigmata are situated, a ciroumstanoe which soggeste the suspicion that the Scorpion is deriyed from some foam possessing more numerous limbs.
The minute oral aperture leads into a small pTrifoim lateraUj-eompressed sac (Fig. 86, a) with ohitinous eUuitio walls. Muscles pass from these to apodemes of the sternal wall of the head, and doubtless act as divaiicators of the wall of the sac. As the Scorpion sucks out the juices of its prey, it is probable that the elastic sac acts aa a kind of buccal pump — the nutritious fluid rushing in when the sides of the pump are separated, and being squeezed into the (Bsophagus when the elasticity of the walls brings them back to their first position.*
The oesophagus (Fig. 86, h) is an exceedingly narrow tube, ' which springs from the tergal and posterior aspect of the sac just mentioned, traverses the nervous ring, and then, passing obliquely upwards and backwards, enlarges into a dilatation which receives the secretion of two large salivazy glands, by a wide duct on each side. The alimentary canal narrows again, and becoming a delicate cylindrical tube which widens posteriorly, passes straight through the body to the anus. The numerous ducts of the liver open into the anterior part of this region of the alimentary canal, and it receives two delicate' Malpighian tubuli.
The liver is a vast follicular gland, which occupies all the intervals left between the other organs in the enlarged part of the body, and even extends for some distance into the narrow posterior somites.
The eight-chambered heart (Fig. 86, H) is a lai^ger and more conspicuous structure than the alimentary canal, above which it lies, in a pericardial sinus situated in the middle line of the tergal aspect, between the eye-bearing shield and the tail ; each chamber is wider behind and narrower in front, and has two valvular apertures, by which blood is admitted from the pericardial sinus at its postero-lateral angles. It
• Hoxley, <*Oii the Mouth of the Scorpion." (< Quart. Jouni. of Microscopical Science,' 1660.)
377
giy«s off small lateral arteries, and ends in fnmt aad beliind in a wide aortic tnmk. Of theae the anterior is larger than tlie ceaophagns, and both aort«e give oif biaacdiea which are diatribnt«d widely through tiie body. A large tnmk liea on the tergal aspect oi the gangliimic chain, and is niut«d with the anterior doisol aorta, by a lateral aortic arch, on each side of the bodj. The veins, on the other hand, are irregnlar passages, the blood of whic^ is oarriod to two afferent pwItmnMry rimtsw, one for each act of respiratory
These respiiatorj orgsiu are fonr pairs of flattened sacs, which open externally by as many stigmata, on the et^na of the four posterior free thoracic somites (Fig. 85, xi-xit)
Fig. 87.
Fig.e
(After]
d.)
; reiplnlory lMfl«t* of Scorpb a
in front of the tail. Each lies with one flat aide eternal and the other tergal, in front of its stigma, and its walls are bo folded as to divide ite cavity into a multitude of sub- divisions, each of which opens into the oommoa obamber which commnBicates with the exterior by the stigma(f%. 87). The organ, in fact, somewhat reeemblea a porte-motmaie irith many pockets. The blood circulates in the folds and, after being thus exposed to the inflnence of the air, is carried by efferent puimonary nniiae* to the pericardial sinuB. Expiration ia effected by muHclca which pass vertically be- tween the sterna and terga of the free eomites.
The bilobed cerebral ganglion supplies nerves to the eyes and chelicerw, and is connected by thick conunitsurea with.
378 THB AKATOMT OF nTYSBTSBlLLTSI) AVIILLLB.
the pofit-ceeophageal ganglion, a large oral mass, whence branches are given to the Tnaxillm and following aomitee. A long cord formed by two closely-applied oomzniasnies passes to the three ganglia placed in the twelfth to the fourteenth somites. There are four ganglia in the ab- domen, two distinct cords passing from the last to its extremity. The visceral nervous system is represented by an oesophageal ganglion receiving roots from the cerebnd ganglion, and giving branches to the alimentaxy canaL*
Two lateral ovarian tubes, connected by transrerse ana* stomoses with a median tube, end in two oviducts, which open by a fusiform vagina on the first free sternum (ix). The tubular testes end in a pair of deferent ducts, on which, before their union at the common orifice, two long and two short cseca are found, the former playing the part of vesiculffi seminales. Both male and female organs lie imbedded in the hepatic mass in the posterior thoracic region, their ducts passing forwards. Partial yelk-division takes place, and the ova undergo development within the ovarian canals, in a manner which is very fiimilar to that of AsUteus. Thus there is no metamorphosis, and the young differ but little from the adult in any respect but size.
The Pseudo-scorpions (Chelifer, Ohigium) resemble the Scorpions in form and in the nature of their appendages, but they have no aculeate telson nor poison gland. They possess silk-glands which open close to the genital aperture, and their two pairs of stigmata are connected, not with pulmonary sacs, but with tracheal tubes. According to Metschnikoff, the eggs undergo complete yelk-division, and the young leave the egg provided only with that pair of appendages which become the pedipaJpi.
In the number of the appendages, and in the segmentation of the abdomen, Oaleodei (or Solpuga) agrees with the Scorpions and Pseudo- scorpions. But the three somites which bear the three hinder pairs of ambulatory limbs
* Newport, ''On the stnic- and Maemrons Araehnidm." tore &o., of the nervous and cSr- (* Philosophical Tianaaetion^' cuiatory tyitemB in Myriapoda 1843.)
l"'*
THX ABAinsnrA^
379
(ti, yii, yiii in the Scoipion) retain their distinctnesB, and
there is no cephalo-thorax, in the proper sense of the word.
In form and function the pedipalps resemble the first pair
of ambulatory limbs, while the chelicene are subchelate.
The organs of respiration are tracheal
The PhalcmgidcB {Phalangiv/m, Chnylepius) have chelate
chelicersB, but the pedipalps are filiform or limb-like, and
the articulated abdomen is relatively short and broad.
They have no silk-glands, and their respiratory organs are
tracheal.
Fig. 88.
^Stffk
Fig. 88. — Mygale c(ementana,—A, female of the nataral rise: At, eheli- ceraB ; iv', pedi palpi ; v*, vi', maxillary feet ; vii', viii', thoracic feet ; Cthj cephalo-thorax ; B, the last joint of the pedipalpus of the male much magnified; C. terminal Joint of the chelicera At^ with the poiaon gland ; D, the left pulmonary sac viewed from its dorsal aspect ; Stp, stigma : Pm, pulmonary lamellae ; £, the two arachni- dial mammillae of the left side, the smaller &>1 is situated on the base of the huge one S^2. (After Duges, * Regne Animal.')
While the last-mentioned forms lead from the ArthrO' gastra to the Acarina^ the pulmonate Phrynidoe, or Scorpion- spiders {Thelyphonus, Phrynvs), are in many respects inter- mediate between the Arihrogastra and the Araneina,
The Abaneina. — The Spidei-s stand in somewhat the same relation to the Scorpions, as the brachynrons to the macmroas Cnuiacea, That part of the body which Ilea
t
330 THE ANATOMY OF IKYXBTIBSATBD ANIMALS.
behind the ceplialo-thorax and answers to the free somites of the body of Scorpio is swollen, and presents no distinct division into somites.
The chelicersB are snbchelate, that is to say, the distal joint is folded down upon the next, like the blade of a pocket knife upon the handle. The duct of a poison gland, lodged in the cephalo-thoraz, opens at the summit of the terminal joint. The pedipaJpi are filiform, and, in the males, their extremities are converted into singular spring boxes, in which the spermatophores are received from the genital apertures and conveyed to the female (Fig. 88, B).
The pulmonary sacs, two or four in number, are similar to those organs in Scorpio, and are placed in the anterior part of the abdomen ; a tracheal system is also present, a pair of sternal stigmata, situated either behind the pulmonary saes, or at the end of the abdomen, leading into two more or less branched tubes. There is a complex pharyngeal apparatus, probably having the same function as in Scorpio* The stomach gives off csecal prolongations which may extend far into the limbs. There is usually a dilated short rectum, into which the branched Malpighian ducts open. The nervous system, more concentrated than that of the Arthrogastra, is reduced to a supra-OBsophageal ganglion and a single post-<Bsophageal mass, with four indentations on either side. There are six or eight simple eyea in the anterior part of the carapace. Auditory organs have not been discovered in these or any other Arachnida,
One of the most characteristic organs of the Araneina is the arachnidivm, or apparatus by which the fine silky threads which constitute the web, are produced. H. Meckel,t who has fully described this apparatus as it occurs in Epeira diadema, states that, in the adult, more than a thousand glands, with separate excretory ducts, secrete the viscid material,
* Lyonet*s ^ Anatomie de diff^ piklps of the male spiders,
nnteiesp^eef d'lnsectet" C Mtfm. t *' Mikrographie einiger DrG-
daMiii^iim d'Hiitoire NatureUe,' ten - apparate der nlederen
1889) ooatsins an elaborate ao- Thiere.*^ (Mailer's Arohiv, 1846.)
floapl of this apparatoB, as well See abo Buchbola and Landols.
is of tlis stmoture of the pedi- (Ibid., 1868.)
TBB JJUBimi.. 381
whicb when ezpooed to the air, hardens into eilk. These glands are diviaibU into five different kinds (ociniform, ampnllate, aggregate, tubnlifonn, and tnberona), and their dncta ultimately enter the aix prominent arachnidial mam- m^la, which, in thi« species, project from the hinder end of the abdomen. The superior and inferior Tnummillio are three-jointed, the middle one ia two-jointed. Their terminal r\g. 89, A. Fig. W, B.
Fig. 8
—The heart sod ktMtIsI tcucIi of the u
faces are tmncated, forming on area beeet with the minnte anitfhMdtal pa^Ue b; which the secretion of the glanda ia ponred out.
The males are smaller than the females, and their ap- pioaohes to the latter are made with extreme oantion, as ibej mn the risk of being devoured ; extending their pedi* palps, they deposit the apermatophorea in tihe female genital ap«rtni«, and betake themselTea to flight
382 THK ASATom or nrmBTiBKA.TiD axjxai^
The Anmeina are onparoiu, bnt the development of Uie embiyo takes place as in the Artkrogeutra, and then is no metamorpliOBiB.*
The ACABIHA.— In the Hit«8 and Ticks, the hinder eoniit«s are, as in the Spiders, distinctly separated from obc
Fig. 90.~Iioda cicmm, r>ii»le (uter PagciiitFCher).| — a, nundibabr booklets; A, i/,c^ fourth, third, &ndi«eondjoiDti of the palp; c, book- lets of eternal larfece of proboaeii ; /, tax of tlie praboeoU ; t, ttig- ma ; h, genital aperture ; i, anal vafvee.
another, bnt the; am not separated by anj constriotioii from tbe anterior somites.
• CIspsiWe, "Becherebes em riwlafa flM AlSlBDrin," ISSS.
t ' Anatoraie dcr lUIben,' IBCD.
« Tt
THB AOABIHA. 383
The bases of tlie cbelicerse, and of the pedipalpi, coalesce with the labram, and give rise to a suctorial rostrum (Fig. 90).
There are turaallj several gastric cansa, but no distinct liver. Salivary glands occur in some, and Malpighian caeca are occasionally found. No heart has yet been discovered. Special respiratory organs are sometimes wanting (e. g, Sareopies) ; when present, they are tracheal tubes, springing brush-wise from a common trunk which opens by a stigma. The stigmata are usually two, sometimes anterior and sometimes posterior in position. The ganglia of the nervous system are concentrated round the guUet, as in the Spiders; and the reproductive aperture is situated far forwards, sometimes close to the rostrum.
The greater number of the Acarina are parasites upon other animals or upon plants.
Most are oviparous, but the OribatidtB are viviparous. The course o( the development of the embryo is the same as in the Spiders. The young, when bom, are frequently pro- vided with only three pairs of ambulatory limbs, the fourth pair making its appearance only after ecdysis has occurred.
In some Acarina, a singular kind of metamorphosis occurs.
Thus, in Aiax Bonzif Claparede * observed that, before the limbs appear on the blastoderm, a chitinous cuticula is sepa- rated and forms an envelope, which he terms the " sac of the deutovum." The proper vitelline membrane bursts into two halves, much as in Limulue, and the deutovum emerges. In the meanwhile, the anterior end of the blasto- derm becomes fashioned into two procephalic lobes; while five pairs of tubercles, answering to the rudiments of the cheli- cer»,pedipalpi, the two posterior gnathites, and one pair of thoracic limbs of the Spiders, make their appearance beneath the sac of the deutovum. The rudiments of the chelicerse and pedipalpi apply themselves together, and coalesce into a proboscis. Thus the first larval form is completed. It tears the pseudoval sac, emerges, and buries itself in the
* *<S(Ddien an Acariden." (< Zeitschrift fBr Win. Zoologie. 1868.) *
884 XBB ANATOMT OV IIlVSBTIBBATmD AXDUIB.
bnnchue of th« freebwatermnMel (ITmo), upon vhieh it iB parasitic. The cnticnlar inTestment of Hm fiist lam now becomes diBteoded bj absorption of Tater, and fanna K globular case, the limbs bein^ drawn oat of tiieir rtf the The second larval stage completes itself within tlie aao formed b; this dngnlar ecAjmi. The two palpi are dere-
Flg. 91,—Ammolhra pfomgnoidti, femtle (ktter QnktrefkgM).^ — ■, iEiaph4giu ; d, anteiiDa ; b, iloDuoh with Iti prolongatloa Into tta uitaniue uid limbi « ,- c, reatum.
loped from tbe pedipalpal portion of the proboeoia; two homj books from the oheliceral portion; and, finallf, tite binder pair of thoracic tirobs is added. This seotmd larva gmdnaUf passes into the adult Aiax. Is the Aoama (Xyobia eoarekUa) of tbe Mouse, ClaparUe
THB PYCNOOOKIDA. 385
observed tbat the deutoYum stage is followed by a tritoYum ; the chitinous sac, which invests the embryo within the deutovum, apparently representing the cuticle of the first larva of Atax. In this case, it presents a parallel to the Naupliue cuticle of MysU.
The Arthrogastra, the Araneina and the Acarina (with some doubtful exceptions among the latter) possess the same number of appendages, and do not differ from one another so much as do the different forms of the Copepoda, among the Crustacea. But the remaining groups which are usually included among the Arachnida; namely, the Pycnogonida, the Arctisca, and the Pentastomida, diverge widely from the Arthrogastra and the Araneina, though each exhibits certain approximations to the Acarina.
The Pycnooonida. — These are marine animals with short bodies terminated in front by a rostrum like that of the Mites, but with a mere tubercle in place of the pos- terior thoracic and abdominal somites. The adult has four pairs of enormously elongated, many-jointed ambulatory limbs, in front of which are three pairs of short appen- dages, the anterior of which may be chelate, while the posterior are more or less rudimentary (Fig. 91).
The alimentary canal sends off very long csBca into the legs. There is a short heart, but no distinct respi- ratory organs exist. A cerebral nervous mass is con- nected with a ventral chain of four or five pairs of ganglia. Four eyes are seated upon a dorsal tubercle above the brain. The sexes are distinct, and the testes and ovaria are lodged in the legs and open upon their basal joints.
The embryo emerges from the egg as a larva provided with a rostrum, and with three pairs of appendages, which represent the short anterior three pairs in the adult.* The four pairs of gieat limbs of the adult are produced by out- growths from a subsequent posterior elongation of the body.
* \, Dohm, * Untersuchungen uber Baa and Entwickelang der Arthropodeo.* Enter Hett. 1870.
386 THK ABATOMY OP IKV8BTXB&ATKD AKIItLLB.
The comparison of the embryos of the Pyenogomda with
tiiose of the ABarina, ea^eciallj sach as leave the egg with
three paire of appeuda^a, appear* to me to leave little
Fig. M.
I itylela^
xino).— a.m alb with rfatrcl uUvar7El*D(k; d, MMmWt ii; ff, teitis; 1,^,8,4, "-"^
llnpillw-
(.ovary; f Greaft*)
donbt that the rostrum of the larval Pyanagonmm it formed, u in the Mitee, by the coalesced repreaentRtJTM of tbe
THB PXmrASTOMIDA. 887
chelicene and pedipalpi. If so, the ■even other pairs of limbs are, bj three pairs, in excess of the nnmber found in any Arachnidan. On the other hand, the hezapod larva of the Pyenoganida differs from the hezapod Naupliiu of the Cnuiaeea, inasmuch as the three pairs of appendages of a Ndupliua always represent antennary and mandibular ap- pendages, and these, by the hypothesis, are to be sought in the rostrum of the Pycnogonida.
The fact to which reference has already been made, that the embryo Scorpion has siz pairs of rudimentary appen- dages, attached to as many of the anterior free somites, of which one pair only remain (as the pectines) in the adult, leads me to suspect that the Pycnogonida may represent a much modified early Arachnidan form, from which the Arthrogastra, Araneidea, and Acaridea have branched off.
The Abctisca, orTABDiOBADA, are microscopic animals, found in association with Botifera, in moss and in sand, rarely in water, which present many points of resemblance to the Acarina, The body (Fig. 92) is vermiform, with four pairs of tubercles representing limbs, terminated by two or more claws. The fourth pair is directed backwards at the hinder end of the body, so that if these appendages answer to the hinder pair of limbs in the typical Arachnida, the hinder thoracic, and all the abdominal, somites are un- developed. The mouth is situated at the extremity of a rostrum provided with two stylets, which is so like that of the Acarina, that it may probably be regarded as formed by the coalescence of cheliceral and pedipalpal tubercles. There is a muscular pharynz, leading into a wide alimentary canal, which gradually narrows to the anus. No organs of circulation or of respiration exist. The paired ventral ganglia, which correspond in number with the appen- dages, are large. They are connected by longitudinal commissures with one another, and with a prsB-oesopha- geal cerebral mass which sometimes bears two eyes. The Aretieca are hermaphrodite, the ovarian sac and the two testes opening together into a doacal dilatation in
388 THE ANATOm OT IHVSBTKBKATXD liltr-^JJi
wliich the intestine terminEiteB. The ova are reiaiJveljreTy lar^e. and the cuticle of the parent is cast off and enclosea them when thej ore laid, ua a sort of ephippinm. Com- plete jelk division takes place. The jouug have one-third the size of the adnlt when they are hatched, and they undergo no netamorphoeia, beyond the addition, in some cases, of one pair of limba after birth.* The Pentastomida. — A still more abemuit foi-m is the
Fig. 91-
. eod iif Iha body ; a, ■■■ . , .
c, rudimenlu'y p*1pifann organi. (Arii
parautic Idngwttula, or Penlattmn'um, which is found in ft sexless condition in the lungs and liver of herbivorous mammals and of reptiles, and in the sexual state in the nasal cavities and maxillary antra of Carnivores. Thus, 1^ Lenckart*s investigations have proved, Penlaatornvm
THE PBNTASTOMIDA.
389
toBnioides, which inhabits the latter Cavities in the Dog and the Wolf, is the sexual state of the P. dentictUcUunif which occurs in the liver of Hares and Babbits.*
The Pentastomida are elongated vermiform animals, the bodies of which are divided by close-set transverse con- strictions into numerous short segments. At first sight, they appear to be entirely devoid of appendages, but, on careful inspection, four curved hooks are foimd, two on each side of the mouth, which is situated rather behind the anterior extremity of the body. Each hook is solid, and its base projects into the cavity of the body and gives attachment to the muscular bands by which it is moved.
The mouth is surrounded by a chitinous ring ; a narrow CBSophagus leads from it into a nearly cylindrical, straight,
, Fig. 94.
Fig. 94. — Embryo of Pentastomum tcdnioides,
alimentary canal, which terminates in the anus, in the middle line of the posterior extremity of the body. A mesentery is attached to the whole length of the alimentary canal and holds it in place. A nervous rin^ surrounds the oesophagus, and posteriorly presents a ganglionic enlarge- ment whence nerves ai*e given off to the body. The muscles are striated. The sexes are distinct, and the males are usually much smaller than the females.
The testicle is an elongated sao which lies on the ventral aspect of the intestine, and is connected anteriorly with two vasa deferentia. These terminate on the fore part of the ventral aspect of the body, each having a saccular
* * Baa and Entwiokelungsgesehiehte der FentMtom^ix^ Y^^.
390 THE ANATOMY OF IKYEBTBBBATED ANIMALS.
dilatation wliich contains a very long, coiled, chitinoos penis. In the female, the ovary is also a large sac and the ovi- ducts come off from its anterior end, but the genital aperture is close to the anus.
The ova undergo their development in the ovary. The embryos are oval, but taper to the posterior end. In the middle line, in front, are three sharp protractile , styles, of which the middle is the longest. Two pairs of articulated limbs are attached to the middle of the ventral aspect; each is terminated by a double hooked claw. The embryo of Lingtuitula thus resembles those of the Acarina, on the one hajid, and those of such parasitic Crustacea as Anchorella^ on the other.
In the case of Pentatiomum Uenioides, the embryos, enclosed in their vitelline membranes, pass out of the bodies of the dog or wolf, along with the nasal mucus. Taken into the body along with the food of the hare or rabbit, they emerge from the egg, penetrate the walls of the intestine, and lodge themselves in the liver. Here they become encysted, grow, and go throiigh a seiies of changes of form, accom- panied by repeated ecdyses, until they pass into the state known as Pentastomum denticulatum. If the flesh of the rodent containing P. denticulatum is devoured by a dog, the parasite passes into the frontal sinuses, or maxillary antra of the latter, gradually takes on the form of P. tcenioides^ and acquires sexual organs. The parasitism of the Pen^ tastomida, therefore, is very similar to that of the Cestoidea,
Spiders and Mites abounded in the tertiary epoch, as their remains preserved in amber, show. Various Arthrogastra occur in themesozoic formations, while Spiders and Scorpions of large size have been found in the carboniferous rocks.
The Mykiapoda. — In these Arthropods, the body is
divided into many segments, the most anterior of which
takes on the characters of a distinct head ; and almost all
tiiese segments bear articulated limbs terminated by claws.
la tlie Centipedes (Chilopoda), the segments of the body
Iwre broad stema, and the bases of the limbs are far apart; Ik
THE KTBUPODX. 391
but, in the Millipedes [Chiiognatlia), the stenul region it mdimentaiy, and the bases of the limbs are close together. Uoreover, in the latter group, the miyjoritj of the segments of the bodj bear two pairs of limbs, sai probably represent two somites.
Pig. 95.
n (CA%»4i)-
The bead ia either flatt«ned front above downwards [Chilopoda), or from before backwards {ChiUi^naiha). Some species are blind, but the majority bare ejee, which are generallj aniall and not very numerous ocelli, though, in
* ' Bigne AnimaL' lllnttnUd edition.
392 THE ANATOXT OF II[TEBTKBBA.TKD
BOm« caaea, they are lai^ compound «jei. Tliere ia almje » pair of jointed uiteiuuB.
The miyoritj liave the mouth conaliiict«d for biting, and are prorided with a pair of mandibles, the most important pecnliaritj of which is that they are jointed, and tliaB depart le«a from the type of the ordinary limb than do
Fig. 98— SmfcpBHfro Hop>:l (sfler Newport).
A, duFMl view of the uilerior part of the bodj. a. antcniiM. A, ccphallo wgnient; B, builar segmrnt.
B, imtnl Yiew of theheftil ; n, B, M bsforc.
C, under view of the eephnlio wgineiit, ■howing the uitennK, a ; the e}e>*; tbe l&bmin and the mandibles, iv'. -
D, the lecond pair of gnathitet v', and the finC pair uf appeniJaBCi of the baallai Hginent vi'.
those of insects, while, to the same eitent, they approach the gnathites of the Peripaiidea. The mandibles are more modified in the Chiiopoda (Fig. 96) than in the ChilogiuMa. In the latter, the second pair of gDatbit«B form a broad lonr-lobed plate which plays the part of on underlip, while,
THB MYBIAPODJL. 893
in the ChUapoda, thej are soft and jointed, and nnited at their bases bj a bilobed median process (Fig. 96, y'). In the Chilogncdha the four segments which follow the head are free, and their appendages . resemble ordinary limbs. The anterior pair is tamed forwards and comes into relation with the month, and the tergam of the first somite is often enlarged ; of the other three somites, the appendages of one appear to be always abortive. Thus there are three seg- ments with single pairs of legs. The rest eackbear two pairs.
In the Chilapoda, on the contrary, the head is followed by a basilar segmeni (Fig. 96, B), formed, according to Newport, by the union of four embryonic somites, and carrying three pairs of appendages. Of these the first are limb-like, but are turned forwards beneath the mouth (Fig. 96, D Yi') ; the second pair are the strong recurved poison- claws, and the hindermost pair may become functional legs, resembling those which are attached to the succeeding somites, but are always smaller than the others, and may be altogether aborted in the adult The somites of the body never bear more than one pair of limbs.
The alimentary canal is usually straight and simple, like that of an insect larva. There are large salivary glands, and the intestine is provided with Malpighian tubules.
The heart extends through the greater part of the length of the body, and is many-chambered, there being one chamber for each of the somites in which it lies. Each chamber is somewhat conical in shape, being broader behind than in front, and admits the blood by a pair of lateral clefts, while the blood leaves it, in part, by the communica- tion with the adjacent chamber, in part, by lateral arterial branches. A median aortic trunk continues the heart for- wards, and the lateral trunks encircle the OBsophagus and unite into an artery which lies upon the ganglionic chain. The arterial system in the ChUopoda is, in fact, as complete as that of the Scorpions.*
* Newport, *' On the structure, in the Mvriapoda and macrurous relations and development of the Arachnida." (* Philosophical
nervous and circulatory systems Transactions,' 1863.)
394 THE ANATOMY OF INVERTEBRATED ANIMALS.
The respiratory organs are trachese, which open by stig- mata on the lateral or ventral surface of more or fewer of the somites. In Scutigera the stigmata are situated in the median dorsal line of the body.
The nervous system presents a ventral chain, with a pair of ganglionic enlargements for each segment of the body, the most anterior of which are connected by commiasures, which embrace the oesophagus, with the cerebral ganglia.
The ovary in both ChUognaiha and Chilopoda is long, single, and tubular in form. It lies above the alimentary canal in the latter, between the alimentary canal and the nervous system in the former. The double vaginsB open on, or close behind, the bases of the second pair of legs in the ChUognaiha ; at the posterior end of the body, beneath the anus, in the ChUopoda, Two spermathecss and eoUeterial glands are very generally present.
The testes in the ChUognaiha are tubular glands, which occupy the same position as the ovary and open in the same region. They have lateral cseca, and are connected by transverse ducts. Two copulatory organs, or penes, are developed on the sternal face of the sixth segment which follows the bead, or are connected with the bases of the seventh pair of legs.
In the Chilopoda there is a good deal of variation in th4 structure of the testis. Thus in LUhobius,* the testis is a single filiform tube, connected at the hinder end with two deferent ducts which embrace the rectum. A large caBcum, apparently a vesicula seminalis, opens into each deferent duct. But, in most Chilopods, the testes are fusiform acini, imited by delicate ducts with a median vas deferena Two, or four, pairs of accessory glands are connected with the opening of the male apparatus.
The spermatozoa are enclosed in spermatophores in Seohh pwdra, Crypiops, and ChophUus,
The ChUognaiha copulate. In Olomeris and Pol]fxenus the genital apertures of the two sexes are brought together
: * Favre, ** Anatomie dee organes reprodacteurs des Myriapodes.** •f^Aanalca des Sciences Matiirelles,* 18D5.)
THB MTBIAPODA. 895
during copulation ; but, in Itdua, the penes of the male are charged with the spermatic fluid before copulation takes place, and it is by their agency that the female is impreg* nated.
The Chilopoda have not been observed to copulate, indeed the female shows a tendency to destroy the males, as among Spiders. The male €hophilus spins webs like those of spiders across the x>assages which he frequents, and deposits a spermatophore in the centre of each,
Metechniko£f * has recently shown that, in the ChUogfuUha, the process of yelk-division is complete, and confirms the observation of Newport (Phil. Trans. 1841) that the sternal face of the blastoderm becomes sharply infolded down its centre, in such a manner that the anterior and the posterior halves of that face of the embryo become closely applied together. MetschnikofF further points out that only two pairs of appendages are converted into gnathites; and that a chitinous cuticula, apparently identical with what Newport describes as the "amnion" in lulus, is early thrown off from the embryo. In some species, it developes a median tooth-like process which serves to burst the vitelline membrane. Newport describes a short cord, or funiculus, which connects the anal extremity of the embryo with the so-called "amnion." It is not improbable that this is simply the continuation of the first larval skin into the rectum.
The embryo lultu at first bursts the vitelline mem* brane, and is enclosed only in the embryonic integument. At this period, its body is divided into eight segments, of which the first represents the head. Traces of the antennse are visible on the sides of the head, and the four following segments exhibit papillsB; those of the second, third and fifth segments develope into the three pairs of functional limbs, with which the young myriapod is at first provided.
Between the terminal segment and the seventh, the body grows and becomes divided into six rudimentary new
* ** Embryologie der doppelftUsigen Myriapoden {Chiloynatha)." (« Zeitsohrift fur Wiss. Zoologie,' 1874.)
There is this difference, however, bet' tlio larval inyriapiMl. that since, iu tl ouly twu pairs of <^iiathites, which r mandibles and first maxillae of iusecl appendages of the second segment second maxillsQ of insects; and hen< apparently the oame number of somit there must in reality be one fewer in myriapod larva therefore, notwithsta character, is essentially different from
The sixth and the seventh segment of legs, as do all the newly-formed i worthy of notice that the male copula much as it is situated in the seventh segment in the adult, is developed froi segments of the embryo, and not in added segments. New segments, each pairs of limbs, are developed by su region between the penultimate segmei of the newly -formed segments, until the adult is complete.
In all other Chilognaiha of which 1 been traced, the young, at first, posses #nT»/»firtTift.l Ifiopfl : and one of the four oi
THB IN8BCTA. 397
early stages of development of Oeophiltts have been de- scribed by Metsclmikoff.* Complete yelk-division takes place, and when the young leaves the egg it has a cylindri- cal body, like that of one of the Chilognatha, and possesses many pairs of limbs. Newport f has pointed out that, in Geophilus longicomis, the basilar segment is formed by the confluence of four somites, of the appendages of which only two are ultimately developed. Thus the basilar segment of the head of the ChUopoda appears to correspond very closely with the four somites which follow the head in the Chilognatha. Under these circumstances, the difference in the position of the reproductive apertures in the two groups is exceedingly remarkable.
Fossil Myriapoda occur both in the tertiary and secon- dary formations, and there seems no reason to doubt that the Xylohius sigillarioR discovered in the coal of Nova Scotia by Lyell and Dawson is to be referred to this group.
The Insecta. — Notwithstanding the vast number, and the singular diversity of form of Insects, the fundamental unity of their structure is remarkable, and, in this respect, the group exhibits a striking conti*ast to the Crustacea.
The division of the body into three regions, head, thorax and abdomen, is usually well marked, not only by the peculiar modifications which the cephalic and thoracic somites undergo, but by the attachment of the three pairs of ambulatory limbs exclusively to the latter. The head possesses four pairs of appendages, that is to say, one pair of antennsB and three pairs of gnathites ; and, as a general rule, there is a pair of compound eyes, sessile upon the sides of the head; sometimes simple eyes are added to them. The first pair of gnathites are the mandihlesj which are always devoid of a palp. The second pair are the maxillcsy which, in those insects in which the mouth is least modified, are distinct from one another and laterally moveable ; while
* * Zcitschrift fur Wiss. Zoolo- riapoda, order Chilopoda. (Trans- gie,' 1875. actions of the Linnean Society,
t Monograph of the class My- zix.)
398 THE ANATOMY OF INVEBTEBRATED ANIMALS.
the third pair of gnathites are united together in the median line, and constitate the ItMum of entomologistL In front of the oral aperture is a median plate, the lahntm; while from the floor of the mouth formed bj the lahinm another median process, the lingua, is osnally developed.
It is hardly open to doubt that the mandibles, the tna^yil^m^ and the labium, answer to the mandibles and the two pain of maTJllffi of the crustacean mouth. In this case, one pair of antennaiy organs found in the latter is wanting in in- sects, as in other air-breathing Arthropods, and the existeiiee of the corresponding somite cannot be proved. But if it be supposed to be present, though without any appendage, and if the eyes be taken to represent the appendages of another somite, the insect head wiU contain six somites, the prsBoral sterna being bent up towards the tergal aspect* as in the higher Crustacea.
The three somites which succeed the head are termed respectively prothoraXf mesothorax, and metaihorax. A pair of legs is normally attached to each ; and, when wings exist, they are lateral expansions of the tergal region (corre- sponding with the pleura of Cnuiacea) of the mesothoraz or the metathorax, or of both.
In the abdomen there are, at most, eleven somites, none of which, in the adult, bear ambulatory limbs. Thus, ^^iMmTnlng the existence of six somites in the head, the normal number of somites in the body of insects will be twenty, as in the higher Crustacea and Amchnida,*
One of the commonest of insects, the Cockroach {Blatta {Periplaneta) orientdlia) is fortunately one of the oldest, least modified, and in many ways most instructive forms ; at the same time, it is not too small for convenient dissection.t
In this insect, the head is vertically elongated, flattened
* It It open to quef don whether four lor the heed, three for tht
Hm podienl plates repraeent a thorax, and ten for the abdomen.
■OMite; and toerelbre it mutt be t 8^ f^r an excellent fignn
weoliaflted that the total number anddeecription, Rolleston,* Forms
af somites, the existence of which of Animal Life,' p. IW, plate
be aetoaUy demonstrated In vi.
JasfifltSi Is only seventeen, vis.,
THE COOKBOACH. 399
from before backwards, and connected by a distinct neck with the prothorax. The antennsB are slender, as long as, or rather longer than, the body. Large reniform compound eyes are situated at the sides of the head. The tergal por- tion of the prothorax {pronotwrn) is a wide shield, which overlaps the head, in front, and the tergal portion of the mesothorax, or mesonotum, behind. The legs are strong, and increase in length from the first pair to the last. The abdomen is flattened from above downwards, and bears a pair of elongated, many-jointed, setose styles {cerci) at its hinder extremity.
The males differ very considerably from the females. They have two pairs of wings, of which the anterior are brown, and are of a stiff and homy texture. As they serve to cover the posterior wings, they are termed teffmina. When closed, the left overlaps the right, and they extend back as far as the posterior edge of the tergum of the fifth abdominal somite.
The posterior wings, on the contrary, are thin and mem- branous; and, in a state of rest, are folded longitudinally upon themselves, the folded edge being internal. In this condition they are triangular, the base of the triangle lying close to the posterior edge of the fourth abdominal somite, and the right a little overlapping the left. When forcibly unfolded and made to stand out at right angles to the body, each of these wings is seen to have a nearly straight, thickened, anterior edge, while its rounded outer and pos- terior edges are very thin. The wing is strengthened by radiating thickenings, or nervures, united by delicate trans- verse ridges ; and, when left to itself, it springs back into its folded state with some force.
The abdomen of the male is not very broad. The sterna of the abdominal somites are all flattened; and, to the hindermost, two minute u^jointed styles are attached, while some singular hooked processes are seen, on close inspection, to protrude between the hindermost tergum and the hindermost sternum. The abdomen of the female is yery much broader, especially towards the middle of ita
1*'»'"> TKK ANATOMY OF IN VEKTEDKATKD ANIMALS.
length. Tlio hin<lermust sternum is convex and lx>at -shaped, and its posterior half is separated along the middle line into two halves, united only by a thin and flexible membrane. Sometimes, the great egg-case, which the female carries abont for some time before it is laid, is seen protrading between the posterior terga and sterna. The female has moveable tegmina, but they are very small, inasmuch as they do not extend beyond the middle of the metathoraic, and are wi<Iely separated in the middle line ; they are, in fact, mere rudiments of the anterior wings. The posterior wings appear, at first, to be altogether wanting. But the outer extremities of the metanotum, or tergal portion of the metathorax. present triangular areas, in which the in- tegument is very thin and exhibits markings which simu- late the nervures of the wings. There can be no doubt, in fact, that these are undeveloped wings, and they show, in a very instructive manner, that the wings are modiflcations of that part of the insect skeleton which answers to the pleura, and therefore to the lateral parts of the carapace, of a crustacean.
The convex dorsal wall of the head of the Cockroach (Fig. 97), is termed the epicranium, A median suture, the epicranial ntture, may be seen, especially in young Cock- roaches, traversing it from before backwards, and dividing between the eyes into two branches, one of which passes towards the articulation of each antenna. The basal joint of the antenna is attached to a transparent flexible mem- brane, which occupies an oval space, the antennaiy fossa, and allows of the free play of the antenna. A little pro- jection of the hard chitinous skeleton when it bounds the inferior margin of the fossa, helps to support the joint. On the inner side of and above the antennary fossa, there is an oval fenestra, covered only by a thin and transparent portion of the integument, which allows a subjacent tissue d glistening white appearance to be seen (Fig. 97, 1. II. h). These have been regarded as rudimentary ocelli by some entomologists ; but their structure needs careful examina- tkm before this view can be adopted.
THX COCKBO^CR, 4ftl
The tnuuparent cornea, of the eje. aitnated external to and behind the antennaij foaaa, is elongated, wider abOYe
Fig. V3.— Btatla otienlaliM.—l. II. lid* and fhint vlem of the head; a. the epicnnld lurure, kt the endi of the lateral bnuidie* of which ■re h. the feneelm-, /, tba anteniuc ; g, the efca', &, the labrnai;
- -'-- -naiKliblei «i,Uib aardii; •( tb«»tipo»; ja, the g"-
the palpal of the maxilla ; p, the pUpDi, a, the mentom and lub- mentum of (he labium ; i, Che margins of the occipital forameD ; i, t, inferior ceriieal ■clerilei ; le, lateral cerrical lolerltaij p a,
KT "■"
in. the labium 'aad' the right maiilla, TiVw«l' f^ , letter! ai before except b, lacinia of the maxilla ; /ip, paraglouee ; U, llgnla -, % mentum ; na, labmeDtum of the lahlnm.
than below, and has aconcare uit«Tior, and alightlf convts
402 THE ANATOMY OF IXVERTEBRATED ANIMALS.
posterior margin. The numerous facets into which the oomea is divided, are hexagonal in 8hax>e, and very small.
The broad flattened region of the fore part of the head, on the oral side of the epicranial sntore, is the elypeu9. It is prolonged in front of the month, and with the truncated edge of this prolongation, the flap-like labnun is freely articulated. Behind the labmm are two, very stout, corred mandibles, strongly toothed at their extremities (Fig. 97, II. m n). Each mandible is articulated with the truncated edge of the lateral part of the skeleton of the head, beneath the eyes, which is termed the gena, in such a manner as to be freely moveable towards and from the median line, but in no other direction. The proximal end of the TnnTilla (Fig. 97, m.) is formed by an elongated basal articulation, the cardo, which is directed transversely to the axis of the head, and is connected with the inferior margin of the epi- cranium, or rather with a thin skeletal band which runs round the posterior margin of the epicranium, and is firmly united with it only on its dorsal side. This band forms the boundary of the so-called oecipUal foramen, by which the cavity of the head communicates with that of the neck, the chitinous wall of the latter region being continuous with it. Articulated at right angles with the cardo is the Hipes, or second joint of the maxilla. This is freely moveable in the lateral direction, and its outer distal angle bears the continuation of the limb, or palpus^ formed by two short and three long joints. Two processes terminate the stipes ; of these the anterior and outer, the galea, is soft, rounded and possibly sensoiy in function, while the posterior and inner — the ladnia— is a curved cutting blade with a toothed and spinose inner edge.
The labium (Fig. 97, III.) consists of two incompletely separated median plates, the wbmerUwn behind, and the mewhim in front; upon the latter follows a bilobed terminal piece, the Ugula, each lobe of which is again divided lon- gitadinally into two portions, which have considerable nxnUarity to the galea and lacinia of the maxilla. The outer ii usually termed the parciglosM,
THB COOXBOACH. 403
Between the mentmn and the ligala, on each outer edge of the labium, a small piece, the pcUpiger, is articulated ; it bears the three-jointed labial palpus, which is to be regarded as the proper free termination of the second maxilla. The resemblance between the labium and a pair of wiftTillya which have coalesced, is obyious.
The submentum is not directly articulated with the cranial skeleton, but its posterior edge is close to one of the eerviedl telerUea,* or skeletal elements observable in the chitinous integument of the neck, of which there are alto- g^her seven. One is dorsal, median, and marked bj a deep longitudinal depression. It articulates with the dorsal margin of the occipital foramen. Four are lateral, two on each side (Fig. 97, 1, le) ; these take an oblique course from the dorsal part of the boundary of the occipital foramen, with a tubercle of which the anterior piece is articulated, to the anterior edge of the epistemimi of the prothorax. The inferior cervical sclerites (Fig. 97, I. ic) are two narrow transverse plates, one behind the other in the middle line. Thej appear to represent the part called ffula, which in many insects is a large plate, confluent with the epicranium above and supporting the submentum anteriorly. I think it is probable that these cervical sclerites represent the hinder- most of the cephalic somites, while the band with which the Tnn-TiUm are united, and the gense, are all that is left of the sides and roof of the first maxillary and the mandibular somites ; the epicranial expansions being mainly formed by the upward and backward extension of the ophthalmic and antennary sterna, which arise out of the procephalic lobes of the embryo. In addition to these externally visible sclerites, there is a sort of internal skeleton (endocraniwn or ten- torium), which extends as a cruciform partition from the inner face of the lateral walls of the cranium, close to the articulation of the mandible, to the sides of the occipital
* I me this term, in the senie nous skeleton. It Is to the latter
in which it has been employed by what a distinct ossification is to
Milne-Edwards, to denote any de- the skeleton of a venebrated ani-
finite hardened part of the ebiti- mal.
404 THE ANATOMY OF INYEBTEBSATED ANIMALS.
foramen. The centre of the cross is perforated bj a rounded ax>ertiire, through which the oesophageal nerye-commissores pass. The commencement of the oesophagus traversee the interspace between the anterior processes of the cross ; the tendons of the great adductors of the mandible pass through the lateral apertures ; and the backward continuation of the gullet enters the thorax through the posterior aperture, in- cluded between the tentorium and the margins of the occipital foramen.
In each somite of the thorax, a distinct median sclerite, the stemumt may be observed ; and a much larger tergal piece, the noium. At the sides of the somite are other definitely arranged sclerites, the anterior of which appear to answer to the epistemum and epimera in the OrusUtcea, while the posterior, perhaps, properly belong to the attached limb.
Forked or double apodemes, the a/ntefurea, medifurca, and postfwrca, project from the sternal wall of each somite of the thorax into its cavity. They support the nervous cord and give attachment to muscles.
The legs present a large basal joint, the coxa, between which and the third, termed /entur, a small articulation, the trochanter, is interposed. Upon the femur follows an elongated tibia; and this is succeeded by the tarsus, which consists of six joints. Of these, the proximal joint is long and stout, the three next are short, the fifth is elongated and slender; the sixth, very short, is terminated by two curved and pointed claws (ungties),*
The broad differences in the structure of the abdomen of the male and female have been already pointed out. Of the eight terga externally visible in the female (Pig. 98), the first is shorter than those which succeed it; and the hindermost (Fig. 98, lo) is escutcheon -shaped, deflexed
* Mr. Westwood (* Modern Mr. Westwood tervaB puhUIwt, but
Classification of Insects,' vol. i. p. it is a true joint provided with a
416) says that the tarsi are five* special flexor, the slender tendon
jointed, and that there is a pul- of which, however, traverses
villus between the ungues. The several of the joints of the tanus. lixth joint appears to be what
THB COOKBOAOH. 405
at the sides, thin in the middle, and notched at the end. When this tergum is gently pulled backwards, two other very narrow terga (Fig. 98, 8, 9), of which the anterior overlaps the posterior, and which were hidden between it and the antepenultimate or seventh tergum become visible. The apparent eighth tergum is therefore really the tenth. Beneath the tenth tergum are two triangular J90(2ica2 plates (Fig. 98, 11), one on each side of the anus. Provisionally, I take them to be the sclerites of the eleventh abdominal somite.
The first sternum is confluent with the second, and largely hidden by the cozsb of the metathoracio limbs. The seventh is g^reatly enlarged, and its posterior edge is produced into a boat-shaped process, nearly divided into two portions by an inward fold of the integument along the median line.
Completely hidden by the seventh sternum is a thin plate, narrower in front than behind, where it is produced on each side. Anteriorly, it is articulated with the stemimi of the following somite, so as to form a sort of spring-joint, which ordinarily keeps it applied against the latter, and therefore directed obliquely upwards and a little forwards. The large aperture of the vulva (Fig. 98, v) lies in the middle of this plate. On the sternal region behind the vulva, between it and the anus, arises a pair of elongated processes, divided into two portions, of which the outer is thick and soft, the inner slender, pointed, and hard. They embrace and partly ensheath two other processes having somewhat the shape of knife blades, the anterior fixed ends of which are curved, and, being attached to the sides of the somite to which they belong, are distant, while the blades meet, and are applied together in the middle line. Of these, which may be termed gonapophysea, the study of their develop- ment shows that the posterior bifid pair belong to the ninth somite, while the anterior pair belong to the eighth.
The cerci (x) are attached to the dorso-lateral part of the tenth somite.
In the abdomen of the male Blatta (Fig. 99). the ten terga are readily discemible; but the eighth and ninth are
4*)\j THE ANATOMY OF INVERTEBRATED ANIMALS.
very short, and the fcjrmer overlaps the latter. The tenth tergnm is flat, and has a freely projecting, truncated, pos- terior margin. Articulated beneath ita lateral edge are two multiarticulate cerci (as), similar to those of the female.
Beneath the tenth tergum, and hidden bj it, are the two podical plates (11) between which the anus opens. The first sternum is small, and may easily escape notice. The second to the bixth sterna are of nearly equal width and length. The seventh and eighth are narrower; the ninth still narrower and longer, about half of its length being covered by the eighth. The covered half is dififei'ent in texture from the uncovered, being thinner and more transparent* and its anterior margin is deeply notched. The uncovered half is strong, homy and dark-coloured, convex below and concave above; its free posterior margin is obscurely tri- lobed by two lateral shallow notches. On each side, a slender, unjoioted, setose style, which projects backwards and outwards, is attached to this sternum.
Thus the tergal surface of the abdomen of the male essentially resembles that of the female, while the sternal surface differs in exhibiting two sterna more (namely, the eighth and ninth) without dissection. Hence, while, in the female, the opening of the recto-genital chamber lies between the tenth tergum and the seventh sternum, in the male it lies between the tenth tergum and the ninth sternum.
When the tenth tergum and the podical plates are re- moved, a very singular apparatus, the male genital ar- mature, comes into view. It consists of a number of chitinous processes having the form of plates and hooks, the exact form and disposition of which could be made intelligible only by numerous figures. It may be stated generally, however, that these plates and hooks tenninate processes of the sternal region of the tenth somite, on each side of the aperture of the vas deferens, and therefore, though they are of the same nature as the gonapophyaee of the female, they are not their exact homologues.
The most conspicuous division of the right gonapophysif
Pig. 9S.— Longltudlaftl uid ver- ti»l aection of ft feirole Cock- ruach (Blatta). — i to ii, ■omltet of ths body; 1 to 11, KimiMi of the ■bdomen ; A, uitauiik ; lb, iBbnim ; a, mouth ; i, <eioptugua ; t, crop ; d, pro- ventricului; c, pylario ckc*; /, objLifia TCDlriclej g, iuMT- tionoriiieHalpiglilmacteu; A, inteitliie; i,reotum; e, tuIts; I, Mlivftry glud; », Mlliuy reorptrcle. B; aa error the duct li nude to terminftte above inWnii of beneeUi the lingiu; a, poritlon of heaiti m, emrt- bnl puigU>j N,thDiacic gut-
oru giwguBi 1'
403 THE AKATOITT OV UTVIBTIBSATBD AHtKALB.
is a broad pUt« divided a.t the eitremitj into two portdtma, th« inner of which carvea inwrnrds and ends in two or Uiree sharp apinea, while the outer is coiled npon itself ao aa to resemble a short corkscrew. The left gonapopbTsiB is pro*
Fig. 99.
-LongiliidinAl and vertical Mctlon of tlwBbdonea of ■ mala Moh (BJoda).— 1, S. a. 4, fco., terga and ilena of tin abdo- I, muihiDom-iluped giaiid ; e, ■pertore of the Taa daferaM ;
Tided with a long proceaa like a tonacnlnm, the incDrred extremitr of which is denticolatod.
The alimentary canal of the Oockroach commences br tl>e oral canty, aitnated between the labmm in front, tlw
THX COOKBOACH.
409
mandibles and maxilliB at the sides, and tlie labium, with the large lingua, or hypopharynx, behind. The (Bsophagns, beginning as a narrow tube, passes between the anterior crura of the tentoriimi, and then leaving the head bj the occipital foramen and traversing the neck and thorax, g^radnally widens into the large crop or infflumed (Fig. 98, c), which lies in the abdomen. TMs is followed bj the small thick- walled proventriculus (Fig. 98, d), shaped like a pear, with its broad end applied against the crop. The narrow end of the proventricnlus opens into a wide canal, the so- called chylifie venkricle, or ventrietUue (Fig. 98, /), an elon- gated tube, the junction of which with the intestine, is marked by the insertion of the nimieroTia Malpighian tabes. The anterior end ot the ventriculas is provided with seven or eight csBcal diverticula of unequal lengths (Fig. 98, e), the pyloric cceca. The first portion of the intestine {iieum) is narrow. The next, termed the colon, is very wide, and somewhat sacculated. A constriction marks off the region of the colon from the straight short rectwn (Fig. 98, i), which terminates in the anus, situated at the hinder ex- tremity of the body between the podical plates.*
The aperture by which the mouth communicates with the gullet is small, and situated at the superior and anterior part of the buccal cavity. A broad projection of the
* M. F. Piateaa CRecherches Bar les ph^aomtees de la digestion ches les InsectM,' 1874; ' Mote Bar les phenom^nes de U digestion chex la filatta americaine IPeri* planeta americanci],* 1876; and ^Rechercbes sor les pbenomines de la digestion chez les Myria- podes,' 1876), dividea the alimeiv- taiT canal of insects and myria- pods into a buccal, a median, and a terminal portion. The baccai portion consists of the oesophagus, crop, and proventriculus— wluch last he considers to be a mere strainer and to hare no mastica- tory ftinction. The middle divi- sion lies between the proventri- colns and the insertion of the
Malpighian tubes. The terminal division extends from the latter point to the anus. With the solitary exception of /k/m, the secretions of the alimentary canal are always alkaline, and that which effects the transformation of the albuminoid elements of the feod into peptones appeara to be iiimished by the middle division, which is lined by epithelium, de- void of any cuticie. In carnivor- ous insects digestion may take place in the crop by the flow of the secretion of the middle in- testine into it. The salivary' fluid of BlaUm rapidly eflfects the transformation of Btaroh into sugar.
410 THB ANATOMY OF INYBSTEBBATBD ANIMAIiS.
posterior and inferior wall of the buccal cavity occupies all thfe space between the cesophageal opening of that cavity and the labium, and ends in a free subcylindrical procefls. This is termed hypopharynx or lingua, but it might be well to reserve the term lingua for the free end, and hypopharynx for the attached posterior portion. The anterior surface of the hypopharynx slopes downwards and forwards; its sides are supported by two sclerites, which are narrow and rod-like above and broad below, where they unite in an arch on the dorsal face, just where the free part, or lingua, begins. On the under side of the lingua are two broader sclerites, which also unite and form an arch, which lies over the opening of the salivary duct. The anterior surface of the lingua and hypopharynx is beset with fine hairs.
The two salivary glands, with their receptacles, are greatly developed in the Cockroach.* The glands (Fig. d8, I) lie on each side of the CBsophagus and crop, extending through the thorax, as far as the commencement of the abdomen. Each gland is a white mass, as much as a quarter of an inch long and composed of numerous acini. The ducts which arise from these acini unite first into a single trunk on each side, and then, beneath the sub><£8ophageal ganglion, the two trunks join to form the single short salivary duct which opens beneath the lingua. The ducts of the salivary glands are lined by a transversely ribbed chitinous membrane, so that they greatly resemble tracheae.
The salivary receptacles (Fig. 98, k) are elongated oval sacs, three-eighths of an inch long, each of which is situated at the extremity of a long duct. The ducts unite in front with one another, and with the duct of the gland, to form the short terminal common duct. The receptacle and its ducts have a chitinous lining similar to that of the duct of the glands, but the spiral marking does not extend over the walls of the receptacle.
The pioventriculus has a thick muscular coat, and the
* The salivary glands are well tische und uropoietisohe System described by Basoh, ** Unter- der BlatU orientalis." ('SiUli. sttcbuDgen uber die chylopoie- Wiener Akad./ 1858.) ,
THE COCKBOACH. 411
chitmous lining which is oontinaed into it from the ingluvies is greatlj thickened and produced into six hard, brown ridge-like principal teeth. Posterior to these is a circle of six prominent cushions covered with setaB, and similar sets beset the lining membrane of the funnel-shaped cavitj into which thej project. Between each pair of principal teeth are five smaller tooth-like ridges, of which the median is the largest, and a variable number of still finer longitudinal elevations lie between them.
The proventidculus leads posteriorly into a narrow, thick- coated canal, the tubular extremity of which projects freelj into the much wider anterior end of the chylific ventricle, and constitutes a very efficient valve.
The short and narrow anterior division of the intestine i^ilewn) is separated from the colon by a circular valve, the surface of which is Leset with small spines.
The Malpighian glands are very numerous (20-30), delicate, csecal tubules, of even diameter throughout, and lined by a small-celled epithelium enclosing a central cavity.
The communication between the colon and the rectum is very narrow, but is not valvular. The walls of the rectum itself are raised into six ridges, which project into its interior and are abundantly supplied with tracheas ; these are the so-called rectal glands. Anal glands appear to be absent.
The histology of the alimentary canal has been particu- larly studied by Basch.* From the oral cavity to the funnel-shaped extremity of the proventriculus, it is lined by a chitinous coat continuous with the chitinous layer of the integument, and beset for the greater part of its extent with fine setiform processes. Beneath this is the proper endoderm, consisting of a layer of cells. Next follows a structureless membrana propria or basement membrane; and this is succeeded by two layers of striped muscular fibres, the internal disposed longitudinally, and the ex- ternal circularly. In the proventriculus, the muscular * * Si.zungtberichte der Wiener Aksdemie/ xxxili., 1658, ^
412 THE ANATOMY OF INYBSTEBBATED ANIMALS.
lajers become mucli thicker, and some of those of the outer layer acquire a radial arrangement, while the longitudinal muscles are disposed in bundles t^hich correspond with the six principal ridges. In the chjlific ventricle, the muscular layers and the basement membrane are disposed much as before. The basement membrane presents pits on its free surface in which rounded cells are lodged, and is beset be- tween these by the elongated cells of a cylinder epithelium. The free ends of these present a thick wall, marked by vertical striations. ^1 here is no chitinous layer. The csBca are merely diverticula of the wall of the chylific ventricle.
The intestine, finally, repeats^ the structure found in that part of the alimentary canal which lies in front of the chylific ventricle and is provided with a setose chitinous lining.
Basch found the secretion of the salivary glands and the contents of the crop acid,* and that an infusion of the salivary glands, acidulated with hydrochloric acid, digested fibrin. The contents ot the chylific ventricle were neutral or alkaline; and an infusion of the chylific ventricle at once turned starch into sugar. The same effect was pro- duced by an infusion of the salivary glands.
The heart (Fig. 98, h) ia a, slender inconspicuous tube, which occupies the middle line of the dorsal wall of the abdomen and presents, at intervals, pairs of lateral apertures. The wall of the abdomen internal to the chitinous integu- ment is lined by a soft cellular substance (hypodermis), the outer layer of which represents the ectoderm or epidermis, while the deeper part is the parietal layer of the mesoderm. This last contains a stratum of longitudinal muscular fibres, divided into segments or myotomea^ in correspondence with the somites, and numeroua tracheee. The heart is enclosed in the abdominal wall which surrounds it on all sides, leaving (mly a small pericardial space.f Beyond the slender aortic
* Plateau denies that the sali- t Cornelius (* Beitrage snr
vary secretion of Blatta is ever naheren Kenntniss von Peripla'
acid, and ascribes the occasional neia {Blatta^ oriemtaHt^ 18^X
aoldityof the contents of the crop found that tne pulsations of the
to the food. heart could readily be obaened
THB COCKROACH. 413
canal in which the heart terminates anteriorly, and which passes into the thorax and the head, no yessels appear to be given off from the heart.
Delicate triangular sheets of mnscnlar fibre, the alary muscles, are attached in pairs by their bases to the wall of the pericardial chamber, while their apices are inserted into the hjpodermis. They occupy the interspaces left by the principal dorsal branches of the trachesB, which form arches on each side of the heart.
From the inner face of the abdominal wall, processes are given off, some of which appear to hang freely into the abdominal cavity, while others accompany the nnmerons trachesB which pass to the alimentary cajial. When the abdominal cavity is laid open, its inner lining has a villous appearance and often seems to be full of free granular matter, as the processes very readily break up into fragments. The substance which thus fills up the interspace between the parietes of the abdomen and the contained organs is the corpus adipogwm. It is made up of cells often so arranged as to form a network, and it usually has a milk- white colour, which arises partly from the air contained in the tracheee, and partly from innumerable, strongly re- fracting granules contained in its component cells.
There are ten stigmata on each side of the body of Blatta, eight in the abdomen, and two in the thorax. The latter are situated between the prothorax and the mesothorax, the mesothorax and the metathorax, respectively ; above the attachment of the coxse and beneath the terga. The ab- dominal stigmata lie in the soft integument which connects the sterna and terga of the somites. All the stigmata are situated in conical thickened elevations of the integument. The thoracic stigmata are the largest, and the anterior pair have a distinctly two-lipped aperture, the anterior lip being notched in the centre. The openings of the abdominal
in BlatttB which had recently minate; but allowance most be undergone eodysit. They were made for the diaturbed condition aa frequent aa eighty in the of the inaeota under obaenration.
414 THE ANATOMY OF imTEBTEBSATBD AKDCALS.
jtigmata are more oval and are inclined baclnrards. Im- mediately within each stigma the tracheal tronk into which it opens is provided with a yalvnlar arrangement, by which the passage can be closed or opened.
4
Fig. 100.
Fig. 100.— Bfa/teorwnte/a.— A, the brain with the antennary (a) and optic (6) nerves; c, e,/,^, A,itomato-ga»tric nerves; B, the anie rfor end of the gullet ; C, the crop ; D, the gizsard.
The large trache» which take their origin from these stigmata immediately divide and give off dorsal and ventral hranoheB; the former unite in a series of arches on each «de of the heart, while, on the ventral side, the branches
\
THE COCKBOACH. 415
are connected by trunks which run parallel with the ab- dominal ganglia. Large tracheae pass from the anterior thoracic stigma through the neck into the head and, in the abdomen, the yiscera receive an abundant supply of air- tubes.
The lobes of the corpus adiposum are also plentifully supplied with tracheae, while fine trunks enter the substance of the ganglia and nerves and there ramify. Trachese accompany the nervures of the wings and are abundantly distributed to the muscles.
The nervous system consists of the snpra-oesophageal ganglia (Fig. 100, A), commonly termed the brain, united by thick and short commissures with an infra-cesophageal ganglionic mass, situated in the head; of three pairs of large coalesced ganglia in the thorax, one for the prothoraz, one for the mesothoraz, and one for the metathorax; of six pairs of closely united smaller ganglia in the abdomen ; and of a set of visceral or atomato-gadric nerves. The several pairs of thoracic and abdominal ganglia are united by double commissural cords. In the males the commis- sures which unite the abdominal ganglia are not straight, but are bent, as if it were needful to make allowance for the possible elongation of the abdomen. The supra-oeso- phageal ganglia give off the nerves to the antenns from their antero-lateral angles; while their postero-lateral angles are produced into the great optic nerves. Above the margin of each antennary nerve there is a small rounded tubercle which is in immediate relation with the silvery patch which shines through the fenestra on the inner side of the antennary fossa. Beneath this tubercle, and on the inner side of the antennary nerve, arises the root of the stomato-gastric system of nerves. Each root passes forwards for a short distance, then turns inwards, and in the middle line, enters a heart-shaped ganglion situated on the gullet (Fig. 100, e). From this a median cord passes backwards beneath the brain and enters a ganglion, which is connected on each side with two others (e, e). The continuation of the median cord passes back
416 THE ANATOMY OF INYEBTBBBATBI) AKIMALS.
along the tergal wall of the oesophagns, and where this begins to dilate into the crop ends in a small triangolar ganglion (g), whence lateral branches are given off, which can be traced as far as the gizzard.
The exact form and arrangement of the male organ of generation has only recently been made oat. The most conspicuous of these organs is a mushroom-shaped gland (Fig. 99, t) composed of a great nnmber of short cseca attached to the extremity of the also very short vas deferens. It is lodged in the hinder end of the abdomen, and covers the posterior abdominal ganglion. The contents of the caeca are viscid, granular, and usually brilliantly white. The anterior end of the vas deferens is dilated, and the cseca are arranged in two groups which open into each side of the dilatation. The contents of the vas deferens are also white and viscid, and evidently consist in great measure of the secretion of the cflBca. In the adult male, however, in- numerable spermatozoa with straight rod-like heads, and long flagella are to be found intermingled with the contents of the vas deferens and its dilatation. On the sternal side of the mushroom-shaped gland, between it and the last abdominal ganglion, there is an accessory gland composed of dichotomous monilated tubes, lined by a columnar epithelium, all bound together by a common investment into a flattened elongated mass.
As the duct of the mushroom-shaped gland in the adult male always contains spermatozoa, and no other organ con- taining spermatozoa is to be found, this gland has naturally been taken for the testis. Btg'ewsky,* however, has re- cently pointed out that the true testes are situated in the tergal region of the abdomen, and that they may be found in this region in the young and yet wingless males, though they are much obscured by the corpus adiposum which in- vests them. He traces the efferent duct of the testis to the glands just mentioned. In the adult male the testes atrophy, and are hardly to be discovered among the masses
* Hoftnann and Sehwalbe, * Jahresbericht,* 187.5. The original paper ii in Rnsslui, and I have not seen it
THX COOKSOAOR. 417
of Qie corptu adipoBnm. I have foimd the teatee in the young males in the poeition aeeigned to them b; Bt^eirskj. Thej comd^ ot nnmerona oval or pjriform sace attached bj short pedicles to a common duct.
The ovaries (Fig. 101) are two groups o( eight tnbra, aitn- ated on each side of the hinder hall of the abdomen. The OTBrian tubes, or omrtolet, of each gronp commniiicate with a short OTiduct, which soon unites with its fellow in the
Fig. 101.
Fig. lO\.—BrMa orintlfll* — Feaikle genital orgaoi : a, the |)0*t«rlor ■ibdoniiiul gaoglion j b, tha oTiducts ; c.ii, e, the oviriui tnbca { /, Dte fflunent by vrhich their eilremitiei va oolted ; g, tb« ipeniM- theca; h, tbe colleterial glands.
middle line and opens extenutllj by the Tery short and wide Tagiua. Tbe finely tapering anterior ends of the ova- rioles of each side are continued forwards by delicate cellu- lar prolongations. These finally unite together into one long filament, which can be traced for some distance for- wards among the lobes of the corpus adipoeom. Itis a cellular cord, which appears to be nothing bat a process 2 B
418 THE ANATOMY OV UfYBBTKBBATBD AKIMAI.8.
of the mesoderm. Numerous nucleated cells, from some of which the ova take their origin, while others remain as in- terstitial ceUs, which are eyentually conyerted into an epi- thelium, make up the substance of the slender anterior terminations of the ovarioles. The ova situated behind these enlarge, and become disposed in a single series. Further on, the epithelial cells form a thick stratum ronnd each egg, and possibly assist in the formation of the large ▼itellus with which it is ultimately provided. As the egg adyances towards maturity, the vitellus acquires first a finely, and then a coarsely granular structure, and the ger- minal vesicle and spot, previously conspicuous, are no longer to be seen. Behind the junction of the oviducts with the vagina, and the last abdominal ganglion which lies upon the latter, there is a small sac with a long neck from which a short csdcal process is given off. It has a thick chitinous lining and a muscular investment, and is the spermcUheca. Behind it are two, large, ramified, tubular colleterial glands, which probably give rise to the substance of which the egg case is formed. Their conjoined ducts open behind the spermatheca.
The eggs are enclosed, sixteen together, in strong capsules of a homy consistency, shaped somewhat like a cigar case, and presenting a longitudinal slit, the raised and serrated edges of which are closely applied to one another. It is through this slit that the fully developed young make their exit. The eggs attain J of an inch in length. Each has its own thin but tough brownish shell, the surface of which is beautifully ornamented with hexagonal patches of minute tubercles. They are arranged parallel with one another in two opposite series, one series occupying each half of the ease. The eggs, adapting themselves to the form of the case, are oonvez outwards and concave inwards, and thus, though their ends touch, a median space is left between the two sets. The inner concave face of the egg is that on which the sternal face of the embryo is situated. The female eanies the egg-case about for a week or more, before 4epoeitiiig it. The young leave the eggs as minute active
THB COCntOJLCH. 41^
insects, oolonrless, except for the large dark eyes. Before they are hatched they acqtdre eyes, antemuB, gnatliiteB, legs, and short oerci, which differ only in detail from those of the perfect BlaUa, into which the larva passes by snc- cessive ecdyses. According to Comelins (I. e. p. 29), the Cockroach undergoes seven ecdyses ; the first immediately on leaving the egg, the second, a month later. After the second ecdysis, the insect sheds its skin only once a year ; so that it attains its adult condition only in its fifth summer. The chitinous cuticula splits along the median line of the ter- gal aspect of the head, thorax and abdomen,before it is cast. Thus the Cockroach is said to be an insect without meta- morphosis. For although the male, in the later stages of its growth, acquires wings, and thus does become very sensibly metamorphosed from a merely cursorial animal to one which has, at any rate, the capacity for flight, there is no period in the life of this insect in which the larva paases into a resting condition, during which it takes no food, aud in the course of which it developes its wings. In other words, the Cockroach passes through no pupa state, which the insect enters as a larva, and leaves as an imago, such as is so well known to occur in the course of the develop- ment of Moths and Butterflies. The term meiamorphoM, in its technical entomological sense, is applied only to that succession of changes of which such a definite pupal con- dition forms the middle term.
It is obvious that a metamorphosis, in this sense, is a secondary complication superinduced upon the direct and gradual process of development exhibited by such insects as the Cockroach ;* and that the Metaibola, as insects having a metamorphosis are termed, are, so far, more differentiated than the Ametabola, or those which have no metamoiphosis. Again, in each of these divisions, it is clear that the insects which never possess wings are less differentiated, or more
* Sir John Lubbock has shown panied by a slight change of form
that the ^oong Chioecm {Ephe- m its passage to the aduH state.
wktra) dimtdiatum undergoes more (Trans. Linn. Soc. 1863.) than twimty eodyteai each aecom*
420 THB AKATOMT OF nTYBKTBBBATKD AKIKALS.
embiyoiiio, than those which are winged. And finally, insecta with the parta of the month in the condition of ordinary gnathitea are less differentiated than those in which such gnathites are changed in form and function, or become confluent. The insects which, in this view of their morphological
Fig. 102.
Fig.102. — CampodeattaphyUnui, one of the Tf^$aHura (after Labboek).*
idations, occupy the lowest position in the group, are the CdUembola and Thysanura, the MaUophaga, and the Pedi- euUna, inasmuch as they possess no trace of wings and undergo no metamorphosis.
The CoUembola and Thyscmwra undergo no metamor- * *Monognph on the OoUemboh and V^mmmra^' pL liiL
THTaAinXBA. — PSDICULINA. 421
pliosiB, and are always wingless. The abdomen contains six segments only in the CoUembola (Podwra, Smynihiwrus, ToTtioeeros), in which group the mouth is usually provided with mandibles and mazillsB, though these, instead of being articulated with the sides of the head, are capable of being retracted into its interior.* In the genus Anoma the mouth is suctoriaL
The Thyaanwra (Lepiama, Campodea, Japym) resemble the young Blattce. They have ten well-marked abdominal^ so- mites (CampodeOj Fig. 102), and the gnathites conform to the mandibulate type. The abdomen in McuJietea has a pair of elongated cylindrical appendages attached to every segment except the first ; while Campodea and Japym have seven pairs of such abdominal appendages-f
The CoUembola are provided with a curious tube or sucker, which is attached to the sternum of the first abdominal somite, and gives exit to a glandular process, which secretes a viscid matter. Most of the insects belonging to this group possess a curiously contrived "spring and catch'* attached to the sternal region of the penultimate or ante- penultimate somites of the abdomen, by the help of which they execute their vigorous leaps.
Sir John Lubbock could find no trace of trachese in any of the Collembola except SmtfrUhwrvs, though they are easily seen in many of the Tkyaanura, According to the same authority Lepisma has four Malpighian tubes, while Campodea, Japyx, and many Collembola have none.
The MaUophaga are parasites upon mammals and birds, on the hairs and feathers of which they feed. The head and body are depressed, the eyes simple, the gnathites of the masticatory type. The abdomen has nine or ten visible segments.
The Pedieulina, or Lice, subsist upon the blood of the mammals on which they are parasites. The gnathites are
* Lubbock * Monograph on the has similar appendages attached CoUembola and Thytanura,* p. 37. to each segment along with legs, t The myriapod SeolopendriUa (Lubbock, /.c.)
422 THB ANATOMY OF IKyBBTKBRATX]> AKIMALS.
conyerted into a piercing and sacking apparatoB. The under aide of the head presents a soft protrosible pro- boscis, provided externally with minute homj ho<^u, and trayersed bj a canal which leads into the CBsophagns. The proboscis incloses two grooved chitinons sidles which are applied together hj their concave sides; and, within the sheath thns formed, lie two finely-pointed chitinons aet» which can be moved up and down in the sheath.* . The proboscis is, in all probability, formed by the union
Fig. 103.
Fig. lOS.— ^ctIei iitTra.— a, the aqoatie apterom larra. B, one of the transitional stages between this and the perfect insect C. (* ^^^P® Animal/)
of the labrum with the second pair of maxiUss, while the two halves of the homy sheath are the mandibles, and the setsB, the first maxillae. The prothorax, mesothorax, and meta- thorax are hardly distinguishable, and the abdomen has nine visible segments.
The Orihoptera (Pig. 103) and the Hemiptera (Fig. 104) are ametabolous. The majority have two pairs of similar or more or less dissimilar wings in the adult state, and
* Qeisifeldt, * Ueber die Mondtheile dcr Sangenden Inseetan,' 1853.
OBTHOITKBA. — HBHIFTEK^ 428
in the apterona fotms it ia probable that tlie wings on aborted, not tTpicalljr absent. In the Orthofi&n* (the' Termites, Cockroaches, Orasahoppers, Cricketa, Day-fliea, Dragon-fliea and Earwiga), the month ia constracted npon the same plan aa that of Blatta; but the Phytopoda or TkifaanTptera (Thript and ite allies), small winged inaecta which live chieflj in fiowen, present a modification whioh ia tranaitional to the Hemipt«ran month (Gerstieldt, I. e.)- There ia a proboscis directed backwards and formed bj the anion of the labnun with the labium, which last ia provided with palps, though thej are sometimes tstt smalL Tha Fig. 10*.
Fig. \0\.—Aphii ptiargoitii. ApteroiU ■gsmogenelio form.
maxilli'! are palpigerons and are united at their bases with the labium. The mandibles are atjliform setce enctoeed in the proboscifl.
In the Semtpfem.t all of which sack the blood of n.i>imnl» or the juices of plants (Bags, FUnt-lice, Cieiida), wings may be present or abaent, and the eyes are usually com- pound. The visible abdominal somites may be rediuwd to six. The gnathites are modified to form a piercing and suctorial apparatus, which ia similar, in many respects.
• The Tl^iaaura and the PAy- with the Nruroptta. Kpoda arc ot'len ualted with the t The MalUfkaga and the
OHhoptcra in modem eluaiflca^ Ptdiculiiia are onlwd wlih llie ~ K^tors by lome aulhon-
424 THE 1.ITAT01IT OF IHTEBTKBEATXD AimULS.
to that of the Pediealina. There ia & Twiadly aharp arad pcmt«d labmiii, while the mandiblea and maxilhe are mere tubercles, ennnoant«d by long chitinoiM pointed styles, of which therefore there are four. The lahiom ia OBtuJly represented by a median, jointed, fleshy, elongated body, the snt«rior face of which presenta a longitudinal groove in which the m&ndibles and ntinT'lli'' are enclosed. Neither . the maiillie nor the labium are provided with palps.
Thns, in the series of ametabolons insects there are some
with Biasticatory, others with suctorial months. It is I7
no means clear that the gnathitee of the aoctorial montii of
the Hem^iera are to be r^arded as modificatiotis of
tie. 10^-
H- lOS. — Hydrtmkilia puma. (■ Ri^ne Animal.')
B, pup.-*.
masticatory gnathites of the type exhibited by the Oriho- ptera. The absence of palps is a very significant fact, ang- geating that the Hemipteron mouth is the extreme term of a series of modifications for the commencement of which we must go back to the Myriajmda.
The metabolons CoUopfera, or Beetles (Pig. 105), hare masticatory mouths of the same general type as those of the OrthopUra ; with which they are closely connected through the Earwigs. The two constituents of tJie labium are how- ever much more completely confluent than in the Ortkoplera,
NBUBOPTEBA. — I>IPTB&A« 425
There are nBuallj two pairs of wings, the anterior pair being conyerted into stiff homj elytra ; these take no part in the act of flight, but serve as covers to the metathoracic wings, which, in the state of rest, are folded np beneath them. The number of apparent somites of the abdomen is often much reduced. In the metabolous Neuroptera (Ant-lions, Caddis- flies, Scorpion-flies), in some of which the insect is more or less active during the pupa state, the parts of the mouth are, for the most part, very similar to those of the Ortho- ptera. In two groups of Neuroptera, however, the mouth becomes suctorial. Thus in the Trichoptera, or Caddis-flies, the labrum is elongated and grooved posteriorly; the mandibles are aborted, the bases of all the gnathites are united, and the labrum is a spoon-shaped body. In the Scorpion-flies (Panorpina), there is, according to Gerstfeldt, a proboscis formed in front by the elongated clypeus and labrum, and behind by the coalesced maxillsB. The man- dibles are small, and the first marillm much elongated. The ordinary four palps are present.
The Neuroptera have two pairs of wings of a delicate reticulated structure. The metathoracic wings may or may not be folded.
What appears to be a further development of this type of mouth is found in the Lepidoptera (Butterflies and Moths). The labrum and the mandibles abortf and the labium is represented only by a triangular plate which bears two large palps. On the other hand, the maxillse, the palps of which are always very small, are often immensely elongated and applied together by their channelled inner faces, thus constituting a sucking proboscis (Fig. 106, 107). The wings, similar in character, and covered with minute scales, are rarely absent. Both pairs are used in flight.
In the metabolous Diptera (Flies and Fleas, Fig. 108), the mouth is constructed upon the same plan as that of the Hemi- ptera, so far as the conversion of the labium into an organ of suction is concerned ; but usually the metamorphosis of the gnathites is carried still further, and the maTillte have palps. Thus in the Fleas, which are parasitic on mammals and
42S THE AHATOMT OF IHTUtnBKATID AKIMALS.
birda, what appears to be the labmm is an elongfttod, alcnder st^le, which lies between the two elongated nutadiUM. The first maiilla are broad triangular plates, each with a four-jointed palp. The second marilhe (labinm) are re- presented bj a short median lamella, which boands the Fig. 106. Fig. 107.
Fig. 106.— The biad, A, md p«msnf the month, B C, of ^Ajjtf iifftattri-^ii, antenna; b^ epicn
/, nund'ible; p'mixMt; A/niuilkVy' pilpiu t, labial palpiu. B, bue of the HKillla witi
tbBH
(.Uter Newport.)
m; C, lateral view of
t, papilUe; 4, hooVs wUch
month behind and is provided with two. long palps, wfaicb resemble knife blades and are imperfectly divided into tooT joints. The three somites of the thorax are distinct, and the two hinder ones have lamellar appendages, whieh posnblf represent wings. The abdomen has ten somites.* * 8aa L. Lutdols, ' Anstomla dee HnDdeflobet,' 1S66,
&-
427
In those dipterouB inaects wliioli are tenned Ftipipara^ which are apterouB, or nearly so, and parasitic npon mammabs, birds and bees, a circular wall, or short pro- boscis, inrests the other parts of the month. There are, first, two lateral, protrasible, homj plates; secondly, an anterior and a posterior seta; the latter stronger and grooved longitudinally in front. Between these is a single fine seta. Gerstfeldt considers that the last answers to the hypopharynx ; the second pair, to the labrom and the second Tni^Tillm ; the first pair, to the first maxillm ; and that there are no mandibles.
The ordinary Dipiera, which possess one j^air of functional wings attached to the mesothorax, resemble the Hemipiera in possessing a usually fieshy proboscis, often tumid at its
Fig. 108.
Fig.lOS.^Syrphusribesu.—A^lamL, B, papt. C, imago. (* Regne Animal.')
extremity, which is formed by the confluent second maxillse. As in Hemiptera also, the labrum is a more or less elongated pointed plate, and the mandibles and maxillae are usually ter- minated by chitinous cutting setsB (Fig. 109). But the bases of these parts are constantly united together; there is a pair of maxillary palpi, and often a median, more or less styliform structure, usually considered to be the hypopharynx. It seems doubtful, howeyer, whether this may not be formed by the coalesced terminations of the maxillsB. In the common House-fly, the labrum, mandibles, and maxiUse coalesce at their origins to constitute the base of the proboscis, which is mainly formed by the confluent second maxillm. Its longitudinal grooved anterior face is overhung by the
428 THE ANATOMY OF INTSBTSBSATSD ANIMALS.
elongated stjliform labmm. The gnathitee here exhibit almost the extreme modification of the piercing and sucking type of mouth.
FinaUj, the metabolous HymenopUra with, usually, two pairs of reticulated scaleless wings present a series of modifications from the essentiallj masticatory mouth of the Ants to the partly masticatory and partly suctorialt
Fig. 109.
Fig. 109. — ErhtatU floreiu.—d^ front of the head ; e, labrum ; /, man- dible ; g^ maxilla and palpus ; t, labium ; t* extremity of the labium semrately and more magnified ; t"**, inner surface of the paragtoMV ; i*^, the rows of hairs on the inner surface ; ly the ligiila ; «, the cardo and submentum. (After Newport.)
or rather, lapping mouth, such as is met with in the Bees. In the latter (Fig. 110) the labrum is small; be- neath it, a median fleshy lobe — the epipharynm — OTcr- hangs the minute apertiure of the mouth. The mandibles are strong with wide, almost spoon-shaped, extremities. The part of the maxilla which appears to answer to the lacinia in BlaUa, is shaped like a knife-blade and lolds upon the stout sHpea, Like a clasp-knife in its handle.
THI HmXOFTBK^
The ahort and almost mdimentai; palp is attached to the eztremitj of the atipee. The cardinei are t(aig and
Tig. no.— (DpFCTftgnre.) FectioDof thchcwlot Jlamhu. t.oeella*; Ct kntemu ; d^ cl jpnu ; tf, labrum ; y, muidiblA ; ff^ cjriphUTiu ; hy rnulUa ; t, urdo ; j, 4, /, submentum 4iid menlum ; n, in', laUal pal- pal; n, paraglMsa; o, lingua or mediin proceai of tha Ugala; ■, DodpltBl fomnen ; 1, 3, sclariln of the hjpopharjnx.
(LcftlowcrBgure.) Tenninil porlion of amasilia.
(Middle lower figure.) Epipharjrax and hjpopbaryni nuRnifled ; 1,S, ■dertlai of the hypopharyni ; 3, cat and of the niopliagni ; 4, 5, •elerltea tn Ihe wall of the CHophigni and ildea of the mouth ; fi, Kp- likfl pnqeetlon of the hypopbaryni ; f, epipharyni.
(Right lower figure.) a. Quadrate iclerita coDneoted by a MaDgnlar pteoe with c, one of the lancet of Ihe sting ; b, doct of the pobon gland ;/, gnwTed median piece In which the laucea play; h, ODa of die latenfietoae palplform sheath-piecei ; g, genllal aperttm.
■lender and give rise to a hinge joint, wlierebj the maxillia and labiom can be folded back, like a carriage
430 THE ANATOMY OF HrYSBTSBSATBD ANIMALS.
step, under the head. The mentum is lar^, the labial I>alp8 long and slender ; there are two large paraglosssB and, between them, a median, annulated, setose, cylindrical organ proceeds, which either represents the lingaa, or is an independent prolongation of the ligula. Fnnctionallj, this organ is a tongue, and enables the bee to lap up the honey on which it feeds. The mandibles and maxillfB are em- ployed Hs cutting and modelling implements, but appear to have little or nothing to do with mastication, properly so called.
The gnathites and the mouth are abortive in some insects, as the Day-flies, which take no food in the adult condition. The development of the different divisions of the alimentary canal varies greatly. Salivary glands are very generally present. In many suctorial insects, the ingluvies is a sac opening by a long duct into the gullet ; a distinct proventriculus, provided with chitinous ridges, may be present or absent. The ventriculus appears to be always devoid of an inner cuticula. It may be devoid of csBca or beset with short cseca throughout its whole extent. The number of the Malpighian tubes, which are sometimes branched, varies from two to a multitude. In many cases they have been found to contain uric acid ; bat no biliary matter has yet been proved to exist in them. Anal glands are frequentiy appended to the termination of the rectum, and may secrete an acrid or stinking fluid.
In some larvsa {Myrmecoleo, Dytiscus) there is no proper median oral aperture, but canals which open on the ex- tremities of the mandibles lead into the oesophagus, ^e alimentazy canal has no posterior opening in the lar^s of many Hymenopiera, of Myrmeeoleo and of the Pttpipara, The salivary glands secrete the silken material in which the larvse of the Lepidopiera invest themselves ; while, in Myrmeeoleo and the HemerchidcB, it is the rectum which fnzniahes the silk.
The poison of the Hymenoptera is a fluid strongly im* pregnated with formic add, which is secreted bj a special
« «
OYIP08ITOB8 AXm STIKOS. 431
gland and poured into a reservoir connected with the sting.
In many winged insects both pairs of wings are de- Teloped and take equal shares in flight {Hymenoptera, Lepidoptera, Newropiera), In the Coleoptera, the anterior pair are converted into homj wing-covers {ehftra), and the posterior pair, much larger than the anterior and folded up under them when the insect is at rest, subserve flight. In the Diptera the posterior wings are represented only bj short processes, the hctUeres, In ike Strepgipiera, on the other hand, it is the anterior pair of wings which abort. In all orders of winged insects, individual cases of complete abortion of the wings occur either in the female alone, or in both sexes.
The posterior abdominal somites often undergo extensive modifications ; thej may be small and retracted within the anterior somites, or they may even become more or less completely abortive. In many insects, processes of the somites in the genital region of the female, which answer to the gonapophyses of BlaUa, are converted into organs which assist in the deposition of the eggs and are termed ovipaniora. The saws of the Saw-flies and the stings of other Hymenoptera are to be regarded as specially modified ovipositors. The laborious and thoughtful investigations of Lacaze-Duthiers * led him to the conclusion that all these organs are constructed upon the same plan; that they are developed from that somite of the abdomen which lies immediately behind the opening of the vulva; that this opening is always situated between the eighth and the ninth somite ; and is therefore separated by ihiee somites (the ninth, tenth, and eleventh), from the anus.
According to Lacaze-Duthiers, in those insects which are provided with an ovipositor, saw or sting, the ninth somite always consists of a single median tergal sclerite, to the inferior angles of which are connected two small more or lees triangular pieces, each of which carries a long styli-
* ** Recherches ear Tarmure g^nitale femelle des InaectM.*' (' AnsAleB det Sdencet NatureUes/ 1S4»-1853.3
432 THS ANATOMY OF INTSBTBB&ATBB AWTMAT^.
form appendage. There is a smgle median sderite, wbich is the most important part of the boring apparatus; two small sclerites are united with the lateral angles of this piece, and there are two other elongated sclerites which constitute a valvular sheath. Thus, according to Lacaxe* Duthiers' view, in the sting of Bombtu (Fig. 94) k is one of the elongated lateral sternal sclerites, which with its fellow forms a sheath for the rest of the apparatus ; / is the median sternal sclerite; it is pointed and grooved on its sternal surface; while c, one of the lances, is a prooesB of the tergal half of the somite. Each lance is sharp and slender, and its tergal edge fits upon the margin of the groove of the median style, in such a manner as to be able to slide backwards and forwards upon it. The sternal edges of the two lances meet in the middle line, and, together with the median sternal piece, enclose a canal which serves to convey the secretion of the poison gland into the wound made by the sting. In the operation of stinging, the median piece serves as a sort of " director " for the two lances.
However, recent investigations into the development of stings and ovipositors,* e.g. the sting of the Hive-bee, and of the Wasp and the ovipositor of an Ichneumon-fly (Cryphu migrator), show that while the median grooved piece and the two sheath-pieces arise from papillsB developed upon the sternal surface of the ninth abdominal somite of the larva, the lances are the result of the metamorphosis of papills seated on the sternal surface of the eighth somite; and these papillae are so similar to those from which the limbs are developed, that it becomes (to say the least) probable, that they represent true appendages of the somites to which they are attached, rather than mere modificati<Mi8 of the sclerites of the body- wall, as Lacaze-Duthiers supposed
* Kraepelin, ** Untersuchungen des SUchels und der L^e-
Ciber den iBaa, Mechimismuif und scheide." (* Zeitschrift fur Vfin,
Entwickelangsgeschichte des Zoologie,'1875.) See also the ob-
Stachels der Bienenarti^en servations of Packard, *On the
Thipre** C Zeitsehrift fiir Wist. develupinent aud poaiUon of the
Zoolofrie,' 1873); and Dewitz, iJymenoptera,' 1866. ^Ueber Bau und Entwickeinng
THB COPULA.TOBY ORGANS OF IK8ECT8. 433
them to be. In like maziner, the examination of the develop- ment of the ovipositor of Locusta viridiasima has proved that, of the three pieces of which each half of it is com- posed, two are developed from the sternum of the ninth and one from that of the eighth somite. But the two median pieces of the ninth somite do not unite together to form a single piece grooved below, as in the hymenopterous sting or ovipositor. And observations which I have made on the development of the gonapophjses of BlaUa, lead me to the conclusion that the posterior bifid pair are developed from the ninth, and the anterior curved pair from the eighth somite. In this case the latter will be the homo- logue of the lances of the Bee sting.
Thus it would appear that, while there can be no doubt as to the general unity of plan of ovipositors and stings, the view of Lacaze-Duthiers must be modified. It must be admitted that these apparatuses appertain to the eighth and ninth somites, and not to the ninth alone; and that there is much reason to suspect that their chief constituent parts are modified limbs.
The male copulatory organs * are often very complicated, and their homologies have not yet been fully determined. Kraepelin (I, c.) who has examined the development of these parts in the Drone, and the modifications found in herma- phrodite Bees, is led to the conclusion that they are developed from the eighth and ninth somites of the abdomen, and therefore are the homologues of the parts of the sting in the female. In the male Blatta, however, it is obvious that the male copulatory apparatus belongs to a more posterior somite than that upon which the female gonapophyses are developed.
The heart usually has the form of a flattened tube, closed at its posterior end, but, in front, continued into the aorta,
* The male LibelMida possess ordmtry position^ and hence, be-
a peculiar copulatory apparatus fore copulation, the male has to
developed upon the sternum of bend the extremity of his abdo-
the second abdominal somite. men* upwards in order to load
The genital apertore has the this apparatus with spermatosoa.
2 V
42i THE ANATOMY OF INYEBTBBBATBD AKIK ALS.
which may be traced as far as the cerebral ganglia^ and appears to give off no branches. The sides of the tube present slit-like openings {ogHa), which ybtj in number from two to nine pairs ; and, when there are several pairs, each pair answers to a somite of the abdomen. The mai^ins of the ostia may be simple, or may be produced inwards into folds which play the part of yalves. Moscnlar or UgamentouB fibres may extend from the hypodermis to the dorsal aspect of the heart, and serve to suspend it in place.
The alary muscles, which in most insects are fan-shaped, and lie in pairs, opposite one another, on each side of the heart, either unite in the middle line, or are inserted into a sort of fascia, on the sternal aspect of the heart, to which organ they are not directly attached.
The septum between the pericardial cavity and the general cavity of the abdomen thus formed, is termed by Graber* the pericardial septum. From their anatomical relations, therefore, the alary muscles can have nothing to do with the diastole of the heart, the pulsations of which indeed, go on just as well when the alary muscles are cut through. Graber throws out the very probable suggestion that the contraction of the alary muscles causes the peri- cai'dial septum to move towards the axis of the body, and by thus enlarging the cavity of the i)ericardium, facilitates the flow of blood to the ostia of the heart. The same investigator asciibes a special i-espiratory function to the abundant trachea) which are distributed to the walls of the pericardium, and which, undoubtedly, must tend to facili- tate the aeration of the returning blood.
In many insects, a septum, provided with transverse muscles, overlies the abdominal nerve-cord and separates a ventral blood sinus, in which the cord lies, from the abdominal cavity. The sinus is open in front and, as the muscles of the septum contract rhjrthmically from before
♦ "Ueber den v''op"^wi^oni- 187.^), and **Ueber den pulti- sehen Apparat der Insecten ** renden BauchsinuB der Inftecten." (< Zeltschrift fur Wise. Zoologie/ (Ibid., 1876.)
THB BBSPnULTOBT OBOANS OF INSBCT8. 435
backwards, they tend to drive the blood which enters it to the posterior end of the body.
In the respiratory system of insects the number of stigmata is observed to vary from one to ten pairs. As a rule, none are found in the head,* or between the head and the first thoracic somite, and they are usually absent from the terminal somites of the abdomen. A very common number is nine pairs ; the first being situated between the mesothorax and the metathorax, and the rest between the following somites. Only two pairs of stigmata are found in the lAhelluUdoR and Ephemendos, and they are seated upon the thorax. In Nepa and Banaira, there is only one pair of abdominal stigmata, in addition to those in the thorax, and in the larvsB of Tipulidce and of HydrophiluSt the stigmata are reduced to one terminal abdominal pair. The stigmatic openings are usually situated upon the sides of the abdomen, but in some Coleoptera (e. g. Dytiscus) they are dorsal, and in many Hemiptera they are situated on the ventral aspect of that region of the body. Either the lips of the stigmatic aperture itself, or the walls of the tracheal trunk which arises from it, are so disposed as to constitute an occlusor apparatus, provided with a muscle, by the contraction of which com- munication with the external air can be cut off. This occlusor apparatus, long ago described in certain insects by Strauss- Durckheim, Newport, Burmeister, Siebold and others, has recently been specially investigated by Landois and Thelen,t who describe it as usually consisting of four essential parts : the how (Verschluss-biigel), the lip (Verschluss-band), the lever (Verschluss-hebel), and the muscle. The bow is a thickening of one-half of the circumference of the chitinous lining. The lip is formed by the other half of the circum- ference, and the lever is a chitinous process connected with one end of the bow, or with the lip. When the lever is single, the muscle which is attached to it passes over the lip
♦ Sir John Lubbock found the t " Der Stigmenverschlu's bei
two spiracles pf Smynthurus, to be d en Insecten. (* Zeitschrift iiir
situated on the under side of the Wisscnschattliche Zoolngie/
head, immediately below the an- lb67.)
teniue. 2 F 2
486 THB ANATOMY OF IKYERTBBSATSB AKOCAL8.
and is inserted into the opposite end of the bow. When it contracts, it therefore presses the lip against the bow. When two levers are present, they are attached to opposite ends of the lip and bow, and the muscle extends between their extremities. The effect of its contraction is to throst the free edge of the lip against the bow.
The tracheal trunk which arises from a stigma may ramify without communicating with the rest ; but, usually, the trachese which proceed from each stigma enter into more or less extensive anastomoses. Yery commonly the main trunks of each side give off wide anastomotic branches, which unite and form a longitudinal trunk on each side of the body, while transverse trunks often connect the main trachese of opposite sides.
In many insects, especially those which possess great powers of flight, more or fewer of the trachese become dilated into sacs, in which the spiral marking of the chitinous lining is interrupted or disappears. In Bees and Flies, a vast air-sac is thus develop^, on each side of the abdomen, from the longitudinal anastomotic trunk.
The aquatic larvae of many Orthoptera (EphemeridcB, Agrion, Calopteryx) and Neuroptera, and of some Dipiera, Lepidopteray and Coleopiera, though provided with a fully- developed tracheal system, possess no stigmata. The somites of the abdomen or of the thorax are, however, provided with delicate foliaceous or filamentous processes, into which branches of the trachese enter and ramify. The air contained in these trachese is therefore separated from that dissolved in the water only by a very thin layer of integumentary tissue, and an exchange of gaseous con- stituents between the two readily takes place. These are often called hranchioe, but they are obviously of a totaUy different nature from true branchise. The larvse of some Dragon-flies {Libellula and ^schna) present yet another form of respiratory organ. Although they possess a pair of thoracic stigmata, these appear to have little or no functional importance, but respiration is effected by pump- ing water into and out of the rectum. The walls of the
THE SOUNDS PBODUCED BT INSECTS. 437
j« produced into six double series of lamellae, in the r of which trachese are abundantly distributed, and play the same part as the tracheal branchise just ned. These rectal respiratory organs, in fact, appear b complicated form of the so-called " rectal glands " ure so generally met with in insects, chief agent of the movements of expiration and tion in insects is the abdomen, the capacity of which ) diminished by the approximation of its terga and and the shortening of its length by the retraction posterior into its anterior somites; while it may irged by movements in the opposite directions, bhe cavity is enlarged, air rushes in at the stigmata, en it is diminished, if the stigmata are open, expira- 3ur8 ; but, if the stigmata are shut, the effect of the Dry act must be to drive the air into the ultimate itions of the trachese. The movements of inspira- d expiration vary in rapidity with the condition of K;t. In the Bee, Newport observed that in the state they were as few as forty, but that they rose to one i and twenty with muscular exertion. Eur-sacs doubtless assist flight by the diminution of tcific gravity of the insect, which follows upon their, on.
sounds produced by insects* are, in a great propor- cases, effected by the friction of hard parts of the lent one against the oth^. Thus the Grasshopper 3 femur of the hind leg against a ridge on the anterior ad the chirp of the Crickets and Locusts is produced friction of the elytra. The parts which thus rub r are provided with serrations and ridges, which M>n8tant and characteristic disposition. The longi- »etles produce a sound by the friction of the tergum )rothorax upon a process of that of the mesothorax. Dung-beetles by rubbing the coxae of the hind legs the hinder edge of the third abdominal sternum.
lAndois, **Die Ton- and Stimm-Appante der Insecten." rift fiir Wiss. Zoologie,* 1867.)
438 THB ANATOMY OF INYERTEBBATEB ANIMALS.
Further, sounds ai*e necessarily produced by the extremely rapid vibration of the wings, which characterises the flight of many insects. Landois, however, found that the thorax of a Bluebottle fly continued to buzz after the separation of the head, the wings, the legs, and the abdomen. The separation of the halteres weakened the sound but slightly. The acoustic apparatus, in fact, lies in the immediate neighbourhood of the thoracic stigmata. The main trunk of the trachesB dilates into a hemispherical sac, which opens externally by the stigmatic orifice. The sac presents a hooplike thickening, to which are attached free chitinous folds or processes, and it is to the vibration of these that Landois ascribes the sound. The vocal organ of the Fly would thus appear to be a modification of the occlusor apparatus of the stigmata, just as the organ of voice of mammals is a modification of the occlusor apparatus of their i^espiratory opening.
In the Cicad<2 the vocal organs are, according to Landois, the posterior thoracic stigmata. These open into chambers, in the walls of which tense membranes are so disposed as to intensify the sound by their resonance.
As in the Crustacea^ so in insects, the central nervous system varies very much in the extent to which its com- ponent ganglia are united together. In most Orthoptera and Neuroptera and in many Coleopteraf the thoracic and ab- dominal ganglia remain distinct and are united by double commissures as in Blatta. In the Lepidoptera, the thoracic ganglia have coalesced into two masses united by double commissures ; while in the abdomen there are five ganglia, with single or partially separated commissural cords. The concentration goes furthest in some Diptera and in the Strepsiptera, in which the thoracic and abdominal ganglia are fused into a common mass.
A system of stomato-gastric nerves similar in its general arrangement to that of Blatta^ is very generally present.
A special system of nerves, termed respiratory or trantvene, is found in very many insects, both in the larval and in the perfect condition. The principal nerves of this system are
THB AUDITOBY ORGANS OF IK8BCT8. 4S9
arranged in pairs on the sternal aspect of the body, and their outer extremities anastomose with branches of the ordinary peripheral nerres and are distributed to the muscles of the stigmata. Their inner ends unite into a plexus, which lies OTer the interval between two of the ganglia of the central nervous cord, and they are connected by longitudinal cord^ with one another, and with th^se ganglia.
In insects, as in other arthropods, the branches of the nerves which are distributed to the integpunent, and espe* cially those which pass to the bases of the larger or smaller setcD with which the integument is provided, frequently end in minute ganglia. Hensen has shown that in the Orustcieec^ s^TTiWar' setsB in all probability have an auditory function ; and Leydig, Hicks, Lespes, Landois, and others, have as- cribed functions of special sensation to these structures in insects. But whether these setse, on the antennse or elsewhere, 'subserve eith^ hearing or smell, is still very doubtful ; and the only organs which can safely be regarded as auditory in insects, are those which occur in Grasshoppers (AerididoB), Crickets (Achetidoe), and Locusts (LocugHdce), and which were first accurately described by Von Siebold.* Recently, they have been studied by Leydig, Hensen, Ilanke,t and Oscar Schmidt,^ but it must be confessed that much obscurity still hangs over their minute structure.
In the AcrididcB, the chitinous cuticula of the metathorax presents on each side, above the articulation of the last pair of legs, a thin tympaniform membranous space sur- rounded by a raised rim. On its inner face, the cuticular layer of the tympaniform membrane is produced into two processes, one of which is a slender stem ending in a hollow triangular dilatation. A large tracheal vesicle lies over the tympanic membrane, and between its wall and the latter, a nerve derived from the metathoracic ganglion, passes to the region occupied by the processes, and there enlarges into
♦ »Archiv fttr Naturgeschi- 1875.) chte,' 1864. X Schmidt, "Die Oehrorgane
t ^ Beitriige zu der Lehre von der Ueuschrecken." Q Archiv den UebergaDgs-Sinnesorganen." fiir Mikr. Anatomie/ 1875.) (' Zeitachrift fur Wiss. Zoologie,'
440 THE ANATOMY OF INYEBTEBBATED ANIMALS.
a ganglion, tlie outer face of which, beset with nninercms glassy rods arranged side bj side, is in contact with the tjmpaniform membrane. A nerve arising from this gan- glion passes along a groove to the ' stem ' and ends in a ganglion in its dilatation. From this ganglion certain fine filaments proceed.
In the AcheHdce and LocustidcBy the tibisB of the fore legs present similar tympaniform membranes which are easily seen in the common Cricket, but, in other forms, become hidden by the development over them of folds of the cuticle of the adjacent region of the limb. Two spacious tracheal sacs occupy the greater part of the cavity of the tibia, and a large nerve ends in a ganglion in the remaining space. Upon this ganglion a series of peculiar short rod-like bodies are set.
The compound eyes of insects differ only in detail from those of the Crustacea.
In the ocelli, or so-called simple eyes, a sclerotic, a cornea, a lens, a vitreous humour, and a choroid coat, have been distinguished, and the whole organ has been compared to the vertebrate eye. But the * lens ' appears to be always a mei*e thickening of the cuticle which constitutes the cornea, and the so-called * vitreous humour ' is partially or wholly made up of crystalline cones analogous to those which are found in the compound eye. In this respect the ocellus of the insect resembles the simple eye in Arachnida and Crustacea.*
Many insects, as the Glow-worm and Lantern-flies, are remarkable for their power of emitting light.
According to Schulze f the males of Lampyris splendidula possess two photogenic organs, which lie on the sternal aspects of the penultimate and antepenultimate abdominal
♦ Leydig, * Das Auge der Gliedcrthiere,' U64. Landois, *« Die Raupcnaugo " (« Zeitsdirift fur Wiss. Zoologie,' 18t6), and ''Zur Entwickelungsgeschichte der faeetdrten Augen von Tene- bris moiitor" (Ibid., 1^67).
t ** Zor Kenntniss der Leucht- organe von Lampyris splendi- duiar r*Archiv fur Mikr. Anatomie, 1855.) See also Kdl- liker, * Wurzburg Phys. Med. Ge- sellschaft,' 1857.
THB PHOTOOBNIO OSOAKS OF Iir8BCT8. 441
Bomitee. Each is a thin, whiidah plate, one face of which is in contact with the transparent chitinons cnticnla, while the other is in relation with the ahdominal nerre-cord and the viscera. The sternal gives ont mnch more light than the tergal face. The photogenic plate is distinguishable into two layers, one occupying its sternal and the other its tergal half. The former is yellowish and transparent, the latter white and opaque, in consequence of the multitude of strongly refracting granules which it contains. TiachesB and nerves enter the tergal layer,, and for the most part tra- verse it to terminate in the sternal layer, which alone is luminous. Each layer is composed of polygonal nucleated cells. The granules are doubly refractive, contain uric acid, and probably consist of urate of ammonia (Kolliker). Hence the cells of the layer which contain them are termed by Schulze the "urate cells," while he calls the others the " parenchyma cells." The branches of the tracheee which ramify among the parenchyma cells end, like those of other parts of the body, in stellate nucleated corpuscles, one pro- cess of the corpuscle passing into a ramification of the trachea. Schulze is inclined to think that the other pro- cesses end in parenchyma cells.
The nerves of the photogenic plates are derived from the last abdominal ganglion ; they branch out between the parenchyma cells into finer and finer branches, which even- tually escape observation.
The female reproductive (M*gan8 of insects consist of the ovarian tubes, or ovarioles^ with their so-called peritoneal investments, and of the oviducts, which unite into a vagina ; while a spermatheca, and, g^ierally, accessory glands open into, or close to, the vagina.
The ovarioles may be few or very numerous. Each consists of an external structureless membrana propria, within which lies a solid columnar mass composed of cells. The anterior, usually tapering, end of this ovarian mass is composed of protoplasmic substance in which nuclei are imbedded, but in which the contours of the cells which they indicate are not distinguishable. Further back, some of
442 THE AKATOMY OF INYEBTEBBATSD AKOCAUS.
these nuclei enlarge, become surrounded by an acciunu- lation of protoplasm, and constitute the primitive OTa. Each primitive ovum is separated from its fellow by a layer of nucleated protoplasm which thus forms a capsule around it. In some insects, such as Blaikiy, the capsule is hardly distinguishable in those ova which lie between the smallest and those of middling size, which follow the former in order from before backwards. But, in the larger ova which succeed these, the cells of the ovicapsule rapidly enlarge in a direction perpendicular to the surface of the ovum, and constitute a very well-marked epithelial layer. I am inclined to believe that, for some time, an addition is made to the vitellus of the egg by these epithelial cells, and that they, in fact, play the part of vitelligenous cells. But however this may be, before long, a delicate structureless lamella appears on the surface of the vitellus and encloses the egg as a vitelline membrane. The epithelial cells of the ovi- capsule next secrete from their surface a thicker, often ornamented, layer of chitinous substance, which constitutes the chorion, and the egg is complete.
The ovarian mass, therefore, as Waldeyer has justly pointed out, corresponds with one of the epithelial tub^ of the ovary of a vertebrated animal, and the ovicapsules answer to Graafian follicles.
In some insects, as Aphis, the indifferent tissue of the anterior end of the ovarioles gives rise not only to ova and ovicapsular epithelium, but to large mtelligenous cells. These stay in the dilated anterior chamber of the ovarian tube. But each ovum is originally connected by continuity of substance with one of these cells, and the pedicle of connection may be traced even to the second and third ovum. It seems probable, therefore, that these " vitelligenous cells," for some time, supply material to the growing ova.
In most insects, similar vitelligenous cells are found ; but they are situated at the anterior end of each ovicapsule, so tfaat^ as the column of ovicapsules lengthens by the addition of new ovicapsules to its anterior end, the vitelligenous oeUs axe interposed between every two ova. The vitelline mem-
THB OYABIA OF IKSBOTS. 443 '
brane and tlie chorion first invest the posterior extremity ' and the sides of the ovmn ; and, for some time, leave an opening at the end of the oynm adjacent to the vitelligenons cells. This opening is usually only partially closed, and what remains of it constitutes the aperture or apertures, termed the micropyle, through which the spermatozoa enter when the egg is fecundated. The vitelligenous cells usually remain outside the ovum, and eventually undergo degeneration ; but, in many Diptera, they become enclosed within the coats of the ovum and their substance is merged in that of the vitellus.
Dr. A. Brandt has proposed the term panoistic for ovaries of the first mode, and meroisHc for those of the second and third modes of development of the ova here described. So far as is at present known, only the Orthoptera and the Puliddce possess panoistic ovaria.
The peritoneal coat of the ovarioles is a cellular structure, containing many trachese and, frequently, muscular fibres. It is usually extended beyond the anterior end of each ovariole into a filamentous process, which after uniting with those of the other ovarioles of the same side, is continued into the pericardial tissue. At its opposite extremity it passes into the walls of the oviduct, which are muscular and are lined by an epithelium.
The development of the ovaria has been traced in Diptera and Lepidoptera. Each ovary is, at first, a rounded mass of indifferent tissue, from which a filiform prolongation is given off backwards; this has not been traced into con- nection with any other organ, and appears to terminate by a free end. The mode of origin of this rudimentaiy, or primary, ovarium is unknown, but the first step towards the formation of the genital organs is the separation of the peripheral indifferent tissue from the central portion, and the division of the latter into as many elongated solid cellular bodies as ovarioles are to be formed. The peri, pheral cells become the peritoneal layer. Each cellular rudiment surrounds itself with a structureless membrane, and then elongates into an ovariole, some of the cells filling
444 THE ANATOMY OF INYEBTBBBATBD AKIMALS.
the posterior end of which then become differentiated into the first primary oTom and its capsule, with or with- out yitelligenous cells. The contents of each ovariole must therefore be regarded as a column of generatiye cells, which instead of burrowing in the stroma of an ovaiy, and becoming divided into ovisacs, as in a vertebrated animal, grows straight backwards, and, as it grows, becomes divided into ovisacs, of which the oldest and most advanced is the hindermost.
Nothing is certainly known respecting the origin of the vagina or the oviducts, though it may be suspected that the posterior prolongations of the primary ovaries give rise to the latter.
The development of the testes takes place in the same manner as that of the ovaries, but the contents of the testi- cular tubes become converted into spermatozoa. The origin of the vasa deferentia is unknown.*
In most insects, the vitellus undergoes partial yelk- division. In some Podwridoe, however, complete division has been observed. The development of the blastoderm takes place in the same way as in other Arthropods, and the cephalic end of the embryo terminates in two procephalic lobes. In many insects, the periphery of the blastoderm, external to the longitudinal thickening which gives rise to the sternal region of the body, and which may be termed the
* The account given above of the structure of the ovarian tubes in Blatta and AphU^ is based on my own observations, which are in pretty cloee accordance with those of A. Brandt, <' Ueber die EIrShren der Blatta {Penplaneta) ormUalit** («Mem. de TAcad. St. Petersbourg,' tome xxi., 1874). The literature of the subject is •omewhat extensive. See espe- cially Leydig, '<Die Eierstock QDd die Samentasche der Insec- ton " (< Nova Acta,' xxziU., 1867) ; Lobboek, '* The ova and psendova oflnaeets" (Phil. Trans., 1858); I, ** Die nachembryonale
Entwickelung der Museiden ** ('Zeitschrift fur Wise. Zoologie,' xiv.); Bessels, *' Entwickelung der Sexualdriisen bei den Lepi- dopteren " (* Zeitschrift fur XViss. Zoologie,' 1857); and Von Sie- bold, * Beitrage xur Partheno- genesis der Arthropoden,* 1871. The various forms uf the micro- pvle and the structure of the chorion are dealt with by Leuc- Icart, in his elaborate memoir, *' Heber die Micropyle und den feineren Bau der Schalenhaut bei den Inselcteneiem " (*Muller*8 Archiv/ 1855.)
AOAMOOENBSIS IN IN8B0TS. 445
Hemdl hand (' Keimstreif ' of the Gterman embryologists), gives off a lamina which grows inwards over the sternal face of the embryo, and eventaally forms a complete investment thereto. The lamina may be formed by a single layer of cells, or it may, from the first, be a fold of the blastoderm and thus consist of two layers, the inner of which is continnons with the sternal band, and the outer with the blastoderm which invests the tergal surface of the vitellus. In the latter case, it becomes strictly comparable to the amnion of a vertebrated animal; and, when the folds have united in the middle line, the investment in question is distinguishable into an outer membrane, which answers to the lamina serosa, and an inner, which corresponds with the amnion proper of the vertebrate embryo. In some cases, the vitel- line substance fills up the interval between the la/mina serosa and the amnion, so that the sternal band and the latter form a sac plunged into the interior of the yelk.
The development of a more or less complete amniotic investment has been observed in Orthoptera (Libellula), Coleopiera, Hemiptera, Hymenoptera, Lepidoptera, and Dip' tera, but it does not appear to be universal.
Agamogenesis is of frequent occurrence among insects, and occurs under two extreme forms ; in the one, the parent is a perfect female, while the germs have all the morpho- logical characters of eggs, and to this the term parihe7U>' genesis ought to be restricted.* In the other the parent has incomplete female genitalia, and the germs have not the ordinary characters of insect eggs.
In Coccus (Lecanium) hesperidvm, in Chermes ahieiis and pint, no males have yet been observed; but the perfect females produce ova, out of which only females proceed. It is probable that many species of gall insects (Cynips) are in the same predicament.
*■ The excellent * Beitrage zur the statements in the text respect- Parthenogenesis ' (1871) of Yon ing Agamogenesb in Insects. , Siebold is my chief authority for
446 THE ANATOMY OF INYEBTEBBATED ANIMALS.
The TmimpFegnated, apterous, caterpillar-like females of the Lepidopterons genera Psyche and Solenobia, lay eggs out of which only females issue. The males occur bnt rarely and locally, and, fix)m the impregnated eggs, males and females issue in about equal numbers.
Leuckart discovered that the ovaries of so-called nenters among wasps, hornets, humble-bees, and ants, often contain more or less well-developed eggs, and that in the wa^pB and humble-bees, such eggs are laid and develope young, the sex of which was not ascertained. Yon Siebold has ob- served that the neuters of Polistes gallica are distinguished from the perfect fertilisable female, by little more than their smaller size, and that they possess completely deve- loped female organs. These neuters, or rather, small females, laid eggs which developed, and gave rise only to male Polistes. The unimpregnated females of a Saw-fly, Nematus ventricosus (the larvsB of which are known as gooseberry caterpillars) regularly lay eggs, which develope and produce male offspring.
The terms arrenotohous and thelytokous, have been pro- posed by Leuckart and Yon Siebold to denote those par- thenogenetic females which produce male and female young respectively.
In the case of the Hive-bee, it has been ascertained that the queen either impi'egnates, or does not impregnate, the eggs when they are laid. The spermatheca, in which the spermatic fluid, introduced by the single act of copulation which takes place, is contained, contracts as the eggs pass along the vagina, in the former case, and remains passive in the latter. The unimpregnated eggs give rise to mal^ or drones ; the impregnated eggs to females, which become neutera with imperfect reproductive organs, or queens, with perfect organs, according to the nutriment which they receive.
In the Aphides, ova deposited by the impregnated females in the autumn are hatched in the spring, and give rise to forms which are veiy generally wingless, and bring forth living young. These may be either winged or wingless, and
AaAMOGENBSIS IV APHIDB8.
447
are also yiviparous. The number of Bnccessive yiyiparous broods thus produced, has no certain limit, but, so far as our present knowledge goes, is controlled only by tempera- ture, and by the supply of food. Aphides kept in a warm room and well supplied with nourishment, have continued to propagate viviparously for four years.
On the setting in of cold weather, or, apparently, on the failure of nourishment alone, in some cases, males and females are produced by the yiyiparous forms. The males may possess wings, or may be deyoid of them. The females appear inyariably to be apterous. Copulation takes place and the eggs are laid.
Sometimes yiviparous forms co-exist with the male or female forms, and some viviparous Aphides are known to hybemate.*
The viviparous forms differ essentially from the ovi- parous forms in the structure of their reproductive organs. They possess neither spermathecse nor colleterial glands, both of which, as Von Siebold first demonstrated, are present in the females. The young are developed within organs which resemble the ovarioles of the true females in their disposition and may be termed psetidovaries. The terminal or anterior chamber of each pseudovarian tube is lined by an epithelium, which encloses a number of nu- cleated cells. One of the hindermost of these cells enlarges and becomes detached from the rest as a psevdovwm. It then divides and gives rise to a cellular mass, distinguish- able into a peripheral layer of clear cells and a central more granular substance, which becomes surrounded by a struc- tureless cuticula. It is this cellular mass which gradually becomes fashioned into the body of a larval Aphis. A portion of the cells of which it is composed becomes con-
♦ Huxley, "On the Agamic Reproduction and Morphology of Apnis.'* P Linnean Transac- tionfl,' 1857.)
The papers of M. Balhiani (*Ann. des Sciences Naturelles' 1869, 1870, and 1872) should be
consulted, not only on account of their richness in details, but for the peculiar views tvhich the author entertains respecting the nature of the reproductive pro- cess in the Aphiaes,
448 THE AKATOMY OF INYBBTBBBATBD AKIMAL8.
verted into a psendoyarimn, and the development of new psendova commences before the young leaves the body of its parent. It is obvious that this operation is comparable to a kind of budding. If the pseudovum remained adherent to the parental body, the analogy would be complete.*
The agamogenetic multiplication of CecidamyiaABrvm is an essentially similar process. Professor Nicolas Wagner, of Kasan, f discovered that the larv» of a Dipterous insect belonging to the genus Cecidomyia, or to a closely-allied f orm, (Miagtor) multiply agamogenetically in the autumn, winter, and spring. In summer, the final terms of the successive broods of grubs thus produced are metamorphosed into males and females, which copulate and lay eggs. From these, larvae which exhibit the same phenomena, emerge. In this case, the young are all developed from germs which are found lying loose in the perivisceral cavity of the parent, the body of which they destroy and burst in order to become free. Leuckart, Metschnikoff, and Gkuiin,^ have sho^ini that these germs are detached from the pseud- ovarium, which occupies the place of the rudimentary ovarium ordinarily found in larvsB ; and that each represents the egg-chamber of an ordinary insect ovariole with its epithelial capsule, ovum and vitelligenous cells.
In the ordinary process of growth of an insect, from the time it leaves the egg until it attains the adult con- dition, every marked .change in the outward form of the
* Leydig C* Die Eierstock und die Samentasehe der Inseoten," * Nova AcU,' 1867) affirms that in November he htut met with Aphidea in which, in the same animal, some of the ovarian tubes contain fully formed ova, and others pseud - ova, undergoing their ordinary method of develop- ment. Unfortunately no infor- mation is afforded as to whether these aphides possessed a sperma- theoa, and snowed evidence of impregnation or not The oo- corrence pf agamogenetis along*
side of sexual propagation is in itaelf nothing unprecedented, e.ff. Pyrosoma.
t K. E. von Baer, "Berichf ('Bulletin Acad. St. P^ten- bourg,' 1863.)
t Tieuckart, ** Die ungeschlecht- 11 one Vermehrung der Ceoidomy- ienlarven " (* G5ttinger Nach- richtcn/ 1865); K. von Baer, " Ueber Prof. Nic Wagoer^s Ent- deckung,'* &c. (* Melanges biolo- ffiques tirtfs du Bulletin de I'Aead. Imp. des Sciences de St. Peten- bourg/ 1865).
THB MBTAMOBPHOSB8 OF IN8BCT8. 449
body, or of its appendages, is accompanied bj a diedding of the cnticula. In some cases the modification effected at each ecdjsis is very slight, and the monltings of the cntide are nmnerons, amounting in a species of Day-fly {Chloeon)y described by Sir John Lubbock, to as many as twenty. In such a case as this, the structure of the adult is gradually substituted for that of the lanra, and the organs of the larva, for the most part, pass into those of the adult.
The like holds good of some insects which undergo metamorphosis, that is to say, in which a quiescent pupal condition is interposed between the active larval and the active imaginal states. Herold and Newport have de- scribed at length the series of changes by which the elongated ganglionic chain of the Lepidopterous caterpillar is con- verted into the much more highly concentrated nervous system of the Butterfly; and Weismann has shown by what gradual steps the apodal Core^^ra-larva acquires the character of the Dipterous imago. But, in the Flesh-flies (Musca), and probably in many other members of the division of the Diptera to which they belong, the apodal maggot, when it leaves the egg^ carries in the interior of its body certain regularly arranged discoidal masses of in- different tissue, which are termed imaginal disks.* Of these, twelve are situated in the thoracic region, two on each side of each thoracic segment, while two others lie in front of the pro-thoracic disks. These imaginal disks undergo little or no change until the larva encloses itself in its hardened last-shed cuticle, and becomes a pupa. But they then rapidly enlarge ; each of the sternal thoracic disks g^ves rise to a leg and to its half of the sternal region of the thorax, while the tergal disks develope into the tergal halves of the corresponding somites, with their appen- dages, the wings and the halteres. The anterior pair of disks originate the head and proboscis of the fly. As the imaginal disks develope, the pre-existing organs contained
* See the remarkable memoir of Weismann, * Die naohembryonale Entwickelung der Mosciden.'
2 a
450 THK AirATOKI OT UmBTBBBATKD AHIIUX8.
in tbe ha&d and thoisx of the lura, mtdergo complete or partial reeolntioiL On the other hand, tihe abdomcm erf the flj ia produced hj the contdnnooB modifioatioit of tlie coiutitneata of the larval abdomen. As in the Onutaeta, bo in Jiuedo, the paisaitic haint is Fig. 111.
Fig. t ll.-Tbe lert-bBod fi{ altrrimia coacaiuiog tiro dcsiIt hfti flgnrv, a newly borti Urrt of Stfb^
mmma. A, vcolrml nuftec of ihc tbonx; B, tha ■bdomen; mandiblM ; t, labial plata and moatti; t, tuItbj 1,2,3, the thr thoraeic Mgnianta onitsd. (Afler NawporU)
rwmto an adult famali of Stwhpt batcbed em, and the Hgbt-baod ib^ on a Dalr of Andnma TVm-
aooompanied hj extreme modification of form. In ttiis respect the Strapa^itera, which are pansitio opon Bmb, present A remu^aUa hiatory. Thef«mAle(Fig. llljhsa the fonn of a hw wiUi ■ ihort neck, and nerer leaTes the bodj o/C&a^menoptemainKUcbaheiapanntic, The mala*.
1 1
THB PUtASITISM OF INSBCT8.
451
on the contrary, are exceedingly active insects provided with a single pair of wings, which are attached to the metathorax, while the mesothoraz has a pair of twisted appendages in the place of wings.
The larvffi of both males and females when they leave the egg, are minute active hexapod insects (Fig. Ill), with mdimentary mandncatoiy organs, and are found creeping about between and on the hairs with which the abdomen of their host is provided. In this condition they are carried into the nests of the bees, and they attack the larvsB of the latter, boring their way through the integument into the ab- dominal cavity of the grub. Here they cast their cuticle and become changed into sluggish apodal grubs, provided with a mouth, with rudimentary jaws, and with an alimentary sac, but devoid of an anus. About the time that the Hymen- opterous larva x>asses into its imago state, the Strepeipteral larva thrusts the anterior end of its body (the so-called cephalo-thorax) between two of the abdominal segments of the bee, so that it projects externally. The male becomes a pupa, and eventually makes its way out as a winged insect The female on the other hand undergoes little change of outward form, but presents an opening, which plays the part of a vulva, and enables the male to effect the fecundation of the eggs. These are developed within the body of the female, and make their way out by the cleft in question.*
The Ichneumon-flies deposit their eggs within the bodies of the larvffi of other insects, and the grubs thence hatched devour the corpus adiposum of their host. The larvsB of some of these parasites {PlaiygcuteTf Teleas), described by Qanin f are extraordinarily unlike other insect larvsB, and have a certain resemblance to Copepoda.
* See Yon Siebold. <*Ueber Strepsipteren" (* Arehiv fur M»- torgetohiehte,' 1843), and New- port, ** Katural History kc. of the
Oil-beetle, MelSe '* (Linn. Trans. 1847). t * Zeitsohr. for Zoologie,' 1869.
^ ^^
452 THB AKATOMT OF nfYXBTXBSATXD AHIMALS.
CHAPTER Vm.
THE POLYZOA, THE BSACHIOPODA, AND THE MOLLUSCA.
HowsvEB diverse in outward appearance and in complexity of organisation the multitudinous forms of a-nimftli^ which have been described in the preceding four chapters (Chap. lY. to YII.) may be, the student passes from one to the other, by easy and natural gradations, from the simple Turbellarian at the bottom, to the most highly differen- tiated Arthropod at the summit of the series. But with the higher Crustacea, Arachnida and Inaecta the scale ends ; from none of these are we led to any higher form of f^niinal life.
The Cuttle-fish, the Whelk, the Snail, and the other innumerable forms of animals with univalye, bivalye, and multivalve shells, which are commonly known as MoUusca, are so widely different, not only from the AtihrO' poda, but from all the higher members of the group of Worms (Chap. Y.) that any connection with these seems, at first, to be wanting. The segmentation of the body, which is so conspicuous a feature of the greater number of the members of the series which ends with the Arthropods, is absent ; limbs are wanting ; instead of the equality of the neural and haemal faces of the bilaterally symmetrical body, and the consequent remoteness of the oral and anal aper- tures, which is usual among the Arthropods and Worms, these two faces are usually unequal. The haemal face is often produced into a longer or shorter cone ; the anus is, as a rule, approximated to the mouth ; and, very often, the hffimal face of the body is asymmetrical.
The higher MoUuBkB, in Iaa^ ioxm. the final term of a
Aj^
THB POLTZOA.
453
series of their own, which commences in the Polyzoa, with animals which have many resemblances to the Boti/era,
The PoLYzoA or Bbyoz.oa. — In outward form these animals bear a general likeness to the Sertnlarian Hydrosoa, with which they were formerly confounded under the name of " Oorallinee." Like the Sertularians, they almost always form compound aggregations, produced by repeated acts of gemmation from the primitively single embryo, and have a hard cuticular exoskeleton, which remains when the soft parts decay. The compound organism thus formed is
¥i^, 112.
Fig. 112.— A portion of the polyzoariom of FlunuOeUa npent (after Albnan).*
termed a Polyzoarivm (Fig. 112), and each zooid which buds from the common stock is a Polypide, The outer, chitinous or calcified, cuticular exoskeleton, is termed the edocyst, and, as the rest of the body of the polypide is contained in, or can be retracted into, the hard case thus formed, it is commonly termed a " cell."
The proper ectoderm, with the parietal layer of the mesoderm which lines and secretes this cell, is termed the endoeyst. The mouth is situated on a disk, termed the
"» * Monograph of the Freth-waiec PoVfiovs ^^^^*
lophopkore, at the tree end of the poljpide; anfl the margina of the lophopkon are produced into a nomber of ricblj ciliated tentaevJa. At the oisl aperture, the ectodemi paasea into the endodermal liniiig of the a]i-
Flg. 113.
tli«didioi , ,
tioe; i, inuii n, miuelnj v, ftmlcalui. (After AIIomd.)
ft borne b; Itomaeh ; K, iotm- ; z,itatoblMU; *,
mentary canal, which is aJmoet alwajB divided into three jwrtionB, a long and wide pharjm, a Bpacioos stomach, and a iuun>w inteotiite. Tb« \bM«t ia a^-n.-^^ N^bnli. ti.^ aearlf
THB POLTZOA. 455
parallel with the pharynx, and terminatee in an anus situated beside the mouth. As the nervous ganglion is placed between the mouth and the anus, the flexure of the intestine is newral,* and the hsemal face of the body is developed greatly in excess of the neural face. A wide perivisceral cavity occupies the interval between the ali- mentary canal and the parietes of the body, and sometimes the walls of this cavity are ciliated. Very generally, the gastric division of the aHmentaiy canal is connected with the parietes of the body by a sort of ligament, theyunicu/tM, or gckstrO'parietal hand. Circular and longitudinal muscular fibres, which frequently exhibit distinct transverse stria- tions, may be developed in the body- wall; and there are usually special muscles for the retraction of the lophophore within the cell, and others for the closing and opening of the opercular apparatus, with which many species are provided.
The single nervous ganglion is situated, as has been stated, between the oral and the anal apertures. In Serialaria, Scrupocellaria and some other genera, nervous cords and plexuses connecting the ganglia of the several polypides and constituting what F. Miillerf terms a " colonial nervous system," have been described. But it is not yet certain that these cords and plexuses are really nerves. It is doubtful if there are any special organs of sense, unless a lobed process — the epistoma, — ^which over- hangs the mouth in many fresh-water Polyzoa, be of this nature. The ectoderm of that region of the body which lies immediately beneath the tentacula is always soft and flexi- ble ; and when the lophophore is retracted, becomes invagi- nated, so as to form a sheath, by which the tentacles are protected. Sometimes, as in the Ctenostomata^X ^^^ sheath
* In dealing with the morpho- tres, or the pedal ganglia (when
logical relations of the parts of such are separately distinguish-
MoUusks, it is very necessary to able) are placed, neural^ and the
employ a terminology which shall opposite hetmaL be independent of the ordinary j *Archiv fur Anatomic,'
position of the animals. I there- 1 860.
fore term that face of the body \ Pane, ^* 0\>ii«r««.>aoTA «vw ^^
. on which the chief nervous cen- nunute «\"nic\»i^ ol wasi&A ^"^ ^^^
458 THK AXATOmr or HmSTXBBATSD AJnilAlB.
in aornxukded b^ a circle of chitinoim filaments, wliicl), wboB the teotacl«B are retracted. fnmiBh a protectiTe outer oover- iug to them. And, aometimes, aa in the Cheilotlomala,* part of the ectocjst of the poljpe cell ia diapoeed in anch a mannar as to coiiBtitut« a moTeable lid, which shntB domi on dke retiacted poljpide. This opereuittin is placed on the opposite side of the poljpide to that on which the nerrona ganglion ia aitooted- In manj K<enef», the cells are prorided with flagellifonn Fig. lu.
appenda{(es— the vibraci^a (Fig. 114). These are nsnallj srticalated with short dilated processes of the ectocjet, and
UKherfomuofFolTpi" ("Pbll. See for thil gnap Nitacfae'* m- Ttsiw.' 1837). Beicbert, " Veber cent ImporUnt "Beitrigc tat Zoobiibyo* pdhcidM" ('Abh. d. Kennmias der Bryoioei) " (' Zdt- Kflnlgl. Akul. dar WiaMMchafl- >cbrlfl fCr Win. Zoologie, 1869- «B,'B«lin,18«.). 711.
Biuk, 'C*f*logiie of the
JHaitot FolTiM in the Brittih
Mwmam: Chtlastanua,' IKA-M,
JITICUI.4XU AKD TIBEIOUI.^ 457
eseente coxuitaut laahing muvementa. In oihen, bodies shaped like birds' beads, witb a movable mandiUe,- and either Mated upon slender and flexible pednncles or sessile, snap incessantl;. Sometimes these last, which are termed aoiadaTia (Fig. 115), are present along with vibnicula. Fig. lis.
Fig. Ji^i.—Buj/tik
idt, BhDK
iUcell(ji); tbe intestine (i) and tiie itonuh udgnllct (/>; f, n- (rictar mutcles ; if. cf, svicul»rla. One of thrae U boldlng > iunijt« wonn whicb it his aeiied. In front ot tlii> ie tetn an orioall.
iculuiom (rf), viewed Tram the
poljpide »
The dilated bases of the vibracnla contain mnscles bj the contraction of which the flagelliform appendage is moved. In the avictdaria, a huge addnctoi mnndft,'«\a.<^ takes its origin troia the greater p^rt ol ^!ki« Vimet bo:A.«rk>
458 THE AHATOMT OF IN7EBTBBBATSD ANIMALS.
of the "head," is attached bj a slender tendon to the "mandible" on the one side of the binge line, while a smaller diyaricator mnscle is fixed to the other side. The mechanism of adduction and divarication of the mandible is quite similar to that bj which the dorsal Talve of the shell of an articulated Brachiopod is moved upon the ventral valve.
Male and female reproductive organs are usuallj com- bined in the same poljpide. Thej are cellular masses, developed in the funiculus, or in the parietes of the body, whence the ova or spermatozoa are detached into the perivisceral cavity. They sometimes pass thence, and undergo the first stages of their development in dilatations of the wall of the body, termed otnceUs.
Multiplication bj gemmation occurs throughout the group, but the buds usuallj remain adherent to the stock. In Loxosoma and Pedicellina, however, the buds become detached.
Some Polyzoa multiply agamogenetically by a kind of gemmule developed in the funiculvs, provided with a peculiar shell, and termed a statohlaat.
With these general characters, the Polyzoa present an interesting series of modifications. They have been divided by Nitsche into two groups — the Entoprocia, in which the anus lies within the circle of tentacles ; and the Eetoproeta, in which it lies outside this circle. In the former division, the genus Loxosoma* which attaches itself to Sertularians and to other Polyzoa, is particularly noteworthy. It is a small stalked animal, and the superior wider end of the body is an obliquely truncated disk, the margins of which are elon- gated into ten ciliated processes. The mouth is a trans- versely elongated, slit-like aperture on the lower side of the tentacular circlet. A long oesophagus connects this with a globular caecal gastric sac. From the midst of the
• Kowalewsky, '< Beitra^e zur St. Petenboarg,' 1866). Oscar
Anatomie und F.ntwickclungs- Schmidt. ^^ Die Gattang Loxo-
geteMehte det Lozosoma neapoU- anma *' r Archiv fur Mikr. Anat.*
*' CM6n. de TAcad. de 1075).
■> i.
THS POLTZOA. 459
disk, a oonical prominence, the summit of which bears the anus, is situated. The sexes are united, the OTaries and testes being situated on each side of the stomach, and the spermatozoa pass directly into the ovaries. No nervous system has jet been made out in Laxosoma, The animal is fixed bj the truncated extremity of its narrow stalk-like end; and this stalk contains a gland, the duct of which ox>ens in the centre of the face of attachment.
Loxasoma appears to multiply by budding, but the appa- rent buds are really one of two kinds of embryos developed from the impregnated ova. The other kind of embryo be- comes a gastrula, with a large post-oral ciliated disk, like a mesotrochal annelid larva, and its ultimate fate has not yet been traced.
The Ectoprocta are divided into the OymnoloBmaia, which have a circular lophophore, and no epistoma ; and the Phy^ ladolcBmata,* which possess an epistoma, and usually have the lophophore prolonged into two lobes, so as to be horse- shoe-shaped ; whence the term hippocrepian applied to such Polyzoa,
Among the OymnolasnuUa are distinguished; the Cydo- giomaia, in which the opening of the cell is round and has no opercular structures ; the Ctenastomaia {auprd), and the CheilogtonuUa {sfuprd).
All the Phylactolcemata are inhabitants of fresh water; while all the OyrivnolcBmataf except PahuUcella, are marine.
The polyzoarium of Cristatella is free and creeps about as a whole ; and that of LtmulUes is free, at any rate in the adult condition.
In the fresh-water Polyzoa, the impregnated ovum gives rise to a saccular planuliform embryo, which is covered externally with cilia. From one end of this cyatid, one or more polypides are developed from thickenings of the wall of the sac.
•See Dumortier and Van Bruxelles,' 1850); the monograph
Beneden, ** Histoire Naturelle d. of Allman cited above ; and
Polypes composeefl d'eau douce *' Nitsche's " Beitrage." ('Bfem. de TAcad. Royale de
460 THE ANATOMY OF nTYEBTEBRATBD AKIMAIiS.
In the Gymnolsematous genera Bugvla, ScrupoeeUaria, and BiceUaria, the embryo is ciliated, and provided with a mouth and with eje-spots. After swimming about for some time, it loses its cilia, fixes itself, acquires a chitinoiis outer coat, and becomes a mere sac or cystid, in which a poljpide is developed bj gemmation, and gives rise to the first cell of the poljzoarium.
Schneider * has shown that the anomalous Chfphonautes, which he considers to resemble Aetinotrocha, and which is enclosed in a bivalve shell, is the larva of Metnbranipora pUosa, It is provided with an intestine, and with largelj developed ciliated motor bands. But when it attaches it- self, all these organs disappear, and the larva passes into the condition of a cjstid, from which a poljpide is de- veloped, as in the foregoing cases.
Hence, it has been pointed out that the characteristic poljpide of the ectoproctous Polytoa is a structure developed from the cjstid, in much the same waj as the Tcenia-head is developed from its saccular embr jo ; or as the Cerearia is developed from the sporocjst or Bedia; the cjstid of the Phylactolcemata being comparable to a sporocjst, and that of Menibranipora to a Bedia, But, without altogether denjing the justice of this comparison, it maj be suggested that the cjstid is comparable to a vesicular morula, and that the mode of development of the alimentarj canal of the poljpide corresponds with that of the formation of an alimentary sac bj invagination. If this view of the case be correct, the perivisceral cavitj in the Polyzoa is a bias- tocoBle, more or less modified bj the development of the mesoderm.
The onlj known representative of the genus Bhabdo* plewraf is an aberrant Poljzoon which presents manj in- teresting peculiarities. The poljzoarium consists of a creeping stem from which erect branches, each of which
* **Zar Entwickelangsgeschi- f See the papers of Allman
THE BRAOHIOPODA. 461
ends in a circular aperture and constitutes the cell of a poljpide, arise. The cavity of the stem is divided bj transverse septa, and its centre is traversed bj a hollow chitinoas cord, which passes through and is attached to the septa.
The lophophore resembles that of the hippocrepian Phy* IcietolcBnuUa in being produced into two arms, fringed with a double series of tentacula. These arms are longer, narrower, and more cylindrical than in any other Polyzoa, and, thus far, approach the arms of the Brctchiopoda. Furthermore the tentacula are confined to the arms, which are very flexible. Between the bases of the arms there is a rounded or pen« tagonal disk with raised and ciliated edges, which occupies the place of the epistoma in the phylactolsBmatous Polyzoa, The mouth is situated beneath the free margin of this disk, on the opposite side to the anus, and to that towards which the arms are turned. The animal is attached to the bottom of its cell, or rather to the endosarc of the stem, by means oi a long contractile pedicle, by which its retraction is effected. According to Sars, it protrudes itself by climbing up the wall of its tubular cell by means of the disk. Professor Lankester's comparison of the polypide of Bhahdopleura to the embryo Piaidium,* appears to me to be fully justified. The branchisB of Nucula, in form and position, present no little resemblance to the arms of Bhahdopleitra, though these, like the arms of the Brachiopoda, are probably more strictly comparable to the labial palpi of the I^unelli- branchs.
Polyzoa occur in the fossil state from the Silurian epoch to the present day, and the oldest forms are referable to the groups which now exist.
The Bbaghiopoda. — The Brachiopoda are all marine animals provided with a bivalve shell, and are usually fixed by a x>€duncle which passes between the two valves in the
* '* On the developmental history of the Mollusca.*' C PKVI.Ttvsa! 1874.)
462 THE AKATOMT OF INTBBTBBBATBD AHDEALS.
centre of the binge line, or the region which answerB to it, in . those Brachiopods which have no proper hinge. Thej never multiplj bj gemmation, nor give rise to oom- ponnd organisms. The shell is always inequivalye and equi- lateral; that is to saj, each yalve is symmetrical within itself and more or less nnUke the other yalve. The shell is a cnticolar stmctore secreted by the ectoderm, and con- sists of a membranous basis, hardened by the deposit of calcareous salts, sometimes containing a large propcurtion of phosphate of lime {Lingula).
In many Brachiopods, varionsly formed calcareons spi- cnla, or minute plates, are f onnd in the walls of the peri- visceral cavity, and of the greater sinuses; and also in the arms and cirri, and sometimes these unite together so as to form an almost continuous skeleton.*
The body, or rather that part of it which contains the chief viscera, is often small relatively to the valves of the shell, and the integument is produced into two broad lobes, which line so much of the interior of the valves as the visoenJ mass oes not occupy. The free edges of these lobes are thickened, and are beset with numerous fine chitinous sets like those found in Annelids, and Hke them lodged in sacs. Between the two lobes of the mantle, or paUium, is the pallial chamber, bounded behind by the anterior wall of the visceral mass. In the middle line, this wall presents the oral aperture, which is seated in the midst of a wider or narrower area, the margins of which are provided with numerous ciliated tentacula.
In Argiope, the oral area occupies a large part of that lobe of the mantle which is ordinarily termed dorsal, and its margins are simply indented by three deep "Tinatioiig In Thecidium, the sinuations are deeper, and the folds of the oral area thus produced, narrower. But in most Brachiopods the oral area is narrowed to a mere groove, and 18 produced on each side of the mouth into a long spirally-
*Tlicte have been described by champs, *Becherohee ear For- Woodwaid, Leosae-Ihithiert mod nnisatioD du BCanteeu ches lee <yedaHy by Eudct Beilmi^ Bmohiopodes artieiiMe,' 1864.
THS BIU.CHIOPODA. 463
coiled arm, fringed with tentacles; whence the name of Bra4:hiopoda, applied to the group.
In this case, the tentacnla disappear from the anterior margin of the oral disk in the region of the mouth, and are replaced bj a lip-like ridge. Each arm contains a canal, which ends in a sac at the side of the mouth.
In Waldheimia (Fig. 116), the two arms are united together and their distal portions coiled into a horizontal spiraL In many genera, the margins of the oral area or arms are fixed to processes of the dorsal valve of the shell.* In this case the arms are not protrusible ; but, according to the observations of Morse,t they can be straightened and extended beyond the shell in Bhynehonella, which has no brachial skeleton,.
The alimentary canal consists of an oesophaguB, a stomach, provided with hepatic follicles, and an intestine. In the majority of existing genera the latter is short, and ends in a csecimi in the middle line of the body {e.g, WaMkeimia) ; in others it is long, and opens into the pallial chamber on the right side of the mouth {e.g. Lingula, Diseina and Crania),
The alimentary canal is invested by an outer coat — the so-called peritoneum, — by which it is suspended, as by a mesentery, in a spacious " perivisceral" cavity. The walls of this cavity are provided with cilia, the working of which keeps up a circulation of the contained fluid. Lateral processes of this coat — the gcutro^parietal and ileo-parietal banda — connect the gastric and intestinal divisions of the alimentary canal respectively, with the parietes.]:
From the perivisceral cavity, sinus-like, branched pro- longations extend into each lobe of the mantle, and end csBcally at its margins. The lobes of the mantle are pro-
■* See, for excellent figures of Natural History, 1873.)
, these arrangements, and for the { Huxley, ** Contributions to
shells and external form of the the Anatomy of the Brachio-
body in genenU, Woodward's poda'* (Proceedings of the Royal
* Manual of the Mollusca.' Sodetv, 1854); and Hancoek,
t ** On the systematic position '* On tne organisation of the Bra-
of the Brachiopoda." (Proceed- chiopoda" Q PhU. Trans.' 1858). ingg of the Boston Society of
THE AXATOKT OF IVTXBTBBBATED AXI1LLLS. V\g. 111.
. — LaMrkl tItw of the viicen of WaUkcimiii a< BwMoek, ' On the Orgaolution of the Braobiopodi,' I9S8).— «,'doTMt' Ujd of vuali«', b,'*entral' Itjtti <
I WlT
THS BBA.OHIOPODA. 465
walls of the body between the mantle lobes; d!,armB; 0, gullet; a, stomach with cut biliary ducts of the left side ; r, nght hepatic mass ; «, intestine ending csecally below ; v, so-called *' auricles " ; o, the right " pseudo-heart," the left being almost wholly removed ; w. pyriform vesicle fixed at the back of the stomach ; 2, oesophageal
Sanglia; t, j\ adductor ; k, divaricator; l, adjustor muscles; M9 pe- uncles.
bably, together with the ciliated tentacnla, the seat of the respiratory function. The sinuses of the pallial lobes of Lingula give rise to nnmerous highly contractile, teat-like, processes, or ampvlloB, During life the circulating fluid can be seen rapidly coursing into and out of oach ampulla in turn (Morse, Ix. p. 33).
The perivisceral cavity communicates with the pallial chamber by at fewest two, and sometimes four {Bhynchonella) tubular organs, which have been described as hearts,* but are now known to have no such nature.
Each of these organs is shaped like a funnel, the wide portion which opens into the perivisceral cavity, being much plaited and folded, and separated by a constriction from the narrower part, which answers to the pipe of the funnel. The latter, passing obliquely through the anterior wall of the visceral chamber, ends by a small aperture in the pallial cavity. Prof. Morse has observed the passage of the eggs through these organs in Terebratulina septerUrionalis, They are drawn into the open end of the funnel by the action of the cilia with which its surface is covered, and enter the pallial cavity by the aperture just mentioned. It is probable that these " pseudo-hearts " subserve the function both of renal organs and of genital ducts ; and that they are the homologues of the organs of Bojanus of other moUusks, and of the segmental organs of worms.
Between the ectoderm and the lining membrane of the prolongations of the perivisceral cavity in the mantle ; and between the endoderm, the ectoderm, and the lining mem- brane of the perivisceral cavity itself, there is an inter-
* Owen,'"Lettre sur rappareil nodes." (* Annates des Sciences de la circulation chez les Alol- jSaturelles,' 1845."^ lusques de la classe des Brachio-
^66 THB ANATOMY OF IKYEBTSBBATBD AKIMALS.
space, broken up into many anastomocdng canals, wbich I conceive to represent a large part of the proper blood system.
Yesicolar dilatations of the walls of these canals found at the back of the stomach, and in some other localities, in the Brachiopods with articulate shells, have been regarded as hearts, but observations on the living animals, made by various investigators, show that they are not contractile and their function is unknown. Although the existence of a direct communication between the perivisceral chamber and the blood canals has not been demonstrated, it is veiy probable that the perivisceral chamber really forms part of the blood- vascular system.
Muscles for the adduction and divarication of the valves of the shell, and for effecting the other movements of the animals, are well developed in the Brctchiopoda.* They are to a great extent striated.
The nervous system of the articulated Brachiopods, in which it has been best made out, consists of a relatively thick ganglionic band on the ventral side of the mouth, the ends of which are united by a commissural cord, which surrounds the gullet, and bears two small ganglionic enlarge- ments. The latter probably answer to the cerebral, the former to the pedal, ganglia of the LameUibranchiaia, Immediately behind the pedal mass, from which two large nerves to the dorsal or anterior lobe of the mantle are given off, are two elongated ganglia, connected by a commissure of their own, which possibly correspond with the parieto- splanchnic ganglia of the higher Mollusks. The nerves to the ventral lobe of the mantle and those to the pedunde arise from these ganglia.
In the inarticulated Brachiopods, our knowledge of the nervous system is very imperfect. In Lingula, Professor Owen has described two lateral nerve cords, and the obser- vation has been confirmed by Gratiolet and Morse. The
* See Hancock (1* c.). Owen, Palseontographical Soeie^, end Introdaotion to Davidson's < Fos- Transactions of the Zoological tii ArMhiopoda ' (MeiaQlit «{ \1» ^wafity of London, 18S5>.
THE DEYBLOPMENT OF THB B&AOHIOPODJL
467
latter anatomiBt finds similar cords in Diacina, and Gratiolet has described an oBsopbageal ring in Lingtda,*
The reprodnctive organs are lodged in the periyisoeral cayitj or its prolongations, and are apparently always con- tained in processes of the lining membrane of that cavity. It is not clear whether hermaphrodism is the rale or the exception. ThecidiiMn, however, has been shown by Lacaze- Duthiers to be dicecions ; and, according to Morse, the sexes are distinct in Terebraiulma and Digcina,
The development of the Braehiopoda, notwithstanding the important observations of F. Muller,t Lacaze-Dnthiers,^ and especially of Mor8e,§ stood mnch in need of farther elucidation (especially in regard to the earlier conditions of the embryo), nntil quite recently, when the investigations of Kowalewsky || filled up the hiatus in our knowledge for the genera Argiope, Hiecidivm, TerebrctMa, and TerebroMina. The egg becomes converted into a vedcular morula, in which an alimentary sac is developed by invagination, and this sac gives off, as in Sagitta, two diverticula, which become shut off from the alimentary canal, and are con- verted into the perivisceral cavity. The latter, therefore, is an enterocoele. The embryo elongates, and constrictions divide it into three segments, of which the anterior becomes fringed with long cilia, and developes eye-spots. Thus the young Brachiopod acquires a great resemblance to an ordi- nary Annelid larva. The resemblance is increased by the appearance of four bundles of set» on the middle segment, which becomes produced into a sort of hood, the free edges of which are at first turned backwards and bear
• ** Recherches Pour servir a I'histoire des &rachiopodeg.*' (* Journal de Conchyliologid, 1860.)
t ^ Beschreibung einer Bra- ehiopoden-larra." (*Arcbiv fur Anat.,' I860.)
L^'Histoire de la Thecidee.*' in. d'Bitt. Nat.,' 1861.) ^ ** On the early stages of 7>re- bratulma $eptetUrioHali$ " (' Me-
moirs of the Boston Society of Natural History,' 1869), and the memoir already cited«
t Contained in a memoir, pub- ed at Moscow in 1874, for which I am indebted to the courtesy of the author. It is in Uussian ; but I have been able to acquaint myself with its contents, to some extent, by the aid of a friend.
468 THB ANATOBiT OF TNYKBTEBBATBD AKIMAJJI.
these setffi. As the hirva grows, the third segment beoomee truncated at the end, and furnishes a surface (proTided with a shell gland? infra), by which the larva attaches itself. At the same time, the first, or prsestomial segment, atrophies, and the setigerons liood developed from the middle segment is retroverted, rapidly grows, and gives rise to the lobes of the mantle, on which the valves of the shell are developed.
The resemblance of the larval Brachiopod to a Poljzoon, and especially to Loxoaoma, is stiiking, and fully bears out the conclusion as to the affinity of the Polyzoa vnth the Brachiopoda which results from the study of their adult structure. On the other hand, the development of the Bra- ehiopoda no less strongly testifies to their close relations with the Worms.*
In the course of the previous pages the terms dorsal and ventral have been employed in the sense in which they are conventionally used by conchologists. But an interesting question, and one not easy to settle is, what relation do these dorsal and ventral regions of a Brachiopod bear to the neural and haemal regions of a Polyzoon, or to those of a Lamellibranch, or of a Gasteropod P
If we compare one of the articulated Brachiopods, such as Waldheimiat in its shell, with a polypide of a Cheilostomatous Polyzoon in its cell, the dorsal valve will appear to answer to itie operculum, and the ventral valve to the cell. If this comparison be just, the two lobes of the mantle of the Brachiopod must both belong to the dorsal or hsemal aspect of the body; that which corresponds with the so-caUed dorsal valve of the shell being the anterior, and that which lines the ventral valve of the shell being the posterior lobe. And the region of the anterior wall of the pallial cavity
* The acceptance of the view ton Society of Natural Historyy
originallv propounded by Steen- 1 873), does not to my mind weaken
■trup and lo ably ur^ed by Pro- the opinion I have always held
feasor Morse, respecting the afii- as to tneir affinities with the Fo^-
idtiei of the Brachiopods with 20a, on the one hand, and with
tiis Worms (Proceedings of Bos- the higher MoHuteojOn ^e other.
THB BSAOHIOPODA. dOO
which lies behind or below Iho mouth, will anawcr to the neural aspect of the Polyzoon.
On the other hand, if the segments of the body of the larval Brachiopod are true somites, and the discoidal sur- face of the hindermost corresponds with the similarly formed end of the larva of iMcinulatia, as Professor Morse suggests, the dorsal lobe of the mant ie will, as before, repre- sent part of the hsemal surface uf tlic body, but the ventral lobe will belong to its neural saifae<2 — and can no longer properly be termed mantle, but will rather answer to the foot of one of the higher Mollusca.
The Brachiopoda are distinguiHhable into two groups, the ArtieidcUa and the Inarticulata, In the Articulaia, the two valves are united by a hinge, and the ventral valve is usually provided with teeth, which are received in sockets of the dorsal valve. The gullet ascends in the middle line towards the dorsal valve, and the intestine descends to- wards the opposite, or ventral, valve, and there ends in a c£8cum. The dorsal valve often gives rise to spiral or looped shelly processes to which the arms are attached. The valves are brought together by a pair of adductor muscles, which pass directly from valve to valve; and they are separated by divaricator muscles, which run obliquely from the ventral valve to a median process (the cardinal process) of the hinge-line of the dorsal valve. The impressions of the attachments of these muscles on the inner surfaces of the valves have considerable systematic importance. Very often the ventral valve is produced into a sort of spout, through which passes the peduncle by which the animal is attached to rocks. At the sides of the visceral chamber the thickened edge of the dorsal lobe of the mantle passes into that of the ventral lobe.
The substance of the shell is very often traversed by numerous canals perpendicular to its surface, which contain prolongations of the mantle.*
* The structure of the shell has Association,' 1844-1847, and In- been psrtioalarly studied by Car- troduction to Davidson's ' Fossil penier. (* Reports of the British Brachiopoda.') See also King,
470 THB ANATOMY OF IKYEBTEBSATED AKIMALS.
This diyision containB the families of —(1) The Tere5ro- kdicUp, (2) the SpiriferidcB, (3) the BhynehoneUidoB, (4) the Orthidoe, and (5) the Productidoe, of which the second, fourth, and fifth are extinct and almost wholly paUdozoic, no species extending beyond the lias, while the majority of the species of the other two families are also extinct.
The family of the Terebratulidce, \diich is not certainly known to occur in formations older than the Deyonian, is the only one in which, since the end of the palsBOzoic epoch, numerons new generic types appear.*
The Inarticulata have no hinge ; the intestine opens into the cavity of the mantle, the margins of the lobes of which are completely separate. Some have a long pednnde {Linffula), others are fixed by a plug idiich passes through an aperture or notch of one valve (Discina), or by the sur- face of one valve (Crania), There is no brachial skeleton, and the arrangement of the muscles is in many respects different from that which obtains in the articulated division.
Species of all these families, except the 8piriferid4B, OrthidcBy and ProducHdcB, exist at the present day, but they are also represented in the older palseozoic epochs, and LingulcB are among the oldest known fossils, f
The MoLLUSCA. — The term MoUusca may be used as a convenient denomination for the Lamellibranehiata and Odontophora (= Gasteropoda, Pteropoda, and Cephalopoda, of Ouvier), which can be readily shown to be modifications of one fundamental plan of structure. This may be represented by a body, symmetrical in relation to a median vertical plane, at one end of which is the oral, and at the other the anal aperture of the alimentary canal. In the body a ventral, or neural, face, an opposite dorsal, or hcemdl, face, and a right and left side may be distinguished. The neural face usually gives rise to a muscular foot. The in- tegument of the haemal face is generally produced at its
' Trans. Royal Irish Academy,' f ^^ Davidran's * Monographs
1869. of British Fossil Brachiopoda,'
* Saess, '* IJeber die Wohn- in the Palseontographical So-
dtMe der Brachiopoden " (* Sitzb. ciety's publications. it Wiener Akad.; \%bl).
* *,
THB XOLLUBOA. 471
edges into a free fold, and the term mantle, or paUiwn, is applied to the region of the integument thus circnm- scribed^ Between the free portion of the mantle and the reet of the body is a cayity, the p(iUi4d chamber, from the waUs of which, processes which sabeerre respiration, the branehioB, may be developed.
In the medkn line of the surface of the mantle of the embryo, a sheU gland is very generally formed, and from the surface ol the mantle a cuticular secretion, the eheU, is produced.
A systemic heart usually exists, and when present, is situated in the middle of the posterior haemal region, and consists of, at fewest, two chambers, an auricle and a ven- tricle. Arterial vessels often ramify extensively through the body, but more or fewer of the venous channels remain in the condition of lacunss. The blood corpuscles are colourless and nucleated. Distinct respiratory organs may be absent, or they may take the form of branchis or pulmonary sacs. When present, they lie in the course of the blood which is returning to the heart. Beside the heart and the intestine are situated the renal organs, which, on the one side, open externally, and on the other, communi- cate with the blood system.
The nervous system consists of, at least, one pair of ganglia (cerebral) at the sides, or on the haemal aspect of, the mouth, and of two other pairs of oesophageal ganglia (pedoZ and parieiO'Splanehnic). The latter are situated at the sides, or on the neural aspect, of the alimentary canal, and are connected by commissures with the former.
In the majority of the JtfoUiMca, the embryo passes through a stage in which it is provided with bands of dlia or with a simple, bifid, or multifid fold of the integument {velvm), the edges of which are ciliated, developed on the hsemal aspect of the cephalic region of the body, in front of the pallial region.
The special peculiarities of the different groups of the MoUueca result chiefly —
1. From the form of the pallial regioii, «isA^(^<b ^tXass^ ^ the mantle-lobes relatively to t\iA\KA^. .
472 THB AKATOXY OF IKYBBTEBRATBD AJflMALB.
2. From the number and arrangement of the pieces of the
shell to which the mantle giyes rise.
3. From the prox>ortional size and the form of the foot
and the production, or non-production, of chitinoiiB, or shelly, matter by it,
4. From the deyelopment of sense-organs on the anterior
end of the body, and the absence or presence of a distinguishable head.
5. From the disproportionate growth of the hamial re-
gion of the body into a visceral aac, followed bj a change in the primitiYe direction of the intestine, and often accompanied by asymmetrical lateral distortion.
The Lamellibbanchiata.* — In these MoUusVs there are always two large pallial lobes, the margins of which are deroid of set® ; and which are lateral, or right and left, in relation to the median plane. Each lobe gives rise to a piece, or valve j of the shell ; and to these, accessory pieces, developed upon the median hsenud face (FhoUu) or the pos- terior end of the mantle (Teredo) ^ are in some cases added ; or, in addition to its valves, the mantle may secrete a shelly tube (Teredo, Aapergillum). The shell itself consists of super- imx>oBed lamellse of organic matter, hardened by the deposit of calcareous salts. It is a cuticular excretion from the surface of the mantle, and never presents any cellular structure. But from the disposition of its lamells, and from the manner in which the calcareous deposit takes place in them, it may present varieties of structure which have been distinguished as nacreous, prismatic, and epidermic.f
The two valves are generally united over the median line of the hffimal surface of the body by an uncalcified chitinous cuticular matter, termed the ligament, which is usually veiy elastic, and is so disposed, that, when the valves are closed, it 18 either stretched or compressed. In either case, it an-
* For a detcriptioii of the of Animal Life.' tomv of a Lamellibranch in f See Carpenter, article** Shell,'
d$iailf the itodent is referred to Todd's * Qrclopedia.' Huxley, Btudty and Mart&n, * Elemcmtsry ^^ Te^asfAuXKri Qft^kaa^" ibid. r/ and RoUMton, ^ BoTma
THE LAMBLLIBSUrOHIATA. 473
tagonises the action of the adductor muscles and divaricates the valTes when these muscles are relaxed. Conchologists commonly draw a distinction between an internal and an external ligament ; but, in relation to the body of the animal, all ligaments are external, and their intemalitj or exter- nality is in respect of the hinge line, or the line along which the edges of the valres meet. In symmetrical, or eqmvalve, Lamellibranchs, each yalre is concave internally and convex externally; it has, in fact, the form of a very depressed cone, the apex of which, termed the vmbo, is incurved and is situated on, or projects beyond, the hsemal, or as it is termed, dorsal edge of the valve. Moreover it is usually inclined forwards, and situated nearer the anterior than the posterior end of the valve. Sometimes the umbonic cone is prolonged and bent inwards, or may even form a short spiral turn {Isocardia, Dicerae), so that the valve acquires a certain resemblance to the shell of some gasteropoda. As the shell of a Lamellibranch increases in thickness by the deposition of new layers on the interior face of the old ones ; and, in area, by the extension of the new layers beyond the old ones, the summit of the umbo represents the original shell of the embryo, and the outer surface is usually marked by concentric lines of growth, which indicate the boun- daries of the successively added new layers of shell sub- stance.
The applied edges of the two valves are very often pro- duced into elevations and depressions which interlock with one another. The form and arrangement of these teeth and sockets are of much use in systematic conchology.
The muscles which are attached to the valves, viz., the adductors, retractors of the foot and pallial muscles, give rise to impressions on the inner faces of the valves, which are very obvious when the latter are removed and cleaned. With the growth of the animal, the distance of these im* pressions from the hinge line and from one another is necessarily increased, and it is not difficult in some cases (e.g. Anodonta) to trace a faint triangular mark, which has its base in each adductor impreadon ttad\\A «i?g«ss.YDL*OE^^T£a^c^.
iJ* THX UIATOn or nmKTSBSl.TBD AKIMAIA
and wliioh indicates Ute saooeaeir« ahiftinga of pomticm of the muscle.
Fig. 117.
FIr. in.— SmHoiwI diagiwn of ■ frohmter'MBMel MiwdD-to}.— A A, muilla, the right lobe of which la cut »w«t ; B, foot ; C, bnnebi»l ohambarofthv mantle OTity; D, >n>l ehombfr; I.mnterioraddDctoT ■ nmiols; II, pcMterior adduotor mnaele; III, retnelor miuola of the fbot; o, mouth; i, atomach; c, inteatlDc, the turoi of which ara anppDtod to be teen through the aide walla of Ilic mewaoma; ^ netom; i, ansa; /, Tantricle; g, auiida; k. gilla, except i, right ■itarnal Kill, largely cut kway and tamed back; ll, labial palpi: ^ embrS; at, pedal; >, parieto-aplanehDia ganglia; <\ apotan of tti UdiW7 ot organ oi Bo^aiuu-, p,Y»^"**^™^
\t.
THB LAMXLLIBSUrOHIATA. 475
In some cases {e.g, Lima) a LameUibrancli may perform a sort of aquatic flight by the flapping of the yalyes of its shell.
The hard and sharp edged yalyes of the shell in Teredo are probably the agents by which the mollnsk caryes its passages through the wood which it inhabits. Whether the yalyes of the shell of the Pholades and Samiecmce are the instrnments by which they excayate their bnrrows in hard rock, or whether, as has been suggested, the foot, armed with sand, is the boring instrument, is a question which has been much discussed, but hardly brought to a satisfactory decision.
The hsemal face of the body is either flat or slightly arched, whence, in side yiew, the hsemal contour is either straight or conyex. In most Lamellibranchs, the body is symmetrical in relation to the median plane, but in those which haye inequiyalye shells, as the Scallop {Pecten) and the Oyster {Ostroea), the one half is more conyex than the other. No Lamellibranch has a distinct head ; but in those which possess two adductor muscles (e.g. AnodotUa), the region in which the anterior adductor lies and which is situated in front of the month, may be distingniBhed as the prosoma, from the middle region {mesosoma) which giyes rise to the foot ; while the part which lies behind the foot and contains the posterior adductor may be termed the metasoma.
The foot may be rudimentary, but it is usually large, flexible, and employed as an organ of locomotion. The posterior face of the foot not uncommonly presents a gland which secretes a chitinous, or shelly, substance— the byssus.
From the sides of the mesosoma, close to the attachment of each mantle-lobe, the branchise project into the pallial cayity.
In its simplest form, the branchia of a LameUibranch con- sists of a stem fringed by a double series of filaments {e.g. Nucula). The next degree of complication arises from these filaments becoming as it were doubled upon them- selyes at their free ends, the reflected i^qx^oiaVxc^^sc^^^
476 THB ANATOKY OF IKTEBTBB&ATKD AKIXALS.
outer side of the outer, and on the inner side of the inner, series of primary filaments. Bnt the free, or l»gmMtl^ ends of the reflected filaments contract no adhesion either with the mantle on the outer side, or with those of the opposite gill on the inner side. Delicate hands stretch from the primary to the reflected filaments across the interspace which they enclose {MytUus, Pecten). In most Lamellibranchs, the gills are four elongated plates, each of which is in fact a long and narrow pouch, with its open end turned towards the hflBmal face of the body. Two pouches are situated on each side of the mesosoma; one of these pouches is internal, the other extemaL* Their walls are united by transrerse septa ; they are richly ciliated, and are perforated by numerous apertures. As the outer wall of each pouch is united with the mantle, and the inner with its fdlow of the opposite side, behind the foot, the whole branchial apparatus forms a siere-like partition extended between the mantle and the foot (Fig. 117), and thus divides the pallial cavity into a supra-branchial and an infra-hranehicd chamber. Inasmuch as the hsemal edge of the inner wall of each inner bran- chial pouch is, for the greater part of its extent, not united with the mesosoma, but only closely applied against the latter, the supra-branchial and infra-branchial chambers may communicate by the cleft thus formed, as well as by the apertures in the lamellar waUs of the branchial pouches. The anterior part of the supra-branchial chamber is divided into a right and left cavity by the interposition of the mesosoma, on the sides of which the apertures of the renal and genera- tive organs are situated. The products of these organs therefore readily pass into these right and left cavities. The posterior part of the supra-branchial chamber, into which these two lateral divisions open, contains the termination of the rectum, and receives the faeces, as well as the urinary and generative products : it is therefore a sort of cloaca. Its external opening is usually termed the anal opening of
* The external gill pouch is and TeUma only one gill poach, 4)fteD mailer than the internal. the internal, is preaent. Jb Ijpidef of lAcmOf Cytkerea^
THE LAMELLTBRAyCHIATA, 477
the mantle cavitj. The margins of this opening may be produced into a tube which is termed the anal siphon. In front of the anal, or rather cloacal, opening, the margins of the mantle may be completely disunited. Yery frequently, however, they are conjoined, so as to leave only an opening for the exit of the foot, and another behind this, which is termed the branchial opening. The edges of this aperture maybe prolonged into a tube, which is termed the branchial siphon. When a Lamellibranch is in its natural element and undisturbed, the valves of the shell gape sufficiently to allow of the free entrance or exit of water, to or from the pallial cavity ; or, when siphons exist, they are fully pro- truded. The cilia with which the branchise are beset, work in such a manner as to drive the water from the infra- branchial chamber, through the openings of the branchisD, into the supra-branchial chamber. From hence its only way of exit is by the cloaca and the anal siphon, when the latter exists. In order to make up for the water thus driven out, a new supply of water enters by the interspace between the lobes of the mantle, which bound the infra-branchial chamber, or by the branchial siphon. These currents may readily be made obvious by allowing a stream of finely- divided colouring matter to pass slowly towards the bran- chial siphon of a Lamellibranch. It will be seen to be swiftly sucked in, and after a very short time a coloured stream will flow out of the anal siphon. The same agency brings the nutritive matters suspended in the water within reach of the labial palpi, by which they are guided to the mouth.
Whatever form the branchise may possess, they are sup- ported by a chitinouB skeleton, in the form of a partial or complete investment to the transverse branchial vessels.
The mouth is bounded by lips, the angles of which are usually produced on each side into two hibial palpi. Some- times the lips are represented by a circular fold produced into numerous tentacula {Pecten), There are no organs for the prehension or mastication of food. A wide and short gullet leads into a stomach surroundedib^ V^[v<^>^^st^^^t>c^^t^
478 THB AKATOMT OF IKYBBTXBSATBD AHIKALS.
consists of nmnerous c»ca united into ducts wliich open into the stomach. Yery generally a diyerticulum of the pjloric end of the stomach contains a transparent rod-like body — ^the crystalline style.
The intestine usually makes many conTolutions, bat» finally reaching the middle line of the dorsal region of the body, it terminates by the anus in the posterior part of the pallial chamber. The heart lies in the region traversed by the termination of the intestine. It consists of an auricle and a yentricle, or of a ventricle and two auricles, or may be divided into two separate auricles and ventricles {Area). Aortic trunks distribute the colourless blood to the body, whence it is carried to a large median venous sinus ; fron^ this it passes through the walls of the renal organs to the gills, and is returned from these to the auricular division of the heart.* Very generally the ventricle invests the rectum, but in Ostrcea, Teredo, and Anomia the ventricle is quite detached from the intestine.
The renal organs, or organs of Bojanus, are usually two in number, often more or less united together, of a dark colour, situated beneath and behind the pericardium and in front of the posterior adductor muscle, extending forwards on each side of the mesosoma, and traversed by such numerous blood-channels that they have a spongy texture. The walls of the cavernous blood sinuses are lined with cells which secrete the urinary matters from the blood. These take the form of calcareous concretions containing uric acid. The gland communicates at one extremity with the pericardium ; at the other, it either opens directly on to the surface of the body, or into a vestibular cavity which has an external aperture.
In Chtrcea and Teredo the renal organ seems to be present in only a very rudimentary f orm.t
The mesodermal region, between the endoderm and the
* The circulatory organs of demie,' 1855 and 1856.)
the freshwater Mussel have been f See for the stractore of the
very fully desciibed by Lancer. rena organs and manv other
(* Aenksehriftea der Wiss. Aka- points connected with ue ana-
TBI LAICILLIBBJJIOHUTA. 479
ectoderm, is for the mort part occnpied by TMcolar, coimeotive, and moBcnlar tiaaneB, and bj the reproductive organa, ao that there ia no lafge periTiaoeml apace. But there is — 1. The large median eiiina alreadj mentioned, which receives the blood retnmed from all parta of the bodj, and is commonlj termed theoena cava. 2. A apadone pericardial chamber which enclosea the heart. It is in
Fig. 118.
Fig. IIS.— j^KOdoKfa.— Tertleal tnd tmwvene wctlan of tbe bodj uirDugb Ibebcart; /,TeDtricJe; p.iurlclca; c.reotum', p,DtritMi' dlmu; h, inner, pouter gill; a* veitibnie of q, the orgui of Bojuiiu; B, foot, A. A, Duitle lobci.
commiinicatioli with the venoaa ayatem, and, OOIMeqiiently, directly, or indirectlj, with the vena cava. 3. The osTitiea of the renal organs, which naoally freely commtmicftte with
tomf of tha LameUStnfftthiatit, tlie ■criet of Tdiuble papen of Lk- cue-Duttiian. (> Auualei dw
+80 THE ANATOMY OF INTEBTEBBATBD
one another, while they open into the pericardinm oi one blind, ajid on the e»teriur of the body on the t 4-. In flomc Lumellilrranekiaia. canaia open on the exi of the b'jdy. especially on the Burtace of the foot. Ii way the blood ajHt^m is placed in direct, thongh circu (?omniimieation with the surrounding water. Tbee colled tcat(v-rei8eU communicate internally with the rt Byatem, of whit'b, indeed, tbey seem to form a part. probable that all these cavitiea, taken together, repr the pcriviai.-erul cavity, pallial ainiiBes, and pseitilo-heaj a Brachiopod.
Sti'ong bundles of muscular fibres, usuallj nnBtriated. tranHversely from one valve of the shell to the other brin^i them together; while they are divaricated bj elastic reaction of tbo ligament. Of Buch addiKtor mi there may be either one or two. When there are ilHnnjariai, the anterior adduetar lies in front, and o htK'mal eiile of. the tcsophagus ; while the ponterior odd iica in front, but on the neural aide, of the rectum. E the alimentary canal, as a whole, lies between these muadca. When only one adductor mnacle eiiste |J s the poaterior.
TRB ULMBLLIBSAKOHIATA.
481
inUgropMiaie and tinupaUiaie as applied to LameUibrancliB which haye the pallia! line erenlj rounded or notched.
The cerebral ganglia lie at the sides of t^e month, and are connected by a commissure, which passes in front of it. Thej gire branches to the anterior region of the mantle, to the gills, to the anterior adductor muscle, to the labial palpi, and to the parts about the mouth. The j^edal ganglia are situated in the foot ; or in the corresponding region on the neural side of the alimentaiy canal, when no foot is dereloped. Each is united by a commissure with the cerebral ganglion of the same side, and gives off branches to the muscles of the foot. The parieto-splanehnie ganglia lie on the neural face of the posterior adductor muscle. The long commissures which unite them with the cerebral ganglia usually trayerse the renal organ, and lie beneath the floor of the pericardium. Each of these ganglia giyes off a nerye to the branchia of its side, and supplies the posterior and middle part of the mantle. This posterior pallial nerye may anastomose with the anterior pallial nerye from the cerebral ganglion. The ganglia also furnish neryes to the posterior adductor muscle, to the heart, to the rectum, and to the nrasclee of the siphons, when the latter are present. Eyes are neyer developed in the cephalic region of the LamelHbranchs, but, in many {e,g, Fecten) numerous simple eyes terminate papillsB of the margins of the mantle. Auditory sacs are almost inyariably attached by longer or shorter pedimcles to the pedal ganglia.
The Lamellibranchiaia are usually dioecious, but some- times hermaphrodite* {e,g. Cyclas, some species of Cardium and Pecten, Odroea, ClavageUa and Pandora), The genera- be either almost all ovigeroas or almost all spermigeroos : and it appMrs probable that toe pre- dominantly male, precedes the predoninantlv female, condition. See Lacase-Dathiers. **Organea genitanx dea Ac^halea Lamelll- branehes " (' Ann. das Scienoet NatiireUef,'1854).
2x
* The testes and ovaria are distinct in the hermaphrodite Pectines. In Cardhan terratum. adjacent c®ca of the sexual glaml contain spermatosoa or ova, or both products may be developed in the same omcum. In the eoa- mon Oyster the genital cseea in any given individoai are found to
480 THB AKATOMT OF INYEBTBBSATBD AKOCALS.
one another, while thej open into the pericardiom on the one hand, and on the exterior of the body on the other. 4. In some Lamellibranchiata, canals open on the exterior of the body, especially on the surface of the foot. In this way the blood system is placed in direct, though circuitous communication with the surrounding water. These so- called toater-vessels communicate internally with the renous system, of which, indeed, they seem to form a part It is probable that all these cavities, taken together, represent the perivisceral cavity, pallial sinuses, and pseudo-hearts of a Brachiopod.
Strong bundles of muscular fibres, usually unstriated, pass transversely from one valve of the shell to the other, and bring them together; while they are divaricated by the elastic reaction of the ligament. Of such adductor muscles there may be either one or two. When there are two (Dimyaria), the anterior adductor lies in front, and on the hsemal side of, the oesophagus ; while the posterior adductor lies in front, but on the neural side, of the rectum. Hence the alimentary canal, as a whole, lies between these two muscles. When only one adductor muscle exists (Mono- myaria), it is the posterior.
The foot is retracted between the valves of the shell by two or three pairs of retractor muscles, of which the anterior and posterior pairs are usually attached to the shell, close to the anterior and posterior adductor im- pressions. The protraction of the foot appears to be effected by the compression of the blood by tiie intrinsic muscles of the walls of the mesosoma and of the foot itself.
Each lobe of the mantle is attached to the corresponding valve of the shell by a series of muscular fibres, the attach- ments of which give rise to a linear impression, which runs from one adductor to the other, and constitutes the paUial line. When the siphons are largely developed they have retractor muscles, the insertions of which are so disposed as to cause the posterior part of the pallial line to be more or less deeply curved or angulated. Hence the distinction of
THE LAMBLLIBSAKOHIATA.
481
integropalliaie and sinupdUiaie as applied to LamellibrancliB which haye the pallial line erenlj rounded or notched.
The cerebral ganglia lie at the sides of the month, and are connected by a commissure, which passes in front of it. They giye branches to the anterior region of the mantle, to tiie gills, to the anterior adductor muscle, to the labial palpi, and to the parts about the mouth. The pedal ganglia are situated in the foot ; or in the corresponding region on the neural side of the alimentaiy canal, when no foot is deyeloped. Each is united by a commissure with the cerebral ganglion of the same side, and giyes off branches to the muscles of the foot. The parietO'Splanchnie ganglia lie on the neural face of the posterior adductor muscle. The long commissures which unite them with the cerebral ganglia usually trayerse the renal organ, and lie beneath the floor of the pericardium. Each of these ganglia giyes off a nerye to the branchia of its side, and supplies the posterior and middle part of the mantle. Thia poeterior pallial nerve may anastomose mth the anterior pallial nerye from the cerebral ganglion. The ganglia also furnish neryes to the posterior adductor muscle, to the heart, to the rectum, and to the nrascles of the siphons, when the lattar are present. Eyes are neyer deyeloped in the cephalic region of the Lunellibranchs, but, in many {e.g. Fecten) numerous simple eyes terminate papillsB of the margins of the mantle. Auditory sacs are almost inyariably attached by longer or shorter pedimcles to the pedal ganglia.
The Lamellihranchiata are usually dioecious, but some- times hermaphrodite* {e.g. Cyclae, some species of Cardium and Pecten, Ostroea, ClavageUa and Pandora), The genera-
* The testes and ovaris are distinct in the hermaphrodite Pectmes. In Cardium terratunu adjacent c®ca of the sexual gUnd contain spermatoaoa or ova, or both products Biay be developed in the same caecum. In the com- mon Oyster the genital cseea in any given indiyidoai are found to
be either almost all ovigerous or almost all spermigerous : and it appears probable that toe pre- dominantly male, precedes the predosnioantlv female^ condition, bee Laoaae-Dathiers, '*Organes g^nitaux des Ao^phales Lamelli- branches " (* Ann. det Sciences Naturelles,' 1854).
2x
482 THB ANATOMY OF IKTEBTEBSATKD AHIMALS.
tiye organs are ramified glands of simple straotore and similar in both sexes, the ducts of which open into, or dose to, the renal organs.
The process of yelk-division * nsnally gives rise to smaller and larger blastomeree, of which the former, as an epiblast, invest the latter as a hypoblast. At the o^halic end of the embryo of most Lamellibranchs, a velum, or disk with richly ciliated edges and, nsuaUy, a central toft of longer cilia, ii9 formed. On the dorsal face of the embryo the integument rises into a patch with raised edges, which is the rudiment of the mantle. The separation of the shell into twa valves, united by an uncalcified hinge, must pro- bably be ascribed to the manner in which the calcareous matter subsequently added to the- shell is deposited. The foot appears as a median outgrowth of the neural face of the embryo behind the mouth. The branchis have, at first, the form of separate filamentous processes, which are developed from the roof of the anterior part of the pallial cavity, at the^ point of junction of the mantle vritii the mesoeoma, and gradually increase in number from before backwards. Jn those Lamellibranchs which have pouchlike gills, it appears that the processes which are first formed become the outer lamella of the inner g^-plate, their free ends uniting together; the inner lamella of this plate is produced by the upgrowth of a thin lamina, which subsequently becomes perforated, from the united ends of these processes. The inner- lamella of the outer gill is formed of branchial processes, which grow out from the attached ends of the first set; and the outer lamella of this gill is produced in the same- fashion as the inner lamella of the inner gill.f
Recent observations tend to show that in these, as in other Invertebraia, the nervous ganglia are modified ingrowths of the epiblast.
* See Love'n, 'Archiv fur Na- f Lacaie-Ihithien^ <'Sur lede-
tnrgeeohichte,' 1849. De Quatre- veloppement dea branehies des
^'Memoires sur TEmbrvo- Molfusques aoephalea Lamelli-
des Tareto ** (*Anii. dea So. branchea " O ^nn* dea Seienoat
1849). Naturelles,* 4, iv.).
THB DETSLOPMBITT OF LAMBLLIB&AHCHB. 483
The simplest form of development of the LameUibranehu aia has been observed in Pitidiwn.* Bj the prooess of cleavage, the yitelliis is divided into a number of equal blastomeres. The morula thus formed undergoes invagi- nation, and is converted into a gastrula. The blastopore, or aperture of invagination, closes, and the epiblast, or ectodermal layer of the embryo, growing much faster than the hypoblast, or endodermal layer, the latter forms a small shut sac, the primitive alimentary sac (or (Mrchenteron) attached to one point of the inner surface of the much larger ectodermal sac. The mesoblastic cells appear to be derived both from the epiblast and the hypoblast.
The mouth is formed by a depression of the ectoderm at the anterior end of the body, which grows towards and opens into the archenteron. The anus is developed at the opposite end, in the region of the primitive invagination. On the neural face of the embryo, the foot grows out, while the mantle appears on the opposite face ; and, in the centre of the mantle, a transversely oblong depression lined by elongated cells is the 'shell gland.' In the median line this answers to the ligament, and, at the sides, to the middle region of the future valves of the shell ; but the precise share, if any, which it takes in the formation of these parts does not appear. Piaidium has no velum.
The development of one of the fresh-water Mussels (Unto pidorwn) has recently been worked out very fully by BabLt The vitellus divides into two unequal masses, of which the larger is termed by Babl the 'vegetative' and the smaller the ' animal ' cell, somewhat inconvenient names which may be replaced by ' macromere * and ' mieromere.* Each of these becomes subdivided, partly by ordinary fission, partly, as in the case of the macromere, by a process of budding, intoblastomeres, of which those which proceed from the macromere long remain larger and more granular than those which proceed from the micromere. The blastomeres
* Lankester" On the Develop- t C. Rabl, *Ueber die Ent-
nental History of the MoUasca." wickelungtgeschichte der Hsler- (* FhiL Trane.' 1874.) muscheV JeD«« 1876.
2 I 2
484 THE ANATOMY OF INYEBTBBRATBD AKOlAIiS.
arrange themselyes into a hollow sphere — ^the hkuiosphere. This is a yesicular morula, composed of a single lajer of blastomeres, of which those of one hemisphere have pro- ceeded from the micromere, and those of the other from the macromere. Two blastomeres of the macromenJ hemi- sphere remain much larger than the rest. The macromeral hemisphere next undergoes invagination, and its invagi- nated part becomes the hypoblast. The two large blasto- meres just mentioned, which are disposed symmetricallj, one on each side of the median plane at the anterior margin of the area of inyagination, become enclosed be^een the hypoblast and the epiblast,mnd by their division give rise to the mesoblast. This last, therefore, may be regarded as an indirect product of the hypoblast.
The endoklermal sac formed by the hypoblast now loses its connection with the region of the embryo of which it is an invagination, and applies itself to the anterior wall of the body, where an involution of the ectoderm, which gives rise to the oral cavity, takes place. The gre&ter part of the mesoblastic cells become the adductor muscle, which is at first single and answers to the posterior adductor of the adult. There seems to be no shell gland. The shell appears at first as a membranous cuticula, continuous from side to side, and therefore undivided into two valves. Subsequently it becomes calcified and bivalve. The byssus gland is developed as an involution of the ectoderm at the posterior end of the body ; and the ventral hemisphere, or that oppo- site the shell, becomes divided by a deep median fold into the two lobes of the mantle on which the characteristic pencil-like papillae appear. In front of the rudimentary mouth are two ciliated depressions of the ectoderm, which are possibly the rudiments of the nei'vous ganglia.
In Unio and Anodonta the young are hatched in the outer gill pouches of the parent, from which they are so dis- similar that they were at one time considered to be parasites (Olochidivm), The valves of the shell are triangular and have incurved and serrated apices, by the help of which the Urvm, after they leave the parent, attach themselves to
THB LAMXLLIBRAirGHIATA. 485
fisliee and other floating bodies. In this position they undergo a sort of metamorphosis, and eyentuallj fall off and sink to the bottom as minnte fresh- water Mnssels.
On comparing the LameUibranchiata with the Brtxehiopadaf it is obvious that the two have, in common with one another and with the Annelida, the ciliated or veligerous larval form. If the shell gland is, as Mr. Lankester suggests, the homologue of the peduncular gland of LooB08oma and of the Brachiopod larvs, it follows that the peduncle of the Brachiopod corresponds with the centre of the pallial surface of the Lamellibranch, and that the so-called dorsa^ and ventral lobes of the mantle in the Brachiopod corre« epond with the anterior and posterior halves of the mantle in the Lamellibranch. The Brachiopod hinge will there- fore be transverse to the axis of the body, while the Lamellibranch hinge is parallel with it. If this comparison be just, however, the three segments of the Brachiopod larva cannot answer to the segments of an Annelid larva, but the two posterior segments of the Brachiopod larva must represent an outgrowth of the hsBmal side of the bodj; and this would correspond very well with the arrangement of the intestine in the articulated Brachiopoda,
In the simplest forms of the LameUibranchiata, as Trigonia, Nucula, and Pecten, the mantle-lobes are almost, or com- pletely, disunited with one another and with the branchiie, and the latter are either simple plumes or have undergone but little modification. The haemal face of the body is short relatively to its vertical height.
In most Lsmaellibranchs the haemal face of the body is longer; the gills are lamellar, and the mantle-lobes are united with one another and with the gills, so as to sepa- rate a supra-branchial from an infra-branchial chamber (Anodonta). In yet others, the posterior margins of the mantle are produced backwards into short siphons, but the mantle-lobes remain separate for the rest of their extent {Cardivm); in others, the siphons are greatly elongated and the ventral margins of the mantle-lobes unite, so as to leave only a small median aperture for the foot {Pholas), In the
4S6 THE ANATOMY OF HTYEBTSBRATBD ANIMALS.
most modified forms, the body becomes more and more elongated, until, in Teredo^ it is completely vermiform, and the valyes of the shell cover but a very small portion of the body.
The foot is wanting as a distinct stmctnre in CMroea; while in Cardium and Trigonia it is a large mnscnlar organ, by the aid of which the animal is able to leap for some distance. The byssns may be present in the yonng and absent in the adult [e.g. AnodorUa). It may have the form of strong chitinous filaments {Mytihts) or of a plate of homy or shelly texture {Area, Anomia). The inequality of the valves attains its maximum in the HippuriHdcB, in which one valve may have the form of a long cylinder, or cone, while the other is a flattened plate.*
The shells of Lamellibranchs are among the most abundant of fossil remains in all epochs of the world*s history. In the Palaeozoic formations, however, the pro- portion of these moUusks relatively to the Brachiopoda is the reverse of what obtains at the present day, the latter being very numerous, while the Lamellibranchs are com- paratively scanty. The integropalliate are far more nume- rous than the sinupalliate forms in the older rocks. The HippurUidoi of the Cretaceous epoch is the only family of ancient Lamellibranchs which is extinct at the present day ; and the only one which diverges to any considerable degree from existing forms.
The Odontophoka. — In the Mollusks which belong to this division, the mantle, always present in the newly hatched young, may abort in the adult condition. It is never divided into two lobes, though it may be slit or per- forated where it forms the waU of the branchial chamber (Hdliotia, Fisswrella).
Yery generally, the prosoma bears tentacula and eyes ; and a distinct head being thus recognisable, these Mollusks
* For an exoeUent Account of the LamelUbramekiaia finom the ooaeMogieal tide, te«^ood^tx^'%^'^«>iv^ui^^t xSu^lAoUnMa.'
THS ODONTOPHOIU. 487
have been named Cephalophora, in OQntradistinotiQn to the acephalous Lamellibranchs and Brachiopods.
The mantle commonlj gives rise to a shell ; which may either be a more or less calcified cnticidar product of the epidermis, covering the onter surface of the mantle, when it constitutes an external shell, as in the LaimMibranchiaia and Brachiopoda ; or it may be developed within a sac in the interior of the mantle, as an internal shell. In neii&er of these cases is it ever a bivalve shell divided into two lateral portions.* Usuallj it is in one piece (uivalve), but in one group, the ChUonidce, it consists of a number of pieces (not exceeding eight), arranged in longitudinal series along the middle line.
Calcareous matter is very commonly diffused, in the form of granules, through the connective tissue, and often takes the form of spicula (e.^. Doris),
The mesosoma is generally prolonged into a muscular foot, which may be provided with lateral appendages, the epipodia. And, on the hsmal aspect of the posterior portion of the foot, a chitinous or shelly plate, termed the operculum^ may be developed. This operculum appears to be the analogue, if not the homologue, of the byssus of the LameUi- branchs; and is certainly not homologous with either of the valves of the shell of the latter, which are pallial struc- tures. The edge of the mantle forms a free fold which nearly or entirely surrounds the mesosoma; and in one genus, Denialium, the margins of the mantle unite for the greater part of their length : in all the rest they remain free. A space is enclosed between the lobes of the mantle and the mesosoma. Usually this space is much larger on one face of the body, and constitutes the pallial chamber. As a rule, the branchiffi are lodged in this chamber, and the anus opens into it.
In a very few Odoniophara, the symmetry of the body is
* The singular bivalve plates, homologous with the shell of
termed Aptvckusj which occur in ordinary Mollusks, which is re-
the AmmomuidtBy whatever their presented by the chambered shell
nature may be, are obviously not of the cephalopoda
488 THI ANATOMY OV INYSBTSBRATED AKIMAUB.
undisinrbed ; that is to say, the month and the anna are sitnated at opposite ends of the axis of the body, and the hsmal face is not produced into a Tiscend sac (e.g. ChUon^ DenUUiwn). Bnt, in the great majority, such a TiBceral sac is formed. In the Cephalopoda, it co-existB with bilateral symmetry, inasmnck as the mantle and the anns lie in the plane which divides the body into two similar halves. Bnt, in most OdotUophora, the anus is twisted to one side (nsnally the right), and in many, it is situated, together with the pallial chamber in which it is contained, on the anterior face of the body.
The mouth lies at the anterior end of the body, on the haemal side of the anterior part of the foot (except in the Cephalopoda), It may be provided with variously dispoeed jaws, or cutting plates, of a chitinous or calcified substanoa But the structure which is most characteristic of the Odo%' tophora, and which is absent in only very few genera (e.g. Teihys, Doridium, Bhodope), is a peculiar rasping and some- times prehensile apparatus, the odowtophore, or as it is often termed, the tongue, which is attached to the floor of the mouth (Fig. 119, 120).
This apparatus consists of a skeleton; of a eubradular membrane, ^i4nch is continuous with the lining of the oral carity ; of the radula ; and of intrineie and extrinsic muscles.
The skeleton is composed of two principal masses of par- tially fibrous, or completely cartilaginous tissue {odonto^ phoral cartUages), which may be more or less confluent, and are further xmited together in the middle line by fibrous and muscular tissue. Their anterior ends and oral faces are free and smooth, and are usually excavated so as to present a trough-like surface to the subradular membrane* which, rests upon them. Accessory cartilages may be added to these. Behind, the subradular membrane is con- tinued into a longer or shorter sac, lined by a continuation of the buccal epithelium. The radula is a cuticular chitinous product of the epithelium of the subradular membrane. It .]• anned with tooth-like processes arranged in one or many [; and addiUona aro cons^aai^lL^ bein^ made to its pos-
THS ODOKTOPHORA.
489
terior end, wliich ie lodged in the sac of the sabradolar membrane. Thus the teeth are replaced from behind, as fast as they are worn away by friction against the food which thej rasp, at the anterior end of the ribbon.
The intrinsic muscles oi the odontophore are attached, on
Fig. 119.
Fig. U9,—Biiecmwm undatitm.—A^ radol*;^ B, one of the transyerte rowf of teeth, a, anterior, b, posterior end ; c, central, I, lateral teeth. (After Woodward^ * Manual of the MoUusca.*)
Fig. 120.
Fig. 120. — A^ T^ockuM einerarimt ; the median tooth and the^teeth ot the right half of one row of the radula. B, C^nta .EWrqpoo, one row of teeth of the radula. (Woodward, ibid.)
the one hand, to the posterior and nnder faces of the odonto- phoral cartilages, and, on the other, to the subradular mem- brane, some being inserted into its posterior and lateral portions, and others into its anterior extremity, after it has tnmed over the anterior extremities of the principal cartilages. Certain of the mnscnlax \>\m!3\«& «x^ ^J^oi^
490 THE ANATOMY OV XNTBBTEBRATED AHIMAL8.
attaclied to the fore part of the odontophond cartilages themselves. The contraction of these muscles must tend to cause the subradular membrane, and with it the radula^ to travel backwards and forwards over the ends of the car- tilages in the fashion of a chain-saw, and thus to rasp any body against which the teeth may be applied. When xm- disturbed, the radula is concave from side to side, and the teeth of the lateral series, being perpendicular to the sorfaoe to which they are attached, are inclined inwards towards one another. But when the intrinsic muscles come into action, the radula, as it passes over the ends of the car- tilages, becomes flattened, and the lateral teeth are con- sequently erected or divaricated. The extrinsic muscles pass from the odontophore to the lateral walls of the head, and protract or retract the whole apparatus. They may give the protruded extremity of the radula a licking motion, which is quite independent of the chain-saw action doe to the intrinsic muscles.*
The odontophore is developed very early, and it would be interesting to know whether it exists in the young of those few Odontophora, in which it is wanting in the adult state.
Salivary glands are very generally present in the Odontih phora,f and the liver is usually large.
As in the MoUusca in general, the blood corpuscles are
* In my memoir *' On the mor- phology of the Cephalous Mol- luBca" rPhil. Trans. 1852), 1 de- scribed the chain-esw action of the odontophore, as I observed it in the transparent i^o&>u2e< and At- laiUa, while living. But as Troschel has remarked in his excellent monograph (*Das Gebiss der Sehneeken,' erste Lieferung, pp. 19, 20, 1856), I did not sufficiently dwell on tlie frequency and im-
Sortanee of the licking action pro- ueed by the extrinsic muscles. I am atill of opinion, however, that this aotioii cannot be rightlv de- •oribady M a inovtiaeia of t^
radula following secondarily upon that of the cartilages, inasmuch as it is a motion of the whole odontophore. On the other hand, it maybe, as has been suggested to me by Mr. Geddes, who at my suggestion has undertaken a re- examination of the structure of the odontophore, that the flexure of the anterior ends' of the odonto- phoral cartilages, by the intrinsic muscles inserted into them, plays an important part in the motion of the radula.
t In IJolium the salivary secre- tion contains free sulphuric acid.
THB ODONTOPHORA. 491
coloorlees and nucleated. The blood plasma is red in Planorbis.
The heart maybe wanting {Dentalium) or it maj resemble that of the Lamellibranchs in having two anricles {Chiton, HalioHs), and even in being perforated by the rectom (HalioHSf IStrhOy Neriia); most commonly it consists of a single anricle and a single ventricle. In the Oephalopods, it is hard to say whether the two or four branchio-cardiac tronks which open into the ventricle should be regarded as veins or as auricles. An accessory " portal " heart has been described in Doris.* Special respiratory organs may be wanting, their place being taken by processes of the body, or by the walls of the mantle cavity, or by the general surface.
The branchise, when present, are numerous lamellar pro- cesses, or from one to four plume-like gills. Aerial respira- tion is effected by the walls of a pulmonaiy sac, which is a modification of the pallia! cavity.
The presence of renal organs, in the form of one or more sacs situated close to the heart, open to the exterior on one side and, on the other, in relation, usually by means of a glandular structure, with the returning current of blood, is very general ; and, in many cases, these renal sacs communi- cate directly with the blood sinuses through the pericardium. In many Pteropods and Heteropods they are rhythmically contractile.
As in the LameUibranehiata, so in many Odoniophora, simple or branched canals traverse the substance of the foot and open externally by a more or less conspicuous pore, which is usually situated upon its inferior face. These ctquiferoua canals, as they have been termed, appear, in many cases, to open by their inner ends into the blood sinuses, and thus to establish a direct communication between the blood and the surrounding water. In species of Pyrula, Agassiz found that coloured fluids ii^'ected into the pore passed into and filled the blood vessels generally. But it
* Hancock and Embleton, « On the Anatomy of lyorii.** Q PhlL Trans.' 1852.)
4d2 THX ANATOMY OF HTVIBBTBBRATED ANIMALS.
may be donbted whether these canals should be regarded as a special system of yessels, rather than as blood giimaes which open extemallj.
The arrangement of the centres of the nenrons system in Dentaliwn * most nearly approaches that which exists in the Lattiellihranchiata. Two cerebral ganglia lie close together on the haemal side of the oesophagns. A long commisBiiral cord connects each of them with one of the pedal ganglia, which are also closely nnited. A second long commissore passes backwards from the cerebral ganglia, and often presents a ganglionic enlargement at its origin. It unites with one of two ganglia, situated close to the anns, and con* nected, in front of it, by a rather long transverse commissure. The nerves distributed to the posterior half of the mantle are given off from these ganglia, and those to its middle region from the anterior end of the commissure or its ganglionic enlargement. There seems no reason to doubt that the ganglia close to the anus, together with the ganglionic enlargements at the anterior ends of the commissures which connect them with the cerebral ganglia, correspond with the parieto-splanchnic ganglia of the Lamellibranchs, and that the cerebral and pedal ganglia are the homolognes of those so named in the latter Mollusks.
In addition to this approximation of part of the gan- glionic mass of the parieto-splanchnic system to the cerebral ganglia, Dentalium differs from the Lamellibranchs and resembles other Odontophorat in the possession of a system of bucccU nei'ves, which arise from the cerebral ganglia, and in which minute ganglia are developed. The nerves which proceed from the buccal ganglia are distributed to the odontophore and its muscles.
In other Odontophora, the two cerebral and two pedal ganglia, with their commissures, are always to be recog- nised ; but the number of the ganglia which represent the parieto-splanchnic system may be increased, and the anterior ganglia of this system may attain a large size, and may
• See Lacaxe-Du\YAcr%, ^ OT^jKn\%^\Xwi 4Ax\i«aUle.*
THE ODOXTTOPHOIU. 493
come into close relation not only with the cerebral bnt with the pedal ganglia.
In Lymnceus pcdustris* for example, there are fiye such ganglia situated close to the cerebro-pedal ring. The most anterior of these, on each side, is nnited with both the cerebral and the pedal ganglion of its side ; and appears, indeed, like an enlargement npon a second commissure between those two ganglia. The ganglia which constitute the second pair are united, in front, by a short commissure, with the preceding; and, behind, with the fifth or azjgos ganglion. The second pair of ganglia give off the nerves to the right and left sides of the mantle respectively.
In Limax, and apparently in the terrestrial PulmoruUa generally, the arrangement is essentially the same, except that all the ganglia of the parieto-splanchnic system co- alesce into one mass ; between which and the pedal ganglia, the aorta passes.
In HalioiiStf on the other hand, while the anterior parieto- splanchnic ganglia are situated close to the pedal ganglia and are connected with them and with the cerebral ganglia, in such a manner as to give rise to an apparent second cerebro-pedal commissure, the ganglia which represent the second pair in Lymnceus, are situated at the base of the branchia) and are united by a long commissure with one another, and also with the anterior parieto-splanchnic ganglia. Of the latter commissures, that from the left branchio-pallial ganglion goes to the right anterior parieto- splanchnic ganglion, and viee versa.
With respect to the position of the cerebral and pedal ganglia in the Odontophora, the commonest arrangement is that in which the cerebral ganglia are supra-oesophageeal, and are connected by two longer or shorter commissures, on each side, with the pedal and anterior parieto-splanchnic
* Compare Lacaze-Duthien, ganglia of the nervous system
« Du systeme nerveuz des Mol- given in his memoir on the
lusques gast^podes pulmonis otocjrsts. (Ibid.)
aquatiques ** (*Arch. de Zoologie/ f See Lacase>I>athier8, " Snr
1872), and the numerous figures le syst^e nerveox de UalictldA" of the anrnngemeut of the oerebnl
4d4 THE ANATOMY OF nrVEBTBBRATED AKIMAIiS.
ganglia, both of which are infra- or po6t-0B8ophi^;8eaL But in many cases (most Nudibranehicttd), the pedal and parieto- gplanchnic ganglia are approximated to the cerebral gang^ (the latter being snpra-cesophagaRal ), and are united bj long sub-OBSophagseal commissures. In others, as in most Plero* poda, the pedal and parieto-splanchnic ganglia are sab- OBSophagBBal ; while the cerebral ganglia, brought close to them, are united by a supra-<Bsophag8Bal commissure.
Accessory ganglia are frequently developed in the region of the heart and branchise, on tbe nenres of the parieto- splanchnic system.
A complicated system of visceral nerves is distributed over the whole length of the alimentary canal, the genital organs, and various parts of the vascular system, in many Odantophora,*
Two auditory vesicles usually exist, and very generally appear to be sessile upon the pedal ganglia. In the Heieropoday in many Nudibranchi4ita, as shown by Hancock, and in numerous genera of Branchuh and Pulmo^gcuieropoda^ which have been carefully examined by Lacaze-Duthiers^t however, there seems to be no doubt that the auditoiy nerves arise from the cerebral ganglia, even though the vesicles may be situated close to the pedal ganglia.
Olfactory organs certainly exist in the C^htUopoda in the form of saccular involutions of the integument near the eyes; and it is very probable that the integument of the
* See especially Hancock and Embleton, **The Anatomy of Doris" (Phil. Trans.,' 1852).
t *' Otocystes dea Mollusques.**
(* Archives de Zoologie Experi-
mentale,' 1872.) In the memoir
the origin of the acoustic nerves
from the cerebral ganglia is deter-
mineil in so many Puhmopastero'
poda (Limaxj Arkm^ TetiaceUoj
CtausUicu ZonkeMy Helix, Sucemea,
PhytOy LymnauSj Aneyhu) and
Branc/uo^oMtercpoda, {NerHinay
jPalmdmay Cffclodoma^ FUeopau^
Cafyptrtta, i^otioa, Nasta,TVochiM^
JIfvrer, Casaidaria^ Pwrptaray Patdla, Haiioiis, Philme, Af*fyua, LameUaria), that there is a large basis for the generalisation that this mode of origin is imivenal. Moreover, aooording to Lacftse- Duthiers, the same law holds good for the Cephalopoda, Sach being the case, the question suggests itself whether the eonnexioo of the nerves of the otoeysts with the pedal ganglia, which obtains universally among the Lamelli> branchs indicates their real or Qn^^ ^^DA^K v^'\ax«ai origin.
THE ODOirrOPHORA. 495
tentacula, or of the lips, maj subBerve the same function in the Gkuteropods.
£ jee are generally present, and are limited to two, situated in the head. They resemble the vertebrate eye in stmctnre, so far as they possess a concave retinal expansion and usually, in front of this, a vitreous humour, lens, and cornea. But they differ from the eyes of Vertebrataj and resemble those of other invertebrated animals, in that the structures which answer to the rods and cones are situated on that face of the retina which is turned towards the light, while the fibres of the optic nerve traverse the pigment layer to reach them.
The reproductive organs of the Odoniophora present very great diversities of structure. They may be either dioecious or monoecious, and each type of reproductive organs may present various degrees of complexity. Of the dioecious reproductive organs, there are two chief forms; the one in which the duct of the ovarium or testis is con- tinuous with the gland ; and the other in which the duct opens into a sac, into which the ova or spermatozoa are set free by the dehiscence of the follicles in which they are developed. The latter arrangement is met with in the Cephalopoda ; the former appears to prevail among all the other dioecious Odoniophora.
In these, the racemose generative gland is usually situated close to the liver. In the female, the oviduct ordinarily presents a uterine dilatation towards its termination, which is generally situated in the pallial cavity on the right side of the body. In some rare cases (PahuLina, Neritina), a dila- tation or a special vesicular api>endage of the uterus may serve as a vesicula aeminalis ; and, in Paludina, according to Leydig, an albumen gland opens into it.
A penis is not always present. When it exists, it is a muscular process of the mesosoma, to which the semen may be led from the opening of the vas deferens by a groove ; or it may be traversed by the vas deferens which opens near, or at, its apex.
In all the moncecious Odoniophora which Wi^ ^a^ '^^\.
496 THB ANATOMY OV INYBBTBBRATBD AHIMALS.
been thoroughlj examined, there is a generatiTe gland termed the ovotesHs, in which both spermatozoa and o?a are produced. Only in the anomalous genua Bhodepe, (Kolliker), are the spermatozoa and ova formed in distinet c»ca; in all the rest, each csBcum is hermaphrodite, the spermatozoa and the ova being usually developed in dif- ferent parts of the csdcum. The duct of the OYotestia may remain single to its termination at the genital aperture ; or become only incompletely divided into two semicanala {Pteropoda, Pleurophyllidia, Umbrella, Aplysia) ; or it may become, at first, partially, and then completely, divided into an oviduct and a vas deferens (Nudibranchiaiaf I^leuro* hranchiaf Pulmonata).
In the former case, there is but one genital aperture. The common duct usually receives the secretion of a uterine gland which may take the form of a special albumen gland* and a spermatheca opens into it near its outer extremitj ; while, on the male side, a vesicula seminalis, and an eversible penis may be added. — The penis, however, may be distant from the genital opemng, and then a groove on the side of the body leads to it {Aplysia), In the latter case, there are two genital apertures, one for the male and one for the female organs, though they may open into a common ves- tibule. The penis is an eversible involution of the integu- ment, on which the vas deferens opens. A prostate gland is usually connected with the latter, and, near its opening, there may be a saccular appendage, in which a hard pointed body, the tpiculum amoria, is contained (Doris, Helieida), An albumen gland opens into the uterus, and a spermatheca is connected with the vagina.
Spermatophores, by the aid of which the spermatozoa are transferred into the female organs, occur in the Cephalo- poda, and in the Pulmonata. In the latter they are grooved bands, or incomplete tubes of hardened mucus secreted by the penis, which become fiHed with spermatozoa during copulation ; while, in the former, they are closed cases which may have a very complex structure.
Li the great majority of the Odoniophora, the young
THS DSTXLOPlCXirT OF THX ODOVTOPHOR4. 497
res the egg as a veliger reij nxnilar to that of the mdlibranehiatci. The ▼eltun usuallj becomes bilobed, I aometiinee (Heteropoda), its tnargins are produced into aj tentacnliform processes; and, in all Pieropoda and mehiog<uteropod(i^ whether the adnlt possess a mantle i a shell or not, the larva is provided with both, the shell ig at first a simple conical symmetrical cap, developed the middle line of the mantle. The eyes make their earance behind the velnm, and the tentacles in front or upon it.
Hiile the course of the development of the embryo in Odaniophora presents a general uniformity, there are e differences in detail.
a PcUudina,* the blastomeres produced by yelk division
of equal size. They arrange themselves into a vesicular
Tila, which undergoes invagination and becomes a gas-
a of the simplest type. Hie aperture of invagination
ttopore) becomes the anus, while the mouth is formed
« involution of the ectoderm of the anterior end of the
, which extends towards and eventually opens into, the
end of the archenteron or primitive alimentary sac.
lated velum is developed on the hsemal side of the
I ; and a ' shell gland ' appears in the centre of the
hich gives rise to the mantle.
'^ymncBUStf also, cleavage ends in the production of
leres of equal size, whether with or without a tran-
tage of inequality, and the vesicular morula under-
Tagination to give rise to the archenteron. The
nre is elongated, and it appears to be likely that
ior and posterior ends may coincide with, if they
ve rise to, the mouth and anus respectively.
t OdorUophora, the process of yelk division goes on
, and results in the production of large and small
ter, *'On the coinci- the development of the Pond
'he bUstopore and Snail" (^Qaarterly Journal of
?ahidina vivipara" 2dicro9Copical Science,' 1874), and
Journal of Micro- C. Rabl, **I>ie Ontogenie der Siiss-
see,' 1876.) wasser Pulmonatan" ('Jen. Zeit-
r, ** Ohaervationt on ichrifV 1875).
2 K
498 THS AKATOHY OF INVEBTEBRATBD AKIMAIJB.
blastomeres (macromeres and micromeree). The latter form a layer which graduallj extends over the macromeret and encloses them. Ob^onslj this comes to the aame result as invagination ; and the included macromeres and their progeny either become converted into the archenteron with its appendages, and more or less of the mesoblast ; or a portion of them may serve as food yelk.
In the Pteropoda and Heteropoda* and in Ncu9a, NaUoa, and Fu9U8,f the blastopore, or aperture circumscribed by the edges of the micromeral layer as it grows round the macromeres, closes, but corresponds in position to the in- vagination of the ectoderm which gives rise to the future mouth ; and the anus is a new formation.
In such land PtUmonata as Idmax, the process of yelk division gives rise to macromeres and micromeres, and the latter enclose the former. What becomes of the blastopore is not clear, though I am inclined to think that it conre* spends in position with the mouth. The latter is seen very early as a funnel-shaped invag^ination of the epiblast bounded by lateral lips. Behind it, the foot grows out and rapidly attains a considerable size. Its posterior extremity becomes flattened from above downwards, and converted into an orbicular appendage, the opposite walls of which are connected by reticulated muscle-cells. This appendage undergoes rhythmical movements of dilatation and contrac- tions. The macromeres form a large mass enclosed within a spheroidal dilatation of the greater part of the hsemal wall of the body, which deserves the name of yelk-sac even better than the structure so named in the Cephalopoda, inasmuch as it more nearly corresponds, morphologicaUy, with the vitelline sac of vertebrated animals. Between this sac and the foot, the small remainder of the haemal wall becomes converted into the mantle.
The walls of the vitelline sac undergo contractionB which
* Fol, ** Etudes but le develop- f Bobretsky, <' Studien liber
pementdes MollosqueB." (*Arch. dieembryonaie Entwiokelungdar
de Zookwie ezperimentale/ 1875, Gastropoden." (*Ai«hiv iMikr.
1876.) Anat.,' 1876.)
THB DBYSLOPMBNT OF THX ODONTOPROBA. 499
sometimes, but not always, alternate with those of the pedal appendage. On each side of it appears the " primitive kid- ney," consisting of a cnrved elongated series of cells within which concretions are developed, and terminating in a dnct which opens on the posterior face of the vitelline sac, «close to the mantle. The exact mode of origin of the alimentary canal has not been made out ; but, in any case, oidy a very small portion of the endodermal cells can take part in its formation, and the archenteron is, at first, a sac which nearly fills the small projection formed by the rudimentary mantle. The oral involution of the ectoderm gives rise to the odon- tophore, and extends across the base of the foot to open, eventually, into the archenteron.
The fold of the mantle which overhangs the ree^nratory aperture, makes its appearance very early ; and immediately behind it, the intestine is visible as a short tube, which extends from the archenteron to the surface, but does sot, at first, open there.
As development proceeds, a movement of the macromerie part of the vitellus takes place in exactly the opposite direc- tion to that of the food yelk of the Cephalopoda ; that is to say, from the vitelline sac into the constantly enlarging foot. The alimentary canal accompanies it, the anus alone remaining in its primitive position. The constantly lengthening alimentary canal becomes disposed in folds ; between these, the macromeric part of the vitellus, which gradually forsakes the diminishing \itelline sac, disposes itself around the coils of the intestine. Eventually, for the most part, it becomes converted into the liver.
The rudimentary shell first makes its appearance in the form of a few subcrystalline calcareous plates, on the inner side of the ectoderm.*
The development of Helix is similar to that of Liinax ; but the intestine passes into the large visceral sac instead of into the cavity of the mesosoma. The shell is stated by Gegenbaur to be at first internal, as in Limax. In neither
* Compare Gegenbaur, ** Zar Entirickelangtgeschichte der Land- Gasteropoden." (' ZeiUchrift fijr Wisi. Zoologie,* 1^52.)
2 K 2
l
1
which the rudiments of the tentacula while, at the sides, a longitudinal ri< of the mantle, and marks off the mor from the flat foot. No shell is forme In Lymn<BU8,f as has been alrea( undergoes complete division, and t morula undergoes invagination to p Only the middle part of the archeni mentarj canal, however. The latera on the form of rounded sacs, maj no Brachiopods, give rise to the perii; this has not been proved. The mo\ formation of an opening in the cc ectoderm, at a point near the ant Upon each side of the month a trai the ectoderm is developed, and rej Telom in other mollnscan embryos, opposite side of the embryo to that 1 1 ' placed, a raised patch of the ectoden
The foot commences as a papilla month. An involution of the centr gives rise to a shell-gland, but the j kidependently of this, as a cutiou
THK POLYFI.^COPHOBA. 501
the Telnm (onlesB it be represented by the ant«riaT contnc* tile sac) nor the external embryonic shell.
The development of the Cephalopoda is vtxy nnlike that of other HoUosks, and will be dealt with under the head of that gronp.
Fig. 121.
Fig. 131.— I. CUbM WufHaiMtkU. (AfMr U
11. ChitiM diMeeted to show o, the mauth : p, the nerrtnuniiB; so. me
■oru; c, tha Tcntrjela; c', %a uirlde; br, tlialeft bianohlB; m^ the
ovIducU. (AttsT Cuvier.) miV. V. StigMofddvelopnientof C*i««»iMr«u. (AfUrLoT^n.)
The lowest forma of the Odtmtophora are the PolypUieo- phara, or ChUomda, and the Beaphopoda, or DrntaUdce. The bilateral STnunetr; of the body is completely or almost completely, nndistorbed, whjle the hsmal wall is flat, oe nearly eo, and there is n '
502 THE ANATOMY OF INTBSTBBSATSD AKHCALS.
The POLYPLACOPHOBA.— The Chitons (Fig, 121, I.) aie elongated, sluglike ajiimals, having the month at one end of the body, and the anus at the opposite extremity. A ronnded lobesurmounts the mouth, but it bears no eyes nor tentacola* and there is no definite head. The edges of the mantle are thickened, but little prominent, so that the pallial cavity is not much more than an elongated groove, beneath, and internal to, the thickened edge, which is sometimes beset with setffi. In the region in which these sets occur, the surface of the mantle is covered by a thick cnticnla. The setsB, which may be merely chitinons or completely calcified, or partly in the one and partly in the other condition, are developed in sacs lined by the cells of the ectoderm.* In the pallial groove lie the short lamellar processes which re- present the branchifB. The shell ia unlike that of any other Mollusk. It consists of eight, transversely elongated, symme- trical pieces, arranged one behind the other, overlapping in such a manner that the posterior edge of the one covers the anterior edge of the next, and articulated together. Some- times the valves are partially or completely enclosed in the mantle. The heart, composed of a single median ventricle and two lateral auricles, is placed in the middle line, above the rectum, at the posterior end of the body. The aorta is continued forwards from its anterior end, while the anri- cles receive the blood from the branchise. In Chiion pieeut, according to Schiff,t each auricle communicates by two openings with the ventricle, and the two auricles are united behind. The reproductive organ is median and symme- trical, and its two ducts open on each side of, and not far from, the anus.
The embryo leaves the egg as an oval body, surrounded near its anterior end by a circular ciliated band, behind which an eye-spot appears on each side (Fig. 121, III.). The segments of the shell appear while the young Chiion is still locomotive, and the disk in front of the ciliated band
* Relncke, << Beitiage lar Bil- Mnschaftliehe Zooloeie.') dnngigssehichte der ftUyaVv^Vu, t ^ZeitBehriftfurWiBsenchaft-
THB 80APHOPODA. 503
becomes oonverted into the lobe above the mouth (Fig. 121, lY. v.). The Chitons have existed from the Silurian epoch to the present day, apparently with very little modification.
The ScAPHOPODA.* — ^In Dentaliwn, the shell is elongated, conical and curved, like an elephant's tusk, with the apex broken off, and it is open at both ends. The animal has a large mantle corresponding in form with the shell, and also open at both ends, the margins of the anterior, larger, aperture being much thickened. The mouth, placed at the extremity of a sort of cup, the margin of which is fringed with papillse, is situated far behind the anterior opening of the mantle. Behind the oral cup, where the body joins the mantle, is a transverse muscular ridge, from which pro- ceed a great number of long tentacles. These protrude through the anterior opening of the mantle, and play. the part of prehensile organs. Behind and below the oral cup the very long subcyHndrical foot proceeds. Near its ex- tremity are two lateral fleshy lobes which perhaps corre- spond with the epipodia of other Mollusks. The oral cup leads into a buccal chamber containing the odontophore, -whence the oesophagus passes to the stomach. The liver consists of two symmetrically-branched divisions ; and the intestine, after becoming coiled upon itself, ends in a pro- minent anal papilla, in the median line, behind the root of the foot. There is no heart, but the blood fills spacious sinuses. There are no special respiratory organs distinct from the wall of the pallial cavity. The two renal organs open one on each side of the anus. The renal blood sinos communicates directly with the pallial cavity by two apertures, situated close to those of the renal organs. In the nervous system, the conmiissures of the parieto- splanchnic ganglia pass directly to the cerebral ganglia, as in the Lamellibranchs. The sexes are distinct and the
* A very complete and acearate toire de rorganisation, da dtfve«
account of the organisation of loppement, des mceon et des rap-
2/entaUuM is given in the mono- ports loologiqaes des I>entalea^"
graph of Lacaze-Duthiors, ** Hitr IS5^.
504 THB ANATOMY OF INTBBTEBIIATED AKIMAIiS.
genital gland is single and symmetrical, thongh ita duct opens into the right renal organ. The embijo is at fizst surrounded by a number of ciliated rings, its anterior end presenting a toft of long cilia. By degrees, the cilia become restricted to the edges of a disk, into whicb the anterior end of the embryo expands, and which represents the prse-oral ciliated velnm of the Lamellibrancha. The mantle now appears on the dorsal aspect of the bodj, behind this disk. Its ventral edges are free, and it secretes a shelly plate of corresponding form. But, as development advances, the edges of both mantle and sheU unite in the median ventral line, leaving the anterior and the posterior ends open.
The Scaphopoda are an ancient group, remains of them occurring as far back as the Devonian Epoch.
The higher Odontophora (or the Chsteropada, Pteropoda^ and Cephalopoda of Cuvier) fall into two divisions, according to the structure and arrangement of the parts of the foot. In the one division (the Gasteropoda and Pteropoda) it maj be a simple disk, or it may be divided into three portions — an anterior (the propodivm), a middle (the megopodiwm), and a posterior (the metapodiwm) ; and it may be still further complicated by the development from its sides of muscular expansions — the epipodia. But whatever the shape of the foot in these Mollusks, its margins are not produced into prehensile processes, and its antero-lateral portions do not extend beyond the sides of the head, and imite in front of the mouth.
In the other division (the Cephalopoda), the margins of the foot are produced into prehensile processes or arms, and the antero-lateral regions of the foot extend over, and unite in front of, the mouth, in such a manner that the latter is placed in the centre of the disooidal foot.*
* See for a valuable discusdon fiMtorily, controverted, Gren-
ofthehomologiesoftheariDiaiid aoher, **Zar Entwiokelnns^e-
the Amnel of the C^)halopoda^ in schichte der Cephalopoden."
nUoh tiM view h«re taken U C ZeitMhrift fOr WIm. Zookwie.'
THE GASTEBOPODA AH]> PTSBOPODA. 505
In the former division, tliat is in all Pterapoda, in all those Cfaaieropoda which breathe the air dissolved in water Branehiogcuteropoda), and in some of those which breathe ur directlj (Ftdmogcuieropoda) the embryo is, as in the Scaphopoda and Polyplaeophoraf a veliger ; or, at any rate, it has ciliated bands which subserve locomotion. Bnt, in the Cephalopoda^ no such velum is formed, and the animal acquires the general characters of the adult before leaving the egg.
A shell-gland is often, if not always, present in the embryo of the higher Odontophora ; and, in all Pteropods and Bran- chiogasteropods, the mantle secretes a cuticular shell, which, however, may exist only during the larval condition.
If the arrangement of the alimentary canal in a Gephalopod, or a Pteropod, be compared with that which obtains in such a Branchio-gasteropod as Atlanta, it will be observed that, in the former, the oesophagus enters the outgrowth of the haemal region of the body which con- stitutes the visceral sac, to reach the stomach ; and that the intestine passes, at an acute angle with the anterior portion of the alimentary canal, along the posterior face of the visceral sac, to end in the pallial chamber, which is situated on the posterior face of the body. The pedal ganglia con- sequently lie between lines traversing the anterior and the posterior divisions of the alimentary canal respectively; and hence the alimentary canal has a neural flexurey or is bent towards the neural face of the body.
In Atlanta, on the other hand, the intestine, when it leaves the stomach, x>a8se8 along the anterior face of the visceral sac, to reach the pallial cavity, which is situated on the anterior face of the body. Hence lines traversing the two divisions of the alimentary canal would enclose, not the pedal, but the cerebral ganglia. In other words, the in- testine is bent in the opposite direction to that which it takes in the Gephalopod, or has a hcBmal flexure*
The hsemal flexure of the intestine is very characteristic
* Huxiev '< On the Morphology of the Cephalous MoUosoa." <^I^\V.
506 THE AKATOHY OF nTTBBTBBRATBD AKHCAXS.
of the Branehiogcaieropoda, and is completed at an eazij stage of their deyelopment.
In such a slightly modified Odontophoran as ChUon, the heart presents its normal position in the posterior r^on of the hsemal face of the body, and has its aortic end turned forwards. Although the branchiaa are situated at the sides of the body, the blood which passes through them must take a backward course to reach the heart ; and thus the branchise may be said to be virtually behind the heart ; and the animal is truly opisthohranchiaie. It appears to be otherwise with such a Gasteropod as Buccinwn^ in which the gills lie actually in front of the heart ; and the animal is therefore said to be prosobranchiaie. It must be recollected, however, that strictly speaking, no Odonto- phoran is other than opisthobranchiate. The anns repre* sents the morphological hinder end of the body, and the auricle of the heart, into which the current of blood from the branchisB passes^ is never, morphologically, posterior to the branchiae.
This is perfectly obvious in the Cephalopoda. In the position which the animal frequently assumes and in which it is ordinarily represented, the gills are in front of the heart. But if the Mollusk is placed in its morphologioally correct position with the oral face of the arms downwards, it will at once be seen that what is commonly called the ventral face of the animal is the posterior half of its hsemal face, and that the heart lies, morphologically, anterior to the branchiffi.
In such Branchiogasteropods as are proeobranchiate, the giUs come to lie in front of the heart in consequence of their having followed the twisted intestine forwards and to the hsBmal side of the body.
The Ptiropoda.* — In this group of small pelagic n-niTwalfl there is no distinct head, the eyes and the ordinary tentacles
* See Bang and Soulevet, * His- < Untenuchungen uber die Ptero- loire natiirelle det Mollusques poden und Ueteropoden/ 1855. Pitfrqpodes;* and QegenWox,
THB PTEBOPOBA.
507
remaining mdimentary. Anditory sacs are attaolied to the pedal ganglia. Sometimes {Pnewnodermon) two eversible spinoee tentacular organs are developed at the sides of the month, and, in addition, two acetabnliferons tentacles take their origin on the inner side of a cnp-like hood, which surrounds the anterior end of the body.* Cymibulia is stated to possess no radula. The epipodia are large muscular ex- X>ansion8, bj the flapping of which the Pteropods swim; but the rest of the foot is always small, and often rudi- mentary, in correspondence with the small size of the neural face of the body.
The haemal face, on the contrary, is always produced, as in the Cephalopoda, into a relatively large visceral sac; and in some (the Thecosomata) this visceral sac is co-exten- sive with the mantle, which is protected by a shelL In others {GymnosoTruUa) the mantle early disappears, and there is no shelL In Cymbulia, the delicate transparent chitinous shell is internal and is invested by an epithelial layer derived from the mantle. In Spinalis, the foot bears an operculum. Chromatophores similar to those of the Ce- phalopoda occur in THedemannia.
In the Thecosomata, the free lobe of the mantle, which encloses a spacious x>allial cavity, usually lies on the posterior aspect of the visceral sac, as in the Cephalopoda, and the rectum terminates in it, on one side of the middle line. In these there is a simple neural flexure of the alimentary canal, as in the Cephalopods, although the turning of the rectum to one side destroys the symmetry of the body. In Limacina and Spirialis, the intestine appears to be bent round to the anterior face of the visceral sac, the mantle- cavity accompanying it, so that the opening of the mantle is placed on the anterior, instead of on the posterior, face of the visceral sac. There are no distinct gills in the Thecosomata, but the lining of the mantle-cavity subserves
* See for the somewhat similar arrangements in C/ibN«, Cschricht, * Anatomische Untersuchungen iiber CUtmt bonaUs,* 1858, and
Maedonaid, "On the Zoological characters of the living Clvf cau- data: (* Trans. Royal Society q£
I
508 THE ANATOMY OF IITVISBTSBBATBD AKOCAIiS.
the fnnction of reepiration, and is sometimeB prodnced into folds, wliich doabtless aid in the performance of that fnnc- tion. Processes of the body, to which the office of gills is ascribed, are fonnd in some Chfmnosomaia {PneMmodemum Spongobranchia) .
The heart consists of a single anricle and a single Ten- tricle. The anricle lies close to the pallial cayity, and re- ceiyes the aerated blood from its walls. The yentride is sometimes directed forwards (as in all Gymnoswnaia), and sometimes backwards, so that nearly related foims axe sometimes opisthobranchiate, sometimes prosobranchiate. The branches of the aortic trunk soon terminate in lacunae, by which the blood is conyeyed back to the waHs of the mantle-cayity. The renal organ is a contractile sac with delicate walls, which opens on one side into the pallial chamber, and on the other into the pericardial sinus.
The Theeosomaia haye the principal ganglia concentrated aronnd the gollet — the cerebiul ganglia being lateral and united by a long commissure.
In the Oymnosomata the ganglia are more scattered, bat the arrangement of their neryous system needs re-exami- nation.
All the Fteropoda are provided with an ovoUHU. This is a racemose gland, in the ultimate cseca of which both oya and spermatozoa are developed. The spermatozoa make their appearance at the closed end of the caecum and accumu- late in its cayity ; the oya are deyeloped from the epithelial tissue of the c»cum, somewhat lower down; neyertheless fecundation does not take place in the oyotestis, probably in consequence of the oya and spermatozoa attaining maturity at different times. The oyotestis has a single excretory duct, the termination of which may be proyided with a receptaculum seminis and connected with a penis.
The young of the Pteropoda leave the egg proyided with a yelum, with a rudimentary shell, and probably with an operculum. In most of the Thecosomaia, the shell is re- tained and forms the commencement of that of the adult, while the yda diBapi^eBx «3A V2i[i<^ ^^\v^A^ «ac% deyeloped.
THB BIULHCHIOGASTSBOPODA. 509
In CynibvlM, the primary external shell is shed and the chitinouB internal shell is a secondary development. In the Qymno9omaia, the primary shell is also cast off, but is not replaced, and three girdles of cilia are developed on the surface of the body.*
The Silurian genera TentacuUtes, Theca, Pteroiheea, Conit- laria, Ecculiamphalua are referred to the Ptercpoda, but they differ much from all existing foims. Unqnestionable Pteropoda are not known earlier than the tertiary formations.
The Bbancriogastebopoda. — In all the members of this gronp, the development of which has hitherto been studied, the intestine becomes twisted round on to the anterior face of the body, in such a manner that the alimentary canal has a completely luemal flexure, even in the veligerous embryo. Hence, in the adult, the intestine springs from the hsemal or dorsal, and not from the ventral or neural, aspect of the stomach; and the pallial cavity, when it exists, is placed upon the anterior haemal face of the body.
In the embryo, the shell always makes its appearance as a conical, symmetrical, median cap. This embryonic shell usually persists at the apex of that of the adult, the form of which is modelled upon that of the visceral sac, and hence, like the latter, is usually spiral. The embryo is also very generally, if not universally, provided willi an operculum.
The shell and operculum of the embryo disappear in the naked Branchiogasteropods ; but the primitive external shell is sometimes replaced by an internal shell lodged in a cavity of the mantle (e.g. Aplyaia). Usually, the Branchio- gasteropods possess a distinct head provided with a pair of tentacles and with two eyes, which may either be sessile or mounted upon peduncles of their own.
* Gegenbaur, /. c. j Krohn, Tleteropoden," 1860; and Fol, " Beitrage lur Entwickelangs- ** Etudes." O Archives de ZooL geschichte der Pteropoden und Experimentsle^' 1%7^ «sisi V^^>^
510 THE ANATOMY OF INYEBTEBBATEB AKOCAL8.
The mouth may be armed with chitmouB jaw-plateB, in addition to the radula. The heart is genen^j composed of a ventricle and a single auricle, but sometimeB there are two auricles.
The Branchiogasteropoda fall into two distinct aeries, of which the one is hermaphrodite (the genital gland being an ovotestis) and invariably opisthobranchiate ; while the other is unisexual and usually prosobranchiate. In each series, there are some forms which are provided with a large mantle; and others, in which the mantle is altogether abortive {Nudibranchiata, Firola), These chlamydate and achlanvydate Branchiogasteropods correspond with the Theco8<miata and fiymnosomata among the Pteropods.
The chlamydate Branchiogasteropods are usually pro- vided with branchisB, which either take the form of numeroTis lamellse, or of two plume-like organs, sometimes reduced to one functional gill and a rudiment of the second. In the achlamydate forms, true gills are usually absent, though they may be replaced functionally by processes of the haemal body- wall.
Among the Qpidhobranchiata, Phyllidia is nearly symme- trical, the anus being situated at the posterior end of the body, and there is a large mantle, devoid of a shell. There is no pallial cavity, and the branchiae are numerous lamellsCt placed on each side of the body, between the free edge of the mantle and the foot. In Aplysia, the mantle is rela- tively small, and possesses an internal shell ; the branchiae, the anus, and the reproductive apertures are placed on the right side of the body. In this genus, and in OoHero^ pteron, there are very large epipodial lobes, by the aid of which some species propel themselves like Pteropods.
The Nudibranchiata have no mantle, and the anus is usually situated on the right side of the body ; sometimes, however, as in Doris, it is terminal. In the pelagic Phylli- rhoe, the foot aborts, as well as the mantle, and the body has the form of an elongated sac.
The gastric portion ol \2iEi<^ v^imentBry canal becomes
THE PBOSOBSANCHIA.TA. 511
complicated bj division into seyeral portions, some of which are provided with chitinous or calcareous plates, or teeth, in Aplysia, BuUa, and other genera. In manj Nndibranchs, as Eolis, the liver is represented by a much-branched tubular organ, the csecal ultimate ramifications of which end in the elongated dorsal papillsB, The apices of these papillse contain thread cells.
In the series of the Proaohranchiata, the great minority are not only chlamydate, but there is a spacious branchial chamber, and the pallial wall of the body is produced into a conical visceral sac, which contains the stomach, liver, and genital organs. It is usually asymmetrically <y>iled, and is protected by the shell No Opisthobranch possesses a large visceral sac of this kind. On the other hand, no Prosobranch is, like Thyllidia, symmetrical, with the anus at the posterior end of the body. PaiteUa and Fissurella are nearly symmetrical, but the anus is anterior.
The Tro8obr(mchiata have, at most, rudiments of q^podia, but the rest of the foot often acquires a much greater development than in the Opisthohranchiata, and a chitinous or shelly plate — the operculum — is frequently developed from the dorsal or hsBmal aspect of the metapodium. The differentiation of the foot attains its highest degree in the so-called Heteropoda, in which the propodium, mesopodium, and metapodium differ widely in form, — ^the propodium being broad and fin-like, and constituting the chief organ of locomotion in these free-swimming oceanic animals.
In the Limpets (Patellidai), the visceral sac forms merely a conical projection of the hflemal surface, and the numerous lamellar, or fiilamentous, respiratory organs, are lodged between the free edges of the mantle and the sides of the body. In the other chlamydate Frosobrcmehiata, except the Oyclostomaia, there are two plumose gills lodged in a pallial chamber situated on the anterior face of the visceral mass, which is usually large and spirally coUed. Qofm^^ixs^^^^'^s^
512 THE ANATOMY OF mYSBTEBKATBD AKIMAIiS.
in the diyision of the Aspidohranehia, the two branchiae are equal or nearly equal in size. Sometimes one is so much smaller than tiie other as to be nearly abortiye (Otono- branekia). Am^Uaria has a pnlmonarj cayity as well as gills. On the other hand, the CycloBtomakL hare no branchisB, but breathe air by means of the parietes of the pallial chamber, whence they are ordinarily reckoned among the Puhnonata, which they resemble in their terrestrial habits. In many Prosobranchiata, the wall of the branchial chamber is produced into a muscular spoat- like prolongation, termed the siphon, which serres to direct the branchial current. The presence of this siphon is usually accompanied by a notch or grooved process of the shell, and by carnivorous habits.
In the Heteropoda, there is a gradual reduction of the mantle, from Atlanta, in which the mantle and shell have the ordinary proportions, and the departure from the ordinazy Gasteropod type is but little greater than that observed in Strombus and Pteroceras, through Carinaria, in which the mantle is much reduced, and the shell is a mere conical cap, to Firola, in which the mantle and shell are wanting in the adult, and which, therefore, correspond with the achlamydate Pteropoda and Opigthobranchiata,
In many genera of the Cienobranchia, and especially among the carnivorous forms, the mouth is situated at the end of a long proboscis, which contains the odontophore, and a great part of the long oesophagus. This proboecis is protruded and retracted by special muscles.*
The eggs are often laid in capsules secreted by the walls of the oviduct. In NeriUna, Purpura, and Buecinwni, each capsule contains a considerable number of ova, but of these only a few (one in Neritina) become embryos, and devour the rest.f
* See the description of the na littoralis Norvegis,' 11. 1856),
proboscis of the Whelk in Cuvier's and Carpenter " On the develop*
* Me'moires sur lea Mollasques.' ment or the embryo of PWrom
t Koren and Daniellssen, lapillui " (Trans. Mior. Roaety,
** Kecherches sur le dcveloppe- 1854, and * Annals of Nat. Bitt^'
ment dc9 Pectimbruichtft** (^^saxir \^^*l^. C\».\«xMe^ **Anatoiiiie vnd
THE PULMONATA. 513
The parasitic Habit whicH is so rare among tlie Mollusca occurs in the genus Stylifer, which infests Star-fishes and Sea-urchins, sometimes imbedding itself in the perisoma ; and, under a very remarkable and not yet thoroughly under- stood form, in the singular parasite of another Echinoderm, Synapta digiiaia^ termed by its discoverer, Miiller, EntO" concha mirabilis*
In some few of the SynaptcB (not more than one, or perhaps two, in a hundred), elongated tubular moUushigerous sacB are found attached by one extremity to one of the intestinal vessels; while the opposite end either hangs freely into the perivisceral cavity, or may be entangled among the bases of the tentacles, at the cephalic extremity of the body of the Synapta. The sac is closed, but, at its attached end, a long invagination extends into its interior. The cavity of the sac beyond the closed extremity of the invagination contains an ovaiy ; and, beyond this, a certain number of free seminal capsules. The ova are detached from the ovary, and undergo their development in envelopes, each containing many ova, which gradually fill the cavity of the molluskigerous sac. From these ova, embryos, pro- vided with a velum, shell, and operculum proceed. A large pallial cavity is soon apparent ; but, in the most advanced stages of development observed, it contained no bran- chifiB.
What becomes of these larvsB is unknown, nor is it even certain to what group of the Odomtophoray Enioconcha belongs.
The PuLMONATA. — These are odontophorous MoUusks which breathe air directly, by means of a respiratory surface furnished by the wall of the pallial cavity.
In some, such as the Peroniadce (Pig. 123) and Veroni- cellidijB, the body of the slug-like animal is very nearly
Fntwickelangtgeschichte der ken in Holothurien,' 1852. Baur,
yerUinafluviatUu:* (*Archiv fur ** Ueber Symgirta dUgUata^
Anatomie,' 1857.) (•Nova Acta,* xxxl. \8ftV^ * ^IHeEneagungvonScbneo-
514 THB ANATOMY OV IKYEBTEBBATBD ANIMALS.
symmetrical; the anus and tlie lung-sac being eitiiated close together at the posterior extremity of the body. The mantle is large, and extends oyer the whole liyBtn^l or dorsal surface. In all the other PvInunuUat the pnbnonary and the anal apertures lie on the right side of the body, and the mantle is provided with at least the rudiments of a shell. The pallial region is sometimes very small in pro- portion to the rest of the body, and then forms a flattened disk, as in the common Slug ; while, in some lAmaddcB and Testacellidce, and in the JaneUidcB, the mantle is so much re- duced that they are almost achlamydate. In the Snails, the mantle is large and is produced into an asymmetrically coiled visceral sac, in which the stomach, liver, and genital gland lie. The mantle-cavity lies on the fore part of the sac, and the anus opens on its margin. Thus, in all the ordinary Pulmonata, the termination of the intestine is twisted from its normal position at the hinder end, forwards to the right dorsal, or hsemal, aspect of the body.
When the pulmonary sac is posterior, and the pallial re- gion small, the ventricle of the heart is anterior, and the auricle posterior, and the animal may be said to be opigtko- pulmoncUe, On the other hand, when the pallial region is large, and gives rise to a visceral sac, with the concomitant forward position of the pulmonary chamber, the auricle is inclined more or less forwards and to the right side, and the apex of the ventricle backwards and to the left side. The animal is thus more or less prosoptUmonate,
The mouth is commonly provided with a homy upper jaw, as well as with a well-developed odontophore. Large salivary glands are usually present.
The heart consists of a single auricle and a single ven- tricle. The aortic trunk, which proceeds from the apex of the latter, divides into many branches, but the venous channels are altogether lacunar. A renal organ lies close to the pulmonary sac in the course of the current of the returning blood.
There are usually two simple eyes, often lodged in the jsnmmitfi o£ TetRy(^tAl<& Y^nW^o^au
THB FOUIONA.TA, 515
The PulmotMta are hermaplirodite. The generative gland Fig. IIS.
Fig. 12i.— Diagram nblbitlng the dispoiltion of theintMtine, Derront »3»tem, etc., In a cominoa Sii^l (Btliey—a, mouth; b. tnoth; c, iHionttn)hor« ; d. gullet; c, iti dilatation Into a aort of crop; /, ■tomaoh; y, coiled termination of the viicaral mala: the latter it alao cloM to Iht commeocement of Ihe Intettine, whlob will be teen to lie on the neural aide of Ihe teaopbasui; A, t«cCudi ; t, anna; f, lanal aao; ( heart; ii, lung, or miidifieil pallial chamber; n, Iti Wteraal aperture; a, thick edge of the mantle united with the ^dea of the biid; ; t>, foot; r.i, cerebral, pedal, and parieto-eplanchnia gaoglia aggregated round the gullet.
is an oTotMtu, and is compooed ot VmatnAtnSL tc&K£a^^x<sI^
516 THE ANATOMY OF INYEBTEBRATED ANIMAIiS.
the cellular contents of wliicli both ova and spermatoBoa are developed (Fig. 123, III.).
A narrow common duct leads from the ovotestiB, and, soon dilating, receives the viscid secretion of a large albumen gland. The much wider portion of the common duct beyond the attachment of this gland is incompletelj divided by longitudinal infoldings into a sacculated, wider, and a straight, narrower, division. The former conveys the ova, and the latter the spermatozoa. At the end of this part of the apparatus, the wider portion, which represents the oviduct, passes into the vagina, which opens at the female genital aperture, while the narrower portion of t^e common duct is continued into a separate, narrow, vas de- ferens, the end of which opens into a long invagination of the integument — the penis. In Peronia, the vas deferens and the oviduct open together by the genital aperture, and, as in some Bi*anchiogasteropods, a groove, along which the seminal fluid is conducted, leads to the outer opening of the eversible penis (Fig. 123, 1. II.).
In connexion with the female genital aperture, there is always a spermatheca, or sac (which is sessile in the Slugs, but in the Snails is placed at the extremity of a long duct) for the reception of the semen of the other individual when copulation takes place.
The Helicidce alone possess, in addition, the so-called sac of the dart, a short muscular bag, in which pointed chiti- nous or calcified bodies — the spictUa amoris — are formed; and certain glandular ca3ca, generally arranged in two digitate bundles, termed mucovs glands, which give rise to a milky secretion. Sometimes, progiatic glands are developed on the vas defei*ens, which may be dilated in part of its course into a vesicula seminalis.
The ova are impregnated high up in the oviduct, and are invested by a relatively very large mass of albumen and enclosed within a thick, sometimes calcified chorion. The mass enclosed by the latter may be a tenth of an inch or more in diameter, while the proper ovum may haTe not m<nre than a twelfth of that size.
THE PULMONATA.
617
There is no tmstwortlij evidence of the existence of the opisthobranchiate Grasteropods before the epoch of the Trias, but it is to be remembered that the great majority of these animals have no shells. Of the rest of the preceding groups of Odontophora, representatiyes are known as far back as the middle of the Palseozoic epoch, while Pterapoda, HeterO'
Fig. 128.
Fig. 12). — I. Peronia verruculata, — a, anus;/)/, pulmonary aperture; gy genital aperture ; /«. seminal groove ; /i, opening for toe penis.
II. Generative or»:ans of tbe same animal, the ovotestis being omitted. gal^ gland which furnishes a glaixy secretion ; o</, oviduct ; vd^ vas deferens; i, intestine; a, anus; r«, receptaculum seminis; /», aper- ture of the penis ; p\ penis ; c«, seminal duct ; ap. glandular appen- dage; m, retractor muscle of the penis. (After Keferstein.)
III. Blind end of a follicle of the ovotestis of Uetix pomatia. At the apex the spermatozoa are seen in different stages of development, the fully formed spermatozoa floaUng in bundles in the cavity of the follicle. Lower down, ova are developing in the walls of the follicle. (Alter Keferstein and £hlers.)
poda^ and Prosobranchiata occur in the Silurian formations. Among the Prosobranchiata, the PateUidcR and the Aspido- branchia are the characteristic forms of the older f oi-mations,
8
THE CEPHALOPODA. 519
however, is not flat, as in tlie moUosks whicli Have just been mentioned, but is elongated perpendicnlarlj to the neura face, so as to form a sort of sac, invested by the mantle. On the posterior, or anal, face of the sac, the mantle enclose a large pallial cavity, in which the branchisB are pro- tected. On the anterior aspect of the sac, on the contrary the mantle may have no free edge, or, at most, forms a comparatively small flap.*
The integument is provided with ehromcdophores, which are sacs with elastic walls, full of pigment, and provided with radiating muscles, by which they may be ditbwn out to a size many times greater than that which they possess in their contracted state. In their dilated condition, the colour proper to the contained pigment becomes plainly visible, while in their contracted state they appear as mere dark specks. It is to the successive expansion and con- traction of these chromatophores that the Cephalopoda owe the peculiar play of " shot " colours, which pass like blushes over their surface in the living state. These blushes of colour are especially well displayed by young Cephalopods just freed from the egg.
But that which particularly distinguishes the Cephalo- pods, is the form and disposition of the foot. The margins of this organ are, in fact, produced into eight or more processes termed arms, or brachia ; and its antero-lateral portions have grown over and united in front of the mouth, which thus comes, apparently, to be placed in the centre of the pedal disk. Moreover, two muscular lobes which correspond with the epipodia of the Pteropods and Branchiogasteropods, developed from the sides of the foot, unite posteriorly, and, folding over, give rise to a more or less completely tubular organ — the funnel, or infundibuluvi. The open end of the funnel projects between the posterior face of the body and the pallial wall of the branchial cavity, and serves to conduct the water, when it is driven
* Cephalopoda are usually de- ber is placed ventral— a method
scribed as if the oral end of the which seriously interferes with,
body were the upper end, and the the QomYi^«iv%\Q!tk oS. x^€vt x^^*-
lace on which tie pallial cham- Uoitt wV\^ oV2k^x VU^>&9ia&«
520 THE AKATOHT Or IHV2BTEBSATKD ASIXAI^.
out of the latter by the contraction of tbe ansntte in Fig. 1S5.
Pig. iaS.-I>l»K™inni»l!c >eotlon of a ftmale Srpja.—a, BacMil nun (urroundMl by the lips, md showing ihe homj' ja»< «tid ton(iie ; *, asophagua; r; Balivary gluid; i', atoinmch; r, pyloric CKCum; f, thelnteiliae; A,tlie>nui; i,themk-bag; *.ihe fJiceof thetyatooio hwul; /, Ihe liver; n, Ihe hepalio duct of (he letl »id«; o,theov«ry; p, the oviduct ; o, ont of the »perlure« by which the w»ter-cl»nib«i» ■re placed In cummunicatloD with the exterior ; r, one of tlic biu- chiK; (, the principal gingUa BggreKsted round the ce>opb*giu;/, the funnel ; n, the mantle ; lA, the internal (hell, or outlle-bone. I, a, 3, 4, 5, the produced and tnodifled nmrgina of (b> foot, oonitital- ing UMfo-««Vled umiof the Srpia. ordinary expuaUon-, Miiit^isttftiftMiimji.wi.-'oa.'floa'Awaai
THB CEPHALOPODA. 521
forcibly driven out in this way causes it to dart swiftly backwards.
The aperture of the mouth (Fig. 125, a) is provided with a hard chitinous bea]^, like that of a parrot, the two divisions of which are anterior and posterior. Of these, the anterior is always the shorter, and is overlapped by the other.
Within the cavity of the mouth is an odontophore, with its radula (Fig. 126, II.) ; and the long gullet passes back on the middle line to open into the stomach, which is situated towards the middle, or the end, of the mantle-sac. From the stomach, the intestine, more or less bent upon itself, passes towards the neural aspect of the body, and ends in the median anus. Hence the alimentary canal has a well-marked neural flexure (Fig. 125).
Except in NatiHlus, one or two pairs of salivaiy glands are present (Fig. 126, I. sf). The liver (Fig. 126, I. h) is always large; and there are two hepatic ducts (Fig. 126, I. dh)f beset for a greater or less extent with glandular follicles, generally considered to be pancreatic in function. Very often a large, sometimes spirally wound, csecum is developed from the commencement of the intestine; into this the hepatic ducts open.
The heart (Fig. 127, c) is placed upon the posterior face of the body on the hsemal side of the intestine, and receives the blood by bi*anchio-cardiac vessels, which correspond in number with the gills ; and, as they are contractile, might be regarded as auricles. The gills themselves have no cilia, and are, in some cases, if not always, contractile. The arteries end in an extensively developed capillary system, but the venous channels retain to a greater or less extent the character of sinuses.* The venous blood, on its way back to the heart, is gathered into a large longitudinal sinus — the vena cava — which lies on the posterior face of the body, close to the anterior wall of the branchial chamber, and divides into as many afferent
* Mihie-Edwards, ' Rechercbei et experiences sur la ClrouUtioa Anatomiqaes et Zoologiques. chez lea Moliusques/ 1845. Premie parde.* ' ObeervationB
522 THE ANATOMY OF INYEBTSBSATBD AHUCAXS.
branchial vessels as there are gills. Each of these yessels traverses a chamber which communicates directlj with the mantle-cavitj, and the wall of the vessel which comes into contact with the water in this chamber is sacculated and glandular * (Fig. 127, re). Each chamber, in fact, represents a renal organ. The pericardiom, and the sacs in which the teates and ovaria are lodged, may communicate with the pallial cavity either directly or through these chambers. Thus in Sepia officinalis^ Krohnt observed that the renal chambers communicate not only with the cavities in which the branchial hearts are lodged, but with a chamber which contains the stomach and the spiral pyloric appendages; and that all these cavities are distended when air is blown into one renal chamber. In EledonCf on the contrary, he found, and I have repeated the observation, that one renal chamber can be fully distended without the air passing into the other.
In Nautihis pompilitis, there are, as Valenciennes dis- covered, three pairs of openings, which lead from the branchial sac into chambers contained in the interior of the body. Of these chambers there are five; the anterior and posterior pairs are situated on each side of the rectum, and each has its own opening ; the fifth, a very much larger
* On account of the trans- parency of the tissues in the living Loligo media, this species affords an easy opportunity of observing the rhythmical con- tractions of the branchiff! and their afferent and efferent vessels. For this purpose the mantle should be laid open, and the nidi- mental glands carefully removed. The sacculated afferent veins and the branchial hearts contract about sixty times a minute. The pulsations of these veins and of the branchial hearts are not synchronous. The branchial veins and the lamellae of the branchite also contract rhythmi- cally, but 1 could observe no con- traction in the bTaTicV\\a\ix\«n«%.
The portion of the branchial vein which lies between the base of the gill and the systemic ventricle is very short, and it is hard to say whether it contracts iode- pendently or not Mechanical irritation causes contraction both of the afferent branchial veins and of the branchial hearts.
In the living Eledone eirrhosus, I liave olMerved regular rhythmi- cal contractions of the vena eava itself as well as of its divisions, tiie sacculated afferent branchial Tcins, of the branchial hearts and of the branchiocardiac vessels.
t "Ueber die wasserfnhrende System einiger Cephalopoden." (' Archiv fur Anatomic/ 1839.)
TSB CSFB&LOPODA.
chamber, has two openings, one on each. side. It is co- eiteneive with that part of the jnanUe which lies behind
Fig. \iS.—Seuii officnuilii.—l. The >limflDtU7 ud>I witb the ink big; lab, buccal moH ; gb. Inferior buccal guiglion ; ^, poiterior wliTaty gland*; k, o.'9'>phagU9 ; h, liver) dh, hepitlo duct; p, ftomach; v', pj'lariu cKcum; i, tuieiline; a, auu>; Bi, ink bag; ^ap, tplancbnlo rauglioa od Ihe lUimach. (jVfler Kefentnn.)
U. Longitudinal >nd vertical leodou throu|;h the buccal maaai vtri, poateri'ir beak ; mn, anteiiorbeak;mic, buccal membrane; n',llp; c, gmtMoTj (?) oriian ; rd, radula : i, uc af the radula ; i', nXinzy
fUnd ; gl, luperior buccal ganglia. (After Kefenlein.) A tingle traaiTsne row ot teeth from the radula (after Troaohel).
the uuertion at the shell mnaclee and the homy band which cosneote them. It is separatcid Ixoni tii^a '^aAxe&. ^OBiO^wsni
524 THB &NATOUT or iiivbbtbbka.txd andcaia
by their iimer walls, and tbese walk are traveraed by the afferent branchial Teina. Appendagea of these Teina pro- ject on the one hand into the paired ch&mben, and on the Other into the single chamber. The latt«r appendogoB an elongated papillte, while the former are lamellar- Etu^j concretions, composed m&inl; of phosphate of lune, bat which field no trace of oric acid, are nsaallf found in the
Fig. H7.
Fig. liT.— S^ia o^imtiii.—c, ijatemlo hekti ; on, anlerior anrlB, uo', pmteiior (orto; l,ven> ckva; "_ iffrifnt hraiirhl»1 i iiwiU, n, reul oigsni ; i, apuendBges of these Traseli ; 9, 4, \mxm poeuiior veiulbrlngingblcwdlo t)ieftaerentbniicbiBlveHel9;.\6,T, eSWcnt bnnchlal veiieli, brsnchisl veiiia,uid braochio-euilikcarBturioulM tninkt. (AAer UuntCT.)
The nervous system in the Cephaiopoda, as in other Mollatea, consiat« of cerebral, pedal, and parieto-splanchnic ganglia, aggregated around the gullet, and connected by
■Owen, 'Memoir on tha poioU in llie anttomjr of
Pau-ly Kmtilui." V«n dir Navtiha poKpilut" (Proeced-
Eloeven, " B*itr«(t lut An»- ingi of the LianMn Saelcl7,
Uimle vom A'aiiliJiu pompilita" IS^B). 8m tlto KefenteiB,
i'Arohlr fur 'NMUTMcaleht*,' Bronu'i ■KUsmd n. OrdnuDgai,'
«:>. Uuxl«;, *0«i wnM W.\i.lJ.*»r^VTO>13!»0, 1819.
THE CEPHALOPODA. 525
commisBTiral cords. In addition to these, buccal, yisceral, branchial, and pallial ganglia may be developed on the nerves which sapplj the buccal mass, the alimentary canal, heart, branchise, and mantle.
In the Dihranchiata (Fig. 128), the three principal pairs of ganglia are usually large, and so closely aggr^ated together that the commissures are not readily distinguish- able. The optic nerves are very large ; one or two nerves are given off to the superior or anterior buccal ganglia, which have coalesced into one mass, and are united by com- missures, which encircle the oesophagus, with the coalesced inferior or posterior buccal ganglia. The pedal ganglia lie on the posterior side of the gullet, and supply the large nerves to the arms, and those to the funnel, while the auditory nerves are immediately connected with them. Each parieto- splanchnic ganglion gives off a nerve which runs along the shell-muscles to the anterior wall of the mantle, and there enters a large ganglion, the ganglion gteUcttwn, A large median branch, or branches, from the parieto-splanchnic ganglia, accompanies the vena cava, and is distributed to the branchise and sexual organs. The inferior buccal gan- glion sends a recurrent nerve along the oesophagus, which ends in a ganglion on the stomach.*
The nervous system of NatUilus differs in some important particulars from that of the Dibranchiata. The cerebral ganglia are represented by a thick transverse cord, which lies in front of the oesophagus, and from the outer angles of which the optic and olfactory nerves are given off, while nerves to the buccal mass proceed from its anterior edge. The pedal ganglia lie close to the cerebral ganglia, and are tmited by a slender commissure, which passes behind the gullet. They supply all the brachial processes and the funnel with nerves, and the short auditory nerves are con- nected with them. The parieto-splanchnic ganglia are, like the cerebral ganglia, elongated, and together constitute a thick cord, which, tmited at each end with the cerebral gan-
* 8e« Hancock, *' Anatomy of the nervoas fjTBtem of Ommasin'' phet." (< Ann. ^mt Hiitory/ 1852.)
526 THE AN1.TOHT OrjIlIVEBTEBEATXD AIIIIUL8.
glia, forms a hoop roond the gnllet, distiiict from the pedal nerre-arch, and separated from it bj a proceas of tlie caitUa> ginons skeleton. The largeet nerree giren. off from theH ganglia are those which go to the hi^nchise.
Ejea, olfactory org&na, and auditor; Baca are ^w^a present. The ejea of the Cephalopoda ma; beloved in orbitel canttea at the sides of the head, as ia all the Dibranehiala : or ma; be pednncnlated, aa in Nautiba. In the former usae, the eje is enclosed partly b; the cephalic cartilage, to
Pig. 128.
tig. IBS.— 5910 qffai%alu.~The narrodi miM ohiob mnonnds th* gnlUt ; N, Ihe rerebrmi; N', tba pedal; N", Uir. parielo-splknchnic, ganglion ; w. the aorta : or. the trsophiiga* ; a', buccal nervaa ; P*, narrea to the anui; U.nalllal nerrai; g, luperior; y', iafcrloc boocal ganglion. (AJIer Garner.)*
which sometimes special orbital cartilages are added, and parti; b; a fibrous capsole continaoos with these. The fibrons capsule becomes tranaparent otbt the eye, ajid givea rise to what is variotiBl; interpreted aa the representatiTO of the cornea, or as that of the e;elidH of vertebrated animals. This tmisparent coat is Bometimea entire, or presents onl; a small perforation (Octopui, Septa, LoKgo, and the other Myopsidte of D'Orbign;) ^ sometimea it has a * Ttana. 'Unxxwa^oiMq , ISaS.
THB CEPHALOPODA. 527
wide opening, throngli which the cxTstalline lens may pro- ject (Loligophes, Ommastrepsis and the other Oigopsidce of D'Orbigny); and Bometimes it is altogether absent, and the capsule of the eye becomes an open cnp (Nautikui),
In the DihranchicUa,* a great part of the chamber of the capsule of the eye is occupied by the ganglion, into which the optic nerve enlarges after entering it ; by muscles ; and by a peculiar white glandular substance. Lining the capsule, but not adhering to its inner surface, in front, is the silvery tapetum^ formed of two layers. These pass into one another at the edges of the free prolongation of the tapetum, which forms the iris. Longitudinal muscular fibres are interposed between the two layers of the tapetum. Under the tapetum is a layer of cartilage, which forms the inner eapnUe of the eye, extends as far as the iris externally, and is perforated by the fibres of the optic nerve on its inner side. The free edge of the inner capsule gives attachment to a thick rim of connective tissue, containing muscular fibres. This so-called ciliary body enters the deep groove which sur- rounds the lens ; the latter is, in fact, made up of layers of structureless membrane, which are cuticular productions of the ciliaiy body. In shape, the lens is elongated in the direction of the axis of the eye, so as to be almost a cylinder with convex ends, and thus, with its deep equatorial groove, into which the ciliary body fits, it has a wonderful resem- blance to a Coddington lens. The vitreous humour is a transparent fluid. The retina lines the inner capsule, and may be divided into an outer and an inner stratum, separated by a pigment layer. The inner stratum is formed of pris- matic or cylindrical rods, the outer ends of which abut upon the pigment, while their inner ends, turned towards the cavity of the eye, are covered by a thick hyaloid mem- brane. The outer stratum contains the plexus of the fibres of the optic nerves, and numerous cells (ganglionic), sup- ported by connective tissue. The terminations of the nerves therefore must traverse the pigment layer to reach the rods.
* See Hensen, *' Ueber d>B Ange einiger Cephslopoden." Q Zeit- schrift f&r WissenMilisltUelM Zootogle,' 1865.)
528 THE ANATOMY OF INYEBTEBBATED ANIMALS.
It will be observed that tbe apparent reeemblanceB be- tween tbe cephalopodons and tbe vertebrate eye are merely guperficial, and disappear on detailed comparison.
In Nautilus^ tbe eye bas neitber cornea, lens, nor TitreonB bumonr, bnt is a mere cnp, lined by tbe retina. Tbe aper- ture for tbe admission of ligbt is exceedingly small.
Tbe olfactory organs, tbe true nature of wbicb was discovered by Kolliker,* are sometimes pits, sometimes papilla of tbe integument, situated bebind or above tbe eyes. In tbe Teuthidce and SepiadcB tbey are depreesions above tbe eyes ; in tbe Octopoday tbey are eitber depressions or papilla) {Argonauta and Tremoctopus) in tbe same position, but nearer tbe anterior face of tbe body. In NauHhu, tbey are elongated, tentaculif orm, and situated immediately bebind tbe eyes.
In tbe Dibranchiata, tbe auditory sacs are lodged in cavities of tbe cepbalic cartilage, and contain a single large otolitb, composed of carbonate of lime, and of rounded or irregular, but definite and cbaracteristic form. In Nautihu, Dr. Macdonald discovered tbat tbe auditory sacs areattacbed to tbe pedal ganglia, and are not lodged in tbe cranial car- tilage. Tbey contain numerous otolitbs.
An endoskeleton formed of true cartilage is developed in tbe region of tbe principal ganglia, and sometimes fumisbes tbem witb a complete investment. It gives attacbment to tbe most important muscles. In some Cepbalopods addi- tional cartilages appear in tbe mantle and in tbe fnnneL Tbe muscular fibres of tbe C^halopoda are imstriated.
Tbe sexes are distinct, and tbe reproductive organs are unlike tbose of otber Mollusks. Tbey consist, in botb sexes, (Fig. 129) of lamellar or brancbed organs, tbe cellular con- tents of wbicb are mctamorpbosed into ova or spermatozoa, and wbicb are attacbed to one point or line of tbe wall of a cbamber, wbicb communicates v^tb tbe pallial cavity by two symmetrically disposed oviducts, in tbe females of some species ; but, in most female and almost all male, Oepbalo-
-* ' Fintw\oke\ung^g2e&OQ\c\i\ft d«t Ce^halopoden,* 1841, p. 107.
THB CKPHAI.OPODA. 629
pods * it liaa onlj one duot, the temunfttion of whicli is UBiiallj eittUited on the left side, Imt may be se*r the middle line (male Nautiltu), or even on the right side (femde Ifau- ahu). In the female, the ovidact, or ondDcta, present glandular enlai^emente. In addition, two la-wlla-T- nida- mmital gland* are dereloped upon the walls id the ImtnchiaL cavity, and to theee aceeeserj glands may be added. These Fig. 139.
Fig. ti9.—Sipia offiemaHt.— i. nale OTgaiu,!, n, valcots Kiiiln>Jii ; pr, pnwiat* ; Aq*, n phom 1 ji, penii with the gcniUI ■pertqre. (After DoTerDOji.)
r..j
glands secrete a viscid fliud, which inrests the ova, and connects them, when laid, into varionaly shaped aggrega- tionH. In the male, a prostatic gland furnishes the material of the cases, or tpermaiophoTei, in which packets of s|
880 THE AKATOmr OF lyVBKTEBBATgP AITIMALS.
tozoa are contamed, and wBicH sometimes possess a rery complicated structure.
In the Dibranchiata, tbe spermatop&ores are slender ejlindrical bodies wiiicH may reacli half an incli in length. They have an external structur^ess case, thinner at one end than the other, and often ending in a fine filament at ^e- thin end. Within this case, filling its thicker end, and as much as half or two-thirds of the rest of its cavity, is a delicate sac full of spermatozoa*
The rest of the case is occupied by a very singolar elastic body, in form somewhat resembling the sponge of a gon with a spiral screw turned on the handle. The enlarged ' sponge' end of this body iis fastened by a d^cate pro* longation to the spermatic sac, while the ' handle^* being too long to lie straight, is coiled up at the end opposite to the sponge and then fastened to the outer case. When these bodies come into contact with water they nndergo strange contortions, and finally, the thin eud of the case giving way, the spring frees itself, starts out of the case and drags with it the spermatic sac.*
In NauHlue, according to Yan der Hoeven, the spermato- phores have a much simpler structure.
Th& male Cephalopods are distinguished from the females by the asymmetry of their arms, one or more of which,, on one side, are peculiarly modified, or heetocotylised.
Some Cephalopods are devoid of any shell, but most possess a pallial shell, which iis either external or intemaL In the former case, the visceral sac is lodged within that part of the cavity of the shell which lies nearest its open end, and the rest of the cavity iis divided into chambers, which contain air, by transverse sepia. The septa are perforated, and a prolongation of the mantle — the siphuncle — is con- tinued through the series of perforations, as far as the apical chamber of the shell. The internal sheUs of the
• For the minute structare of these ourioufl spermatic cart* ridges, see Milne-fedwards' elabo- rate essay, '* Observations snr les
Spermatopbores des Mollosqnes Cephalopodes " (^ Annalea dtf .Sciences Maturelles,' 18 K)).
THB OBPHALOPODA. 531
CepludopodB may h&Ye very yariouB forms, and may eyen be chambered and sipbnncnlated ; bnt, in this case, the chamber nearest the month of the shell is small, and incapable of lodging the viscera.
Onr knowledge of the development of the Oephalopods is confined to that of the Dibranchiata,^ In these, the yelk undergoes partial division, and the blastoderm, formed upon one face of it by the smaller blastomeres, spreads gradually over the whole ovum, enclosing the larger and more slowly dividing blastomeres. The mantle makes its appearance as an elevated patch in the centre of the blastoderm, while the future arms appear as sym- metrically disposed elevations of the periphery,, on each side of the mantle. Between these and the edge of the mantle, two longitudinal ridges mark the rudiments of the epipodia, while the mouth appears in the middle line in front of the mantle, and the anus, with the rudiments of the gills, behind it. The rest of the blastoderm forms the walls of a vitelline sac, enclosing the larger blasto- meres.
The paUial surface now gradually becomes more and more convex, the posterior margin of the mantle growing into a free fold, which encloses the pallial chamber and covers ova the gills.
The internal shell is developed in a sac formed by ac involution of the ectoderm of the mantle. The epipodia unite behind, and give rise to the funnel, while the antero- lateral portions of the foot grow over the mouth, and thus gradually force the latter to take up a position in the centre of the neural face, instead of in front of it. The yelk-sac gradually diminishes, and the contaiaed blastomeres are finally taken into the interior of the visceral sac, into which the alimentary canal is gradually drawn.
• Rdlliker, < Entwiekelimgt- Wira. Zoologie,' 1876). Lankester
geschicbte der Cephalopoden,' '* ObBcrvatioiiB on the develop-
]841. Grenacher. **Ziir £nt- ment of the Cephalopoda."
wickelimgaffescbicnte der ۥ- (* Quarterly Journal of Mler.
pbalopoden/' C ZeitschriA fur Seieuce/ 1875.)
2x2
532 THB ANATOMY OF INTBBTBB&ATBB AHIXALS.
The Cephalopoda are diyided into two yerj distbict groups, the Tetrdbranehiata and the Dibranehiata,
The TetrahrandUata possess an external chambeored si- phnncnlated shelL The terminal chamber is much larger than any of the rest, and the bodj of the «^tiit«i^i can be almost completely retracted into it. When, as in the only existing genus, Nautihu * (Fig. 130), the shell is coiled into a flat symmetrical spiral, its apex lies on the anterior face of the body, and the outermost chamber, into which the whole body can be retracted, is consequently posterior to the axis of the helix. In NautiUtt, the brachial processes are short, and possess no acetabula such as exist in the Dibranchiaia, but the margins of the foot are produced externally into a sort of sheath, which, in front, has the form of a broad hood with a tuberculated surface ; while, at the sides, it is diyided into many processes of unequal lengths. Behind, the halves of the sheath are separated throughout the greater part of their length by a wide interval, but are united above by a thick muscular isthmus. The central portion of the sheath is a broad triangular hood-like plate, the apex of which is free. It contains two long narrow cavities, each of which lodges a tentacle. The tentacle consists of a slender stem, on which are set a great number of transverse plates, in such a manner that the axis of the stem passes through the centre of the plates. The anterior and lateral regions of the hood are completed by two narrower processes, each of which contains a similar tentacle, and the lateral portions of the sheath are formed by sixteen or seventeen smaller tentaculif erous processes, the surfaces of which are more or less distinctly annulated. When the sheath is opened out, there is seen to be attached to its inner surface, on each side, close to the re-entering angle between it and the lip which surrounds the beak, and along the line of junction of the lateral part of the sheath with the isthmus, a
* Owen, 'Memoir on the Katurelles,' 1856. KeferBtein, ia Pearly Naatilu?,' 1832. Yander Bronn's < Klaasen a. Ordniuigak' Hoeven, 'Annales dee Sciences
THI TXTBABUVCHUTA. 533
thin free qrudrato lobe which oaniea tw«lTe tentacloB. The isthmuB joins the porterior edges of theae outer fan- laetUiferwu lobet, as well ss thoae of the two halves of the sheatii, and it exhibits on its anterior, or inner snifftce, a Ivoad ares beeet with delic&te close-set cured Umime. Two other similar, but much thicker, imier ieniaouli- feitniM lobes, which also carry twelve tentacles, lie between these and the lip. Thej are qnite free from the onter Fig. lao.
FIft. iaO.—yaiililii$fiompiliiu, fimmlc. C, hood; mi, ]kwi ; J, ftunel ; p,p',vaaatie; 6r,Dranohia; jrn, iil<l4DieDtsl glud ; r', r, podtlon of the Tcnil titoeadtgea ; <mn, horaj line ; a, ihell muacle ; oe, OTsn ; gal, avIducaTgland ; i/A'. (iplnuiclc ; a, bUck put of ths ihcll DHur the nuntla p', jbt, proeas of the a>KUa(ii>oiu tkcleloii into the funnel. (Aftei Kefenlelii.)
tentoculiferonB lobes, and unite with the sheath only above and behind. Like the halves of the sbcAth, these two lobee are united behind hy a thick isthmus, the surface of which presenta a nomber of parallel longitudinal lamina. The beak, which b hidden hj the sheath and the lobes, is surrounded bj the >■>'■'' circular lip abread; mentioned, the free margin of which ia papillose. Besides these, there is a
534 THB ANATOMY OF INTBBTBBRATSD ANOCAL8.
short conical tentacnlif erous process above the pednncnliite eye, and another below it. In the male, the internal tenta- culiferouB lobes are wanting, and the outer tentacoliferons lobes are divided into two portions, an anterior which bean eight, and a posterior with f onr, tentacnla. On the left sid^ the four tentacles of the posterior division have undergone much modification, and are converted into a peculiar organ termed the qMuUx, which bears a discoidal follicular gland upon its outer surface. There is thus a kind of hectoootj- lisation in the Tetrahranchiata.
The margins of the united epipodia are not united into a tubulai' funnel. They constitute a muscular mem- brane, narrow on the anterior face of the body, but becoming wide, and folded in such a manner that its posterior edges overlap, behind.
The mantle has a broad anterior fold, which coTers the anterior convexity of the shell, and the region which it thus invests is black. The pallial chamber does not extend for more than three-fifths of the length of the body, and is therefore much less deep than in the Dibranchiata, The anus opens in the middle line on the posterior wall of the pallial cavity, close to its junction with the anterior walL The four branchisB are attached, two on each side of the anus, to the posterior wall of the branchial chamber, and the inner branchia is shorter than the outer. The nida- mental glands, composed of numerous vertical la-mftlln*^ partly covered by a fold of the lining membrane of the pallial cavity, are situated on the posterior waU of that cavity, almost midway between its union with the anterior wall and its tree edge. The paired renal chambers lie immediately above them also, in the posterior wall of the pallial cavity.
The buccal mass is very large, its length amounting to one-third that of the body. The apices of the great homy beaks are obtuse, and are coated with a calcareous de- posit. The oBSophagus dilates into a wide crop and is separated by a constriction from the stomach, the chitinouB liniiig oi irhiok ib ^t^nic^ voii i^i^L-^g^ The pyloric cecum
THB TBTRABRANCHIATA. 535
is small and rounded, and the intestine maJses two bends upon itself before reaching the anus. Salivary glands appear to be wanting, unless certain glandular bodies placed within the buccal mass should be of this nature.
The liver is a loosely racemose gland, divided into four lobes, and is lodged in the anterior part of the perivisceral cavity. There is no ink-bag ; and there are no branchial hearts. The quadrate systemic heart is situated on the left side of the posterior face of the body, close to the janc^ tion of the posterior with the anterior wall of the pallial cavity. It receives four branchio-cardiac veins ; and, attached to it, is a pyriform sac, which, according to Keferstein, opens into the pallial cavity.
The cartilaginous skeleton supports the pedal and parieto- splanohnic ganglia, but does not encircle the gullet, or roof over the cerebral ganglia. Two long processes of the skeleton pass into the funnel and give attachment to its muscles. Two large shell muscles are attached to it ; and, passing upwards and outwards, are inserted into oval chiti- nous patches visible on the outer surface of the mantle, and connected together by a thin ring of the same substance (the anntUua) which encircles the mantle.
The oviduct does not arise directly from the sac in which the ovary is lodged, but from a distinct chamber, into which the ovarian sac opens. A large albumen gland pours its secretion into the ovarian sac. The vaa deferens similarly takes ita origin, not from the sac of the testis, but from a smaller chamber communicating therewith. The com- mencement of the vas deferens is enlarged and glandular. Nothing is known of the development of the Teirdbran' chiata.
The only existing representatives of the Tetrabranehiaia are the different varieties of " pearly nautilus " (NautUua pom^itu), which are found in the Southern seaa, living at the bottom at a considerable depth. The genus is one of the oldest in existence, since it is traceable through the whole series of f ossiliferous rocks as far back as the Silurian epoch.
536 THE ANATOMY OF IKTBBTBB&ATBn AHIKALS.
Along with it, in the Palasozcic formatiaiiB, oooiur nomeroas closely allied forms, which differ from KauUiMM mainly in the different oorvature {LUwUes, QyrooenUf 2Vo* choeeras) or straightness {Chihoeercu, Chsmphoeenui) of the shell, and in the varying position, proportions, and d^;Tee of calcification of the siphoncle.
In the middle of the Palseozoic strata (Deyonian), Tetra- branchs {AmmanitidoB) appear, in which the margins of the septa are strongly bent, whence their edges appear as zigzag transverse lines, folded into lobes and MULdles, when the outer layer of the shell is worn away (ChniaiiieM, Cera- titea) ; and, in the Mesozoic epoch, the lobes and saddles become extremely complicated, while the shells maj be straight, simply carved, or bent, or turbinated {AmmonUeB^ BaculUes, TurrUUes), The Ammonitidm are extraordinarilj numerous in the Mesozoic epoch, but no trace of them has been found in tertiary or quaternary formations.
Associated with Ammonites, and not unfrequently lodged in the terminal chamber of the shell, are the so-called Aptyehi. These are plates of a shelly substance, three- sided, with rounded off angles, and applied together bj their straighteet edges so as to resemble bivalve shells. They consist of two layers, an inner and an outer, of which the inner presents lines of growth, concentric with the angle of each plate which is situated on that side of its broad end which is applied to its fellow. The outer layer is composed of many laminsB, and is traversed by pores. Its free surface frequently presents longitudinal ridges. The heart-shaped plates, undivided by a suture, which are found in some Ooniatites and Ammonites, are termed Anaptychi,
The Aptyehi, when undisturbed, occupy the middle of the posterior wall of the terminal chamber of the Ammonite, and have their bases towards its mouth. Nothing is cer- tainly known as to the nature of the Aptyehi or Anaptyehi^
* See the diaeiiMion of this qoeetion by Keferstein, in Bnmn's • Thierreioh.'
THB DIBIUJrOHIATA. 537
In the Dibranchiata, the margins of the foot are produced into not fewer than eight, nor more than ten, arms, which are provided with aceiabula or suckers. Each acetabulum is a sessile or stalked cup, from the bottom of which rises a plug, which nearly fills the cup, but can be retracted by the action of muscular fibres attached to it. When the margins of the acetabulum are applied to any surface, and the plug is retracted, a partial vacuum is created, and the acetabulum is caused to adhere to the surface by atmospheric pressure. The edges of the acetabula are frequently strengthened by chitinous rings, and these may be serrated (Fig. 124, B), and are sometimes produced into long curved hooks.
The margins of the united epipodia are not only folded inwards, but coalesce so as to give rise to a tubular funnel, through which the water taken into the branchial sac for respiratory purposes is ejected. Yery often, a valve which prevents the flow of water back into the mantle cavity is developed within the funnel. There are two branchisB, and the anus terminates between them in the anterior wall of the branchial sac, on which also the nidamental glands are situated. The apices of the homy beaks are acutely pointed, and not ensheathed in calcareous matter. The liver is usually a compact mass. A peculiar gland, which secretes an extremely dark fluid — ^the so-called ink, — and has the form of an oval or pyriform sac (the ink-hoig), with a long duct which opens into, or close to, the rectum, is lodged sometimes in the Hver, sometimes further back (Fig. 126, 1). The ink is ejected when the animal is alarmed, and gives rise to a dark cloud in the water, by which its retreat is covered. There are two branchial hearts.
The eye is lodged in an orbit and is provided with a lens. The cartilaginous endoskeleton forms a ring surrounding the gullet and enveloping the principal ganglia. There is usually an internal pallial shell. It may be chambered and siphunculated, but in this case, the last chamber is small and hardly larger than the others.
The Dibranchiaia are divided into the Octopoda and the Decapoda. The Octopoda have eight nxma, «aii^^gK3«aR»^ t^^
538 THX AKATomr o
pallial shdL Bnt, Id the female of one genoa (Aryommla, tlie "paper Nautilus," Fig. 131), the extremities ol tlie anterior pair of anus ore greatly expanded, and, bong tamed back over the mantle, secrete an elegant ahellj a which oorera the bodj, and Berrea for the a "
Fig. IS1.
FiK- \%l.—Argomaiita argo.~i., female with the eipandcd Bmi iheir u&turkl podtlon, embneing tba thtll b ; d, Ibe othar lis m a, the fUBoel. B, KcUbulA.
Fig. 133. — Argonauta argo, male, wiUi the Htctocotglur
the egga. In thia genns, and in aome other Octopoda (Otioput eonna, TmnodopuM noloonw and T. QvoyaiMM), the male ia rerj much •mailer than the female, and giTca Xiw to ft flNtoeotighu.
THZ DIBBAHCHUTA. S39
Id Argonauta argo {Figa. 132, 133) it is Ute third aim on the left side which becomes thus modified. At first, it has the form of a sac, within which the slender ter- minal part of the arm is ooUed np (Fig. 133, B>. The sac splits to give exit to the latter (Fig. 132], and its two halves reunite on the outer face of the base of the arm to form a chamber, which becomes filled with spermatophores in a manner not jet understood. Dnring boiubJ onion, the arm
Fig. 153.
t'ig. \Xi.~Arginvnda argo.^'R, nwte, with the heCtoeoCrllMd srm enclcnediii iu uc ; I , S, 3, 1, thi other umi of the right aide; Uiil 1', 3', *', tboM of the left ride. A, the hectoeotyliu deuwhed.
thus chafed with eemeu is detached and left in the mantle cavitj of the female (Fig. 133, A). When first discovered, it was regarded as a parasite and termed Ttitiwc^hahia aedabularit bj Delle Chi^'e, while the correeponding bod; found in an Otiiy^ was called Re/dooolyhu octopodii hj
In ZVomodopw, it is the tlkird axm. cm <i^ t^^A. <gAa
540 THE ANATOMY OV XNYBBTBBRATBD AVnCAUB.
which becomes the Hectocatyhu, In other Odopodt* one or other arm is peculiarly modified, but does not become detached, or serve as a receptacle for the spermato- phores.
The Decapoda have ten arms, two of which are tisnally much longer than the rest, and can be protruded from, or retracted into, sockets. The acetabula have homj rims, which may take on the form of hooks.
Hectocotylisation does not go further than a modification of the form of one of the arms. There is always an internal shell, which is either a pen, a sepiastaire, a phroffmocone, or a combination of the latter with a pen.
The TeuihidoB, or Squids, are characterised by possessing a pen. This is a lamellar chitinous body, strengthened by one or more longitudinal ridges, which lies in a sac lodged in the anterior wall of the body, by the lining membrane of which it IB secreted. The posterior end of the pen is com- monly broad, and its sides may be infolded so as to form a conical cup {Ommagtrephes),
In the SeputdcBf or Cuttle-fishes, the sepiostaire, or ** cuttle bone," which occupies the same position (Fig. 125, «A), is composed of a broad plate answering to the pen, and like- wise infolded at its apex so as to give rise to a short cone, but calcified. On the inner face of this plate, a great number of delicate calcified laminse, connected by numerous short columns, form a spongy tissue, which is full of air.*
In the £^rttZi<^, represented by the solitary genus £[ptru2a,t which is among the rarest of animals in museums, though its shells are found piled up in countless millions on the beaches of the islands of the Pacific, the shell is spirally
• Steenatrup, " Die Hectocoty- len-bildung bei Argonauta und Tremoctf^pat erklart durcb Beob- aditungen abnlicber Bildungen bei den Cephalopoden." (*Archiv fOr Nftturgeschichte,' 1856.)
t The planet of the tuperim- poted parallel lamina form an aonte aa^e with thai oi t]bA '^tm- ttijpal plate of the «epV)«X«it^
The connecting eolamns
f placed perpendicularly to the aminee between which they are interposed, and nay be simple or branched. When the young 8eg^ leaves the egg, the sepiottaire already contains air.
t Owen, 'Zoology of the Samarang,^ 1848.
■ DIBKUiaBUTA. Fig. 134.
j.lM^Bd™i«»,wllhthBrraMlniofthebodTofUie»iiliiiJ /F™, » ipidmn in tlw Mixmin of PrwHcml oJlwrvT^i iJS
542 THE ANATOMY OF DnrBBTBBRATSD AHIXALS.
coiled and divided by septa, perforated by a sipliimcle, into cbamberB. The last chamber of this phragmocone, however, is no larger than its predecessor, and the shell is held in position by lateral processes of the mantle, which are nnited over it, and probably represent the waUs of the sac in which the shell was primitively formed. The last chamber of the shell lies in front of the axis of the helix ; the shell is therefore coiled in the opposite direction to that of Nautilus,
In certain extinct genera {e,g. SpiruliraHra),'^ shell, like that of Spirula, is enclosed in a dense and laminated pointed sheath, like the hinder end of a sepiostaire, or of the pen of an Ommastreplies.
In the BelemnitidoB (Fig. 134), which abounded in the Mesozoic epoch, but have been extinct since that time, a straight phragmocone is enclosed within a more or less conical calcified laminated stractnre, the guard or rostrum, which is continued forwards into a variously-shaped, usually lamellar, pro-ostracum. The pro-oetracum and the rostrum together represent the pen in the TeuthidoB.
The rare specimens of Belemnitidce in. which the fossilised soft parts are retained, show that the arms were provided with hooks, and that there was a large ink-bag.*
The genus Acanihoteuthisf {Belemnoteuthis, Pearce)— one of the BelemnitidcB, in which the guard is almost rudimen- tary, while the pro-ostracum is large and penlike — occurs in the Trias, and is the earliest known Dibranchiate Cepha- lopod. The ordinary BelemnitidcB abound from the Lias to the end of the Mesozoic period, after which they disappear. The Sepiadce first appear in the latter half of i^e Mesozoic epoch ; while the Teuthidca are represented by genera closely allied to existing forms (Teuthopsis, Belemnosepia) as early as the Lias.
♦ Huxley. "The structure of t Owen, "A descriptloii of
Belemnites." (* Memoirs of the certain Belemnites,** &o. Q PhiL
Geological Survey of the United Trans/ 1844.) Kingdom/ 1864.)
THE BCHINOBBSXATA. 543
CHAPTER IX.
THE ECHIKODBBMATA.
The Ecbinoderms are exclusivelj marine animals. They are always provided with a skeleton, composed of calcareous spicula, which commonly unite into networks, and give rise to definite skeletal plates. These generally become connected with one another by joints or sntnres, bnt some- times remain distinct. A more or less spacious peritoneal cavity separates the walls of the body from those of the alimentary canaL The nervous system, in those Echino- derms in which it has been most satisfactorily made out, presents a ring, which surrounds the guUet, and gives off radiating longitudinal cords. A remarkable system of vessels, termed amhulaeral, which also form a ring around the gullet, is highly characteristic of the EchinodermcUa, The most conspicuous and familiarly known Echinoderms — the Star-fishes {Asteridea), Brittle-stars {Ophittridea), Sea- urchins {Echinidea)t and Feather-stars (Crvnaidea) — have a marked radial symmetry; similar parts, usually to the number of five, being arranged around a central axis ; and the body is spheroidal, discoidal, or stellate. The Sea- cucumbers and Trepangs (Holothuridea) are elongated and vermiform ; but the radial symmetry is still traceable in the arrangement of the oral tentacula, the nervous, and the ambulacral systems. It is to be remarked, however, that, in many Echinoderms, the radial symmetry, even in the adult, is more apparent than real ; inasmuch as a median plane can be found, the parts on each side of which are disposed symmetrically in relation to that plane. With a few exceptions, the embryo leaves the e^ «a ^\s^a^«sc^^SS::«
544 THK AII1.T0KT Or DnriBTIBBAnD UltMilM.
Bjmmetrical l&ira, prorided with ciliated bands, and oUier- iriae Bimilar to a worm-larra, which maj b« tonaed u Eekiitop<Bdium. Th« convereioii of the Echinopcediiua into an Echinoderm is effected bj the developm^tt ot an ta- Fig. 135.
Fig. 135. — Dlagnm cihibiting the geaenl plui of tha deTctopmoat of at EMnodcrmt (kftet Hullci).— A, common fom whcBO* tia Termlfonn Bolottinrid (B, B') and (he platcifoim OpUorid m EehiDid (C, C) Iuvb ire derived ; D, D', joongcr and Bsn ■dTinced bM£«« of ths Aiteild (SipnuFw) Jkirn; a, mouth; i^ ■tomuii ; c, inteatins : d, *diu ; >, ciliaUd bud ; c', waond or uti- rloi ciliated circlet of . ~
teroctele, and its couvenion into the peritoneal caTitj and tJie HnbolacEal vj^uan ^1 ^ew^ «nd. neireai and tij At
, - *"
THE HOLOTHITBIDEA. 545
metamotpliosis of the mesoderm into radially dieposed antimeres, the result of which is the more or less complete obliteration of the primitive bilateral symmetry of the animal.
1. The HoLOTHXTBiDEA. The stady of the stmctore of the Echinoderms may best be commenced with the members of this division, which, in many respects, deviate least from such worms as the Oephyrea,
In the SynapUB, for example (Fig. 136), the body is greatly elongated and cylindrical, the mouth being placed at one end, and the anus at the other. The oral aperture is situated in the centre of a circle of tentacula, and the gullet leads from it to an alimentary canal, without marked distinction of stomach and intestine, which extends through the body, and is connected by a mesentery with the parietes of the latter. The waU of the alimentary canal presents external circular, and internal longitudinal, muscular fibres, and its cavity is lined by a cellular endoderm.
The body- wall, or perisoma, consists of an external cel- lular ectoderm, covering a layer of connective tissue within which are circular and longitudinal muscular fibres. The latter are disposed in five bands, attached anteriorly to a corresponding number of the pieces of a calcareous ring which surrounds the gullet (Fig. 136, E). The separate ossicles which compose this ring are usually ten or twelve in number, and the five to which the longitudinal muscles are attached are notched or perforated for the passage of the ambulacral nerves, which proceed £rom the circum- oesophageal nerve to the parietes of the body.
The integument contains numerous perforated, flat, cal- careous plates, to which protruding anchor-like hooks of the same substance are attached (Fig. 136, F). According to Semper, these anchor-like bodies are developed in special sacs with an epithelial lining.*
A spacious peritoneal cavity lies between the parietes
• See, on this and all points ArebipelderPhilippinen."(*Wis- relating to the stmetore of the senschaftliche BmoI^^^ "^^ V.\
foioikwrkUa. the beaotiful mo- ffoloCfciinea.') ocraipb bjaem
iper;''jBeifeiiim
546 THB ANATOMY OF IVYSBTSBBATSB AVIMAIiS.
of the body and the alimentaiy oanal, and the oeDs which line it are more or less extensively ciliated. Pedunculated ciliated cups are attached to the mesentery.
The circtdar vessel of the ambnlaoral system snrronnda the gullet below the calcareous ring (Fig. 1S6, E, k). Pos- teriorly, it gives off various csecal prolongations, whidi de- pend freely into the peritoneal cavity. Some of these— the Polian vesicles — are mere cseca; but, in addition, there are one or more tubular prolongations, the perforated ertremi- ties of which are invested by a calcareous network, and are termed the madreporie caruils. Through the openings in the free end of the madreporie canal, the interior of the ambulacral system communicates with the peritoneal cavity. Anteriorly, the circular vessel gives off branches to the tentacula. These pass between the calcareons ring on the outer side, and the anterior end of the alimentary canal and the nerve ring, on the inner side. As each enters its tentacle, it dilates and sends down a short csBcal prolongation on the outer side of the calcareons ring. The ambulacral vessels are filled with a fluid containing numerous nucleated cells.
Contractile vessels, which accompany the intestine and lie on opposite sides of it, filled with a similar corpnsculated fluid, seem, notwithstanding the difference in their contents, to represent the pseud-hromal vessels of the AtiTi<»l|^iy These vessels do not extend into the parietes of the body.
The nervous system consists of a ring which lies superficial to the circular water- vessel, and from which five principal equidistant cords proceed. These pass through the apertores or notches in the circnm-oesophageal plates already men- tioned, and each proceeds along the middle line of one of the longitudinal muscular bands, to the opposite extre- mity of the body.
The ambulacral nerves appear to be hollow ; or perhaps it would be more correct to regard them as thickenings in the wall of a neural canal, as they are in the Agteridea,*
* AocoTdingtoGreeff^Ueber der Gesellschsft lu Marbuf^ den Bau der Ec^\iiQd«tm«Ti^' X^l'IV vckAtJber csnal lies sniMr- j 3te MittheUimg,^itLun«i>MTvc^XA ^\»i Na ^^ v>^»^atfa^^«c<r« is I
v^tll
THB HOLOTHUBIDIA. 547
The genital gland is single, and opens near the oral end of the body, in the line of the attachment of the mesentery. The branched cecal tabuU of which it is composed contain both OTa and spermatozoa, so that the SynaptcB are her- maphrodite. In the majority of the Holothwridea, how- eTer, the sexes are distinct.
In other Holothwridea, the skeleton may attain a mnch greater development, and even take the form of conspicuous overlapping plates (Psohia), Moreover, the circular vessel of the ambulacra] system not only gives origin to Polian vesicles, madreporic canals, and tentacular vessels, but five canals proceed from it, pass through holes or notches in those circum-OBsophageal plates to which the longitudinal muscles are attached, together with the nerves, and run backwards, along the centre of the area occupied by these muscles, on the deep or inner side of the longitudinal nerve. These are the rcLdial ambulaercd vesseU, In the higher Holothwridea, each radial ambulacral vessel gives off many lateral branches; these enter contractile processes of the body- wall, which subserve locomotion, and are the ambtdacralfeet, auckers, or pediceU. In accordance with the disposition of the ambxdacral vessels, the pedicels are usually disposed in five longitudinal bands, which are the ambulacra. Sometimes {Psolua) the pedicels are sup- pressed in two of the five ambulacra, and the other three are disposed upon a flattened surface upon which the animal creeps.
In the higher Holothurids, the intestine terminates in a
distinct cloaca, into which two hollow ramified organs, which
lie in the perivisceral cavity, open. The ramifications of
one of these are received between the meshes of a special
plexus of the psend-hsBmal vessels. Water is taken into,
and expelled out of, the cloaca and these appendages, which,
doubtless, subserve an excretory function, and are commonly
called respiratory trees. It seems probable that the
the Hdothuridea ; and represents (' Jenaische Zeitschrift.' 1^76^,
the ambulacral groove of the star- however, maintains that this
fishes. Teuscher, ** Beitr&ge sur superficial canal ta axL «sVS&s^'«^
Anatomle der £chinodermen " product.
648 THK AMATOKt OW IVTEBTIBXATtD *»TTIiM. Fig. 188.
A. Lun with the UlaUnl eiUMed Iwnd, ud whMt-ilu^ od- tsracMU i^mtM. Vantnl Tlsir. a, moath sad guUat ; fr, ttf^mfii : * " BdMn tar ttitsitiMRhlchte da E^n^ita difOalm." (• Hon
A«US' lUd^ lau.)
THE HOLOTHITBIDBA. 549
c, intestine And aniu ; <f, mci of the enteroooele (mnamge ehaped bodies) at the sides of the stomach ; e, rudiment of the ambulacral vascular system. B. Farther advanced condition of the larva in which the oral aperture is obsolete (the so-^salled ^ pupa-stage *'\ and the cilia are arranged in zones, i, tentacula ; A, Polian vesicle ; ^ the longitudinal muscles of the body-wall. C, a young ^fnapta, in which the ciliated sones have disappeared ; with its five tentacles and the wheel- shaped calcareous bodies at its hinder end. si, the madreporic canal which now opens into the cavity of the body. D, a young ^fnapta mJuerent with anchor^shaped calcareous spicula, except at the hinder end of the body, where they are small and polygonal. £. Longitudinal section of the anterior end of the body of an adult Synapta digitata. a, perisoma with the longitudinal muscles and radial nerve trunks ; 6, calcareous plates which surround the gullet ; e, tentacular canals: d^ oesophagus; e, radiating muscles of the pharynx; g, divided ends of the ctrcum-oral nerve; K, circular ambulacral vessel with Polian vesicle : i, cavity of a longitudinally divided tentacle, into which a tentacular canal opens ; k^ generative caeca; /, mesentery with the dorsal blood-vessel; oi, * auditory vesicle ' on the radial nerve ; n, longitudinal muscles : o, tentaculi^ pedicels ; /», oral disk. F. Calcareous plate and anonor of Sjfiutfia ttUuerem,
ultimate branches of these organs open directly into the periyisceral cavity.*
The Cwnerian organs are simple or branched appendages of the cloaca, the function of which is unknown. The in- terior of these organs is occupied by a solid substance, sometimes of a viscid nature. In some Holothwridea, the anal aperture is provided with a circlet of calcareous plates.
In many of the higher Holothurids the pseud-hsmal vascular system attains a great complexity, and its branches not only extend over the alimentary canal, but, as has been said above, closely embrace one of the branched excretory organs.
The most aberrant form of this group at present known is the genus Bhopalodina. According to Semper, the body is flask-shaped, and at the narrow end of the flask are two apertures. One of these — the mouth — ^is surrounded by ten tentacula; the other, which is the anal aperture, is encircled by ten papillsB, and by as many calcareous plates. A spacious cloacal cavity, provided with excretory organs, traverses the neck of the flask, and opens by the anal
* Semper, loe. cU, Heft iv.,^, Y^l^
550 THB AKATOICT OF IVYXBTSBBJLTBD ▲]riMAI&
aperture. The gullet is stirroanded bj a ring of ten calcareous plates. The genital duct is situated between the cloaca and the gullet. Ten ambulacra diyerge from the centre of the enlarged aboral end of the bodj, and extend, like so many meridians, to near the commencement of the neck of the flask. In correspondence with each ambulacrum is a longitudinal muscular band ; and it is an especial peculiarity of Bhopdlodina that five of these are attached to the anal circlet, and five to the circnm-OBsopha- geal circlet. Until, howeyer, it has been shown that the circular ambulacra! vessel encloses the cloaca, as well as the oesophagus — which is highly improbable, — ^it is justifiable to assume that the anus of Bhopododina is really, as in the Crinoidea, interradial in position.
The development of the Holothuridea is extremely instmo- tive. Yelk-division gives rise to a vesicular momla, which undergoes invagination, and becomes converted into an oval ciliated gastrula. The opening of invagination becomes the anus, while a mouth and gullet are produced by an invagination of the ectoderm, near the anterior end of the body, which unites with and opens into the blind end of the endodermal sac, or archenteron. The completed ali- mentary canal is thus composed of a gullet, a rounded stomach, and an intestine ; and the cilia of the ectoderm usually become restricted to a single band, bent upon itself, though its general direction is transverse to the axis of the 1^>o^7 (^g* 135, B ; Fig. 136, A). At a subsequent period, this single band may be replaced by a series of hoops of cilia (Fig. 136, B). According to Kowalewsky* the embryo of Pentada doliolum does not become ciliated at all, and that of Psolinus passes from the conditioa in which the cilia are dispersed over the surface, directly into one in which it is provided with five zones of cilia, between two of which the mouth opens. In this condition it singularly resembles the embryo of Comatula, And. indeed, in the further advanced condition of the PsoItiHit, the oral end oi the body, surrounded by triang^nlar eal-
THE BIVXLOPKXKT OF THB HOLOTETaBIBEA. 551
careouB plates, withm which the tentacles take their origin, has a striking resemblance to the oral end of the young Pentacrinoid larva of Conutiula,
The peritoneal cavity and the ambulacral vessels take their origin,* in a very remarkable manner, from the archenteron, before the oesophageal invagpjiation reaches it. The anterior part of the archenteron gives off a cscal process, which, becoming a sac, is constricted off from the archenteron as what Salenka terms a vaao-perUoneal vende. This vedde changes its position to the left side of the alimentary canal, and then sends a narrow duct-like diverticulum towards the dorsal region of the ectoderm, which eyentually coalesces with the latter, the cavity of the diverticulum opening on the exterior by a rounded pore. The vaso-peritoneal yesicle now divides into two portions, one of which, the ambulaeral 9ae, remains connected with the exterior by the duct, and constitutes the foundation of the whole of the ambulacral system of vessels ; while the other, the peritoneal eae, gives rise to the peritoneum. The former becomes five-lobed, grows round the gullet, and gives rise to the tentacular and ambulacral canals with the Polian vesicle, or yesicles ; while the duct, detaching itself from the dorsal wall, becomes the madreporic canal.
The latter divides into two vesicles, which arrange them- selves at the sides of the stomach. The stomach takes on a more cylindrical shape, and these Tesides become the "sausage-shaped bodies" (wiirstformige Korper), observed by Mtdler (Fig. 136, A). They gradually increase in size, and growing round the alimentary canal, unite above and below it. Thus a cylindrical cavity with a double wall is formed, between tiie endoderm and the ectoderm. The inner waU of the cavity applies itself to the alimentary canal, and aided by the mesoblastic cells which appear to be developed frx>m the endoderm, becomes the muscular
* See Metfehnikoir, **8tadien the very ■atisfaetory memoir of
Uber die Entwickelnng der Echi- Salenka, ^ Zor Entwickelang der
nodermen and Memertinen ** Holothurien *' (* Zeitsohrift fur
(< Mem. de F Acad, de St. Patera- Wiia. Zoolog^a^ VQ^i«^« boQif , ziv., 1869) ; and eapociaUy
and peritoneal coat of that Tiaom ; wliile Die outer walli attacbmg itself to the eotoderm, or to the meaobbwtio cdU which line it, ie, with Ihem, conrerted into tlie moaealar and peritoneal inveetment of the parietea of Uie body. The interapaoe between the two ia the peritoneal cari^.
Fig. IST.-DeTelopment of ft Bololbnrid (■Rer Mulliir).— A, Eart; condition of the larva (Aurinlnria) ; g, the dorsal pom trf' h, Iki
— V..1 ■ r, 1-. . '^ lolMtino; p, donal poco ;/,/",
Ita prolongatioiia ; i, caler— — Ircnlu cllUtad bands ; g, i
ambulMral nc; B, Ut«T atage oiroular ambulftcral Totsel wit body ; C, yoong Uaiotkwia wltl
nal i f, P
In the meanwhile, the bodj of the embiro elongate*, the tentaciila are deTeloped around the monUi, the ciliated baad> diaappear, and the Holothniid Echinoderm is complete.
Thna it is clear that the peritoneal cavity of the Hole- thnrid ie an enteroccele, and that it answers to the peri- visceral cavitj of SagiHa, or of the Brachiopoda ; and further, that the amhulacral Tesaels are also modific»ti<mi of the enteroccele. UoreoTer, it ia obvious that tlie atone- turea which are developed between the ent«roccele and the ectoderm and endoderm, answer to those which az« evolTsd from the mesoblast in other aniniBla ; and that the adoU Echinoderm stands in the some relation to tbe JSjAmo* •pvedium as an AimeUd does to its embryo ; the adnlt f onn being due to the peculiar arrangement of the parte de- reloped from ^ mewAAaiA.. '^Iti-^axV.A^liABciiliiopBdiwa
VBM ASTBBIDaA. 553
oast off in the oonne of the deyelopment of the HolO' vridea,
I. The AsTBBiDEA. — A Star-fish is comperable to a dothoiid, the ambulacra of which are restricted to its il half, flattened out so as to have a very short axis; ile its equatorial diameter is greatly increased, and xluced in directions corresponding with each ambu<- rum. The result would be a disk, having the form of a itagon, or of a five-rajed star, with ambulacra only on kt face of the disk which bears the mouth. Hence the bulaeral, and the opposite, or antambulaoral, faces are equal extent.
iiost Atieridea are like five-rayed stars, but some are
itagonal disks (Ooniaaier), and some few {SoUuter) have
tre than five rays. In Britinga, the rays are much more
erent from the disk than usual, and the genus thence
aires an outward resemblance to an Ophiurid.
11 the Atieridea are provided with a skeleton made up
•lates or thick rods, composed of a dense calcareous
t>rk. A deep groove, radiating from the mouth to the
)f the ray, marks the position of each ambulacrum,
he sides of this groove are supported by two series of
^4Uind osndea, which meet and articulate together in
iddle line or roof of the gproove. The ambulaoral
uid canal lie superficial to these ossicles. There are
I tentacula.
five-rayed body of the commonest of British Star-
the Fivefinger (Urtuier, or AHerticanthion, rubens),
I an oral face, in the centre of which the mouth is
and an opposite or ahoral face. The hardly-dis-
anal aperture is situated not exactly in the centre
%ce, but close to it. The mouth, which varies very
size, lies in the middle of a soft membranous oral
deep furrow, the ambulacral groove, occupies the
the oral surface of each ray, and is nearly filled
e Hoffhiann, '* Zur Anatomie der Att«rlden." Q ISUd^t- z«hiv; Bd. ii. 1874.)
£64 THE A.SATOHT OP IHVEBI8BKATBD AKtlUl^. ^
hj contractile sacker-like pedicels, with cirenlar diacoj'ltkl Olds, apparently amuiged in tour loDgitudioal a«ri«K. Tbi: deepest part at the groove is at ite central end. where ita lioing passes iato the oral membrane. The sfa&Uo«reBt part IB at Ita dintal end, where it tra'iuinat^a agaLuBt ■ median projection, the peduncle of the eye. on the aboikl Bide uf which is the single median ovular ttfalarU. Lines drawn from the muutli along ea«b ambuliicrutn tu-e termed radii, nnd the regions oocnpied bj the ambniacra are Buid to be radiid. The parts of the body situated between the ambulacra are inlerradial. The lateral woUb of the unbu- kcral grooves of adjiu^nt ambulacru unite at the cirooin. ference of the oral disk, and give rise to &re atl«rraMal aaglee. On one side of the aboral face of the centre of the body, between the origins of two of the rajs, and then^uri; interradial in posHiou, ia an oial or somewhat pentagonal. eligbtly convex, porousplate, the eurfaceof which is covered with uiuTow meandering grooves. Tbia it the madrtpcHe tiAerde or ttiatireporite.
THB ASTBlUDaA. 555
tached to tlie oencola by ligamentous fibres at one end, id free at the other ; and the calcareons stroctazes oon- ined in the pedieeUarieB. On the antambtdacral wall of le body, the osaicnla are elongated rods of very unequal Qgths, united together in such a manner as to leave poly- >nal, rounded, or elongated, meshes. The sides and roof each ambulacral groove, however, are bounded by two lies of regularly disposed and similar ambtUaerdl omHcUs, bich lean against one another in the middle line above, verge so as to enclose the ambxdacral gproove, and, at leir outer ends, abut upon thick short adamMaeral Mdcles, which lie at the sides of the groove (Fig. 1S9, B). Between every two ambulacral ossides in the same half ! the ambulacrum there is a canal, formed by the junction ! notches in the oral and distal faces of the two ossicles, onsequently there is a half -pore on the oral, and another df-pore on the distal face of each ossicle. The half- re on the oral face is always internal in position to ) half -pore on the distal face, and, as the part of the bulaoral ossicle which lies between the two is thin, row of pores, though it is really single and bent in harp zigzag, appears at first sight to be double. The ts which connect the ambulacral vesicles with the eels, traverse these pores ; and the comparatively large very flexible and extensile pedicels are thus so closely ed together, that they appear to form a double row, «ch side of the middle of the ambulacrum, the circumference of the oral disk, the ossicles of the \aora, diminished in size and closely united together, i pentagon, the angles of which answer to the ends of ibnlaoral gprooves, round the oesophagus. The oon- oater ends of the pair of ambulacral ossicles nearest ath project on the oral face, outside the buccal mem- as five vertical crests, armed with strong spines, are beset with pedicellarise. In correspondence ese, five falciform folds of the perisoma, more or ified, project into the cavity of the body. They are al in position, and extend xip \a \2kift ^iX)^T«^^«iS^..
556 THB ANATOMY OF nrYBBTSBSATSD AXIMAIM.
Their inner edges are free, and look towaids the ■fawMA. with one of them, the madreporic canal and the nuns which accompanies it, are closelj connected.
The tpines are more or lees moveablj united with the oesicola, but there are no such regolar joints mm aie met with in the .EeAiaidea. The pedicettarice are snpportednpoa short flexible peduncles. The skeleton of each consists of two blades articulated with a basal piece. From the centre of this Tciy strong adductor muscles proceed to the inner faces of the blades, and weaker fibres, attached to the exterior and to the outer faces of the bases of the blades, act as divuricators.
The guUet opens into a wide stomach produced into five large cardiac mcs^ the walls of which are subdiyided into many sacculi. Each cardiac sac is radial in position, and may extend a short way into the cavity of the aim to which it corresponds. On the aboral side of these mum, the alimentary canal suddenly narrows, and then dilatfs again into a shallow, but wide, pentagonal pyloric sac, the angles of which are produced into five tubes. Each of these passes along the middle of the aboral face of a ray, and divides into two branches, which run parallel with one another through half or two-thirds the length of the r^j, and end blindly. The branches give off numerous ceeosl dilatations, arranged in pairs on opposite sides, and these hang down into the cavity of the ray. The edges of the pentagonal pyloric sac and the aboral faces of its saocnlated branches are connected by mesenteric folds with the abonl perisoma. The oral faces of the cardiac sacs are similarly connected by pairs of mesenteric folds with the sides of the corresponding series of ambulacral ossicles. The aboral face of the pyloric sac presents an aperture closed by projecting valvular folds, which leads into the short tubular intestine. The latter terminates in a minute anal pore, situated nearly in the centre of the aborsl face of the body. The intestine receives the duct of a cfficum divided into two main branches, each of which hfw many minoT a\)\>diNmoi^. M. \2[^ ^•n-imf^]^ having
THI A8T1BIDBA. 557
its month downwards, is divided into two ludres, by a yertical plane passing throngh the month, the central point of the aboral face, the madreporic tubercle, and the middle line of the raj opposite to the tubercle; and if this ray is anterior ; then the anns opens into the left posterior interradial space, and the cseca lie partly in this and partly in the left anterior interradial space.
The nervous * and vascular systems of the Star-fish are so closely related to one another that they may be best considered together, and as there is least difficulty in malring out their arrangement in the ambulacra, the study of them may be commenced in this region.
When the suckers of an ambulacrum are carefully cut away, a longitudinal ridge is seen to lie at the bottom of the groove between their bases. This ridge is the ambula' eral nerve. Followed to the apex of the ray, it ends upon the eye and its tentacle ; in the opposite direction, it reaches the oral disk, at the periphery of which it divides, and skirting the margins of the disk, joins the branches formed by the bifurcation of the adjacent ambulacral nerves, thus giving rise to a subpentagonal ring round the mouth.
The eye f is a thick cushion-like expansion of the ectoderm continuous with the ambulacral nerve. In it are imbedded many dear oval bodies surrounded by pigment, which appear to represent the crystalline cones of a compound eye.
The tentacle which lies on the aboral side of the eye, resembles one of the pedicels in structure, but has no terminal sucker ; its function appears to be tactile.
In a good transverse section of one of the arms or rays
of the Star-fish, the nerve is seen to be a band-like thicken-
fng of the ectoderm, the cells of which have become
peculiarly modified, but which is continuous laterally with
the ordinary ectodermal covering of the pedicels. This
band-like nerve constitutes the superficial wall of a canal,
* See Wilson, " The nervous Teuscher cited below,
system oftheAsterida^C Trans. f Conf. Uaeokel, *Zeit fur
Linnean Soeietv,' 1862), and the Wiss. Zoologie/ 18GQ. later eontribotions of Professor
558 THB AKATOXT 0¥ IVTXBTBB&iLTSD AVIMAIiB.
which extends through the whole length of the ambnlacniiii and may be termed the amhulacral neural eandL It is diyided by a longitadinal septum. At its oral end, as has been seen, each ambolacral nenre, when it reaches the onl membrane, diyides into two divergent branchiea, whidi unite with the corresponding branches of the other amba- lacral nerves to form the oral ring. Answering to the latter is a wide circular neural canal, into which the ambu- lacral neural canals open.
In the transverse section of the arm, a second and mnch larger canal is seen to lie between the conjoined ends of the ambulacral ossicles and a strong septum, containing transverse fibres, which separates it from the neural ^hm>^1 This is the radial canal of the ambulacral system of vessels. At its oral end it opens into the eurcmnorel ambulacral vessel which lies close to the ossicles to whidi the margins of the oral membrane are attached. Ftom opposite sides of the radial canal, short branches are given off, which pass between the ambulacral ossicles, and ea^ opens into the neck of a relatively large sac, with nraacakr walls {ambulacral vesicle), which lies on the aboral face 6L the ambulacral ossicles in the interior of the ray. Hk neck of the ambulacral vesicle passes in the opposite direction into one of the pedicels. Thus the ambnlacnl vessel communicates with the cavities of all the pedioelf on the one hand, and with the cavity of the ciixmmcnBl ambulacral vessel on the other. Five pairs of wwall eminences, consisting of caeca which open into the dr- cumoral vessel, are seated upon it; and from one part of it, opposite one of the interradial falciform folds alrady mentioned, springs a canal, which, taking a sinuous passes to the aboral face and terminates beneath the dreporie tubercle; this is the madreporic mn^ It jg not a simple tube, but, as Sharpey first observed, its waUs ait doubly involuted so as partially to obstruct its cavity, ani it is strengthened by annular (^deifications. The poies d th« madreporic tubercle place the cavity of the aadxepoiie oamil in yKmTftTiT\\oA\.\aa.-^V^>3tvfe. es^xscvor^ whence it foUowi
^
THS ASTSBIDSA.
559
that the cayities of the whole ambolacral ayBtem mnBt he directly acoeaaihle to the sea-water in which the Star-fish lives. The madreporio canal is invested by the lining mem- brane of the peritoneal cavity. This indoses a sinus, which aooompanies the madreporic canal, and into the interior of which a fold projects.
There is no great difficulty in ascertaining the existence of the structures which have now been described, and all anatomists are agreed as to the nature of the ambu- lacral system. But whether the neural canals are to be considered as a special system of blood-vessels, and the sinus which accompanies the madreporic canal, a heart, as is usually assumed, appears to me to be very doubtful.* I am disposed to think, in fact, that not only these canals, but the circular, or rather, pentagonal vessel which has been described as situated on the aboral face pi the body, around the anus, giving off various branches to the viscera and communicating with the so-called heart, are mere sub- divisions of the interval between the parietes of the body and those of the alimentary canal, arising from the dis- position of the ambulacral vessels and that of the walls of the peritoneal cavity ; both of which, as their development shows, are the result of the metamorphosis of saccular diver- ticula of the alimentary canal, which have encroached upon, and largely diminished, the primitive perivisceral cavity which exists in the embryo.
The peritoneal cavity of the body and rays is filled with
it and with the nearal canslf.
* Since Tiedemann't time, the presence or absence of a blood- vaicular system in the Starfishes has been attemately asserted and denied. The recent investiga- tions of Oreef, ** Ueber den Ban der Eohinodermen ** (* Marburg Sitsunesberichte,' 1871-2X Ho£ man (1. c), and of Tensoher, ** Beitrage sur Anatomic der Eohinodermen " Q Jenaisehe Zeit- ■chrift,' Bd. X.), are in fkvonr of the existence of the ' anal ring,' and of an extensively ramified system of canals, ooiinected with
But it does not appear to me that the facts, as they are now known, Justify tike assumption that these canals constitute a distinct system of blood-vessels. Injections show that an these canals communicate with the ambulacral vessels, and with the exterior, by means of canals in the madreporic tubercle which open partly outwards, partly into the madreporic canal, and partly into the sinus which accompanies it,andcomraunieat«& with the QVxc;!QkmoTiXxA!<Q3niiL^^»MS^«
560 THE AKATOMY OT IFYXBTSBRATED ▲KIMAI.8.
a watery oorpusculated fluid ; a similar fluid is found in the ambulacral vessels, and probably fills all tbe canals whicb liave been described. The corpuscles are nii<deated oells, wHich exhibit amoeboid movements ; and the fluid bo ob- vionsly represents the blood of the higher animal it, that I know not why the preposterous name of " ohylaqueous fluid" should have been invented for that which is in no sense " chyle/' though, Uke other fluids of the living body, it contains a good deal of water. As the cavities of the tubular csBca of the perisoma communicate freely with the general cavity, and their walls share in the general ciliation of the lining of the cavity, it is very probable that they may subserve the function of respiration.
The genital glands are situated in pairs, interradially, at the junction of the body with the rays. Each gland is divided into a number of elongated processes, the common base of which is attached to the face of one of the inter- radial septa, while the processes project freely into the cavities of the arms. According to Hoffmann and Greef, the inner cavities of the genital processes are filled when the vascular system is injected. It is possible, therefore, that the genital glands are merely processes of the meso- dermal layer, in the walls of which the genital products are developed ; in which case there would be a close approxi- mation between the genital glands of the Star-fishes and those of the Crinoids. According to Greef, the external openings of the genital glands are visible in UraHer, in tiie breeding season ; in other Star-fishes, they are conspicuous in the interradii of the aboral face of the body. In LuideOy Ophidiaster and some other genera, the glands extend far into the interior of the arms, and Prof. G. O. Sars* has pointed out that, in Brisinga endecacnemos, the genitalia are numerous distinct glands, arranged in two series, one on each side of the middle line of the central half of
* * Researches on the Struc- proves that BrUmga is a troe
tore ftnd AflBnity of the genus Asterid, and not, as has been siip>
Britmga^ 1870. In this im- posed, a transitional fonn beiwesa
portant memoli ^e vqAiot >3[i<^ AsCeriofea and the Opkimnim,
tarn ssWiOPiuKT or tbx UTnnm. 561 encb ra;. Each of theae OTories or tcatea has « separate
Id Bome Star-fiehea, ae in some Holothoridi, the embryo possea into the Star-fish fonn withont anj free lairal stage. Bat, more naualfy, an Echinopndiom is tonoed in the same wBj BM in the Holothorians, though it presents diSerencea In the arrangement of it« ciliated bands, and especially in their prolongation into nomennw lobex or narrow processes. ua in the remarkable ferm originally named Bipinnaria. (Fig. 135, D D', and I^g. 138.) It baa no calcareous skeleton.
Fig. 138.
ria, BfMr MuUer).— A,
, _____ ^ ■£»): C, 09)B«Hina with
rudiment of tht Stu-Sih, a, moaih; t, <Eaop)uga>| c, itonueh: b', inteitine ; o, snui ; s, iraatnl; jr, don«l (ld« of the Mitaiior end of the body; d, f. ellUled buidi ; k, iuboI divertiaulum fbrmlog the rudimeDt of the unbnlunl viacalar ■jstem, uid opening ti- ternnU]' by the pore jr.
Ancerdiug to Uie obaarTationa of Prof. A. Agassiz,* which have been confirmed bj MetochnikofT and Greef, the ambn- lacral veasela commeDoe aa diverticnla of the stomach, which, beconung detached from the alimentary canal, giye rise to the peritoneal cavity, and to all the snbstance
DMDt of wblah U detcribed in tbli Inportant memoir, ue .^Mmun- OuiMpcUlidiu ukd A. iotUiHU.
• "Embr toh."('Con
Uw«lKT<
ieM.>Thi
of lb* Dailad Bute*,' ■pedei, the develop-
562 THE ANATOMY OF DfYBBTBBBATKD AinXALS.
of the body between the endoderm and the ectoderm.* A portion of one of these diverticohi, howerer, sep&rates itself from the rest, opens externally by a pore, and becomes metamorphosed into the ambnlacral yessels. But this ambulacral diverticnlum does not surround the gallet» and consequently a new mouth is developed in the centre of the ambulacral ring. The lanral mouth and g^nllet are abolished, and the greater part of the body of the Echinopsedinm is separated from that portion which contains the stellate Echinoderm. The latter results from the metamorphosis of the mesoderm, which is modelled upon the different divisions of the enterocoele, and encloses the middle portion of the alimentary canal.f
The Ophiubidka. — The brittle Stars, thongh they re- semble the ordinary Star-fishes in form, differ essentially from them, not only in the structure of their skeleton, but in the characters of the Echinopsedium. The ambulacra ai« confined to the oral aspect of the body, so that, as in the Asteridea, the ambulacral and oral, the antambulacral and the aboral surfaces, respectively coincide. The mouth is situated in the centre of the oral face, but no grooves radiate from it along the ambulacra, which are covered hj a series of plates of the skeleton. The alimentary caiud is a simple gastric sac without cseca, and has no intestine or anus. In contradistinction from the Star-fishee, the prolongations of the peritoneal cavity into the rays are very narrow.
The typical Ophiuridea possess a very complete calcareooi skeleton ; which, on the body and on the exterior of the rays, has the form of plates. On the body, the disposition of these vanes much ; but five of them, which are situated
♦ Probably independently de- ^loped mesoblastic cells contri- ate to the formation of the
metodenn as in the Holothorids. t Greef (/.e.)has worked out the
devetopment of Uratter {Astera-
cttHthton) rubeju^ the larval ftta of which resembles the B^mmarit and Braddolaria of HeUinxloiii described by M&ller. ParSess-
genesis appears to oooor ia tkii tar-fish.
THX OPHIT7BIDBA.
563
interradially in the neigHbourhood of the mouth, are often larger than the others, and are termed »cuia huecalia.
Each ray contains an internal solid axis, composed of a single series of quadrate cueial ossicles (Fig. 139, C, a), each consisting of two lateral halves united by a longitudinal suture and articulated together by tenon and mortice joints ux>on their terminal surfaces. Each of these ossicles (which are sometimes termed verie&roZ) is surrounded by four plates ; one median and antambulacral (Fig. 139, 0, h), two lateral (Fig. 139, B, c) and one median and super-ambulacral (Fig.
Fig. 139.
Fig. 139.— A, ventrtl, B, lateral, views of a ray of Ophiura texhtrata (after Miiller). C, transverse section, a, axial or ^'rertebrar* ossicle of ray ; 6, antambulacral plate ; c, lateral plate ; d, ventral or superambulacral plate. D, section of a ray of an Asterid, Attro- peeten aurantiacut f after Gaudry). a, ambniacral or ** vertebral *' ossicles ; 6, adambuiacral ossicles ; c, c', marginal ossicles ; d, paxillse of antambulacral surface.
139, A, d). The lateral plates may meet in the middle line on both the ambulacral and the antambulacral faces. Between the lateral plates are the apertures by which the pedicels make their exit. The oral aperture is surrounded by five ^cU angles, each of which consists of five pieces. The two
2 O 2
564 THS AKATOMT OF INTBBTBBRATBD AHI1CAL8.
oonstitaents of the axial oesiole whicli lies at the <nral end of a ray become moveably articulated with one another, while each ankyloses with an interambulacralpieoe. Trans- yerse muscles connect the two interambulacral piecea, the oral edges of which are articulated with a long narrow plate, the torus angularis (Fig. 140,/). The free anrlace ol the tarua angularia lies in the walls of a sort of Testibiile in front of the mouth. A number of short flat proccnaeo, the pcdcB angulares, are articulated with it and moved by special muscles. They doubtless perform the function of teeth. Rudimentary representatives of the calcareous ring of the Hohthuridea and of the parts of the lantern of the Sehinidea exist as delicate calcareous plates, which lie on the cixxsular ambulacral vesseL The latter is usually provided with CflBcal appendages, or Polian vesielea. The madreporic canal ends on the surface of one of the tenia buecaUa ; the radial ambulacral vessels run in the arch between the axial oasicles and the super-ambulacral plates. The nerve lies superficial to the super-ambulacral vessel, but is also covered by the super-ambulacral plate. A neural canal lies between the nerves and the ambulacral vessels. The pedicelB are tenta- culif orm, and have no vesicles at their bases. The genital glands are lodged in the disk, and pour their products into the peritoneal cavity, which communicates freely with the exterior by verticidly elongated apertures placed inter- radiaUy on its margins.* According to Metschnikoff, Ophiolepis aquamaia is hermaphrodite.
The early conditions of the embryos of most Ophwmridm are similar to those of other Echinoderms, and acquire the characteristic bilateral ciliated zone; but in some, the embryo does not become an Eehinopcedium, but pannnn directly into the adult condition. Thus Krohn discovered
* Muller, '* Ueber den Bau der Apparently cecal divertieiila cf
Echinodennen"(Abh.Berl. Akad. the circular ambulacral canal,
1853) ; Teuscher (1. c); Simrock, and of the necks of the PtoUaa
^Anatomic und SchUogonie der vesiclei (iNua €mbuiacrttiia coai)
riaeiii vtrau" (* Zeitschrift which traverse the peritoaeal Wiss. Zoologie,* 1876). The cavity in aU directiona. latter writer describes numerous
TKt OPRmslDIA. 565
that the eiiibi70 of OphiolepU eiliata it developed within the bodj caritf of the {lorent, to whioh it odhereB b; a bind of pedicel. Where an Eehinopeedivm etage ezista, the larra is a Pluieus (Fig. 135, 0 C). The donal wall of the boc^ of the embrjo exhibits • median oonioal ontgrowtlt ; along the ooone of the ciliated band BjmmetrioaUj' dia- poaed proceases are developed; and these oatgrowiha ore supported bj a oalcoieooa skeleton, which ia ^ao bilaterall; ajmrnetricaL Uetechnikoff * has made the interesting ob- serration that in an Ophiorid (probablj OphiothriafragUu) the whole aTstem of periTisceral and ainbnlacral cavities
Fig. 1«.
Fix. 140.— A, Ophhirplj ciiiala, oral ikeletoa tmta within (Bftfr Hiillst). — B, dorsal mkrgiiial pUtes; b, ventral platat; d, vartebral owicles; c, intcrunbulacral piecn of oral uigle ; /, tonu uigu- laria ; g, apirturee for oral bmtaclei ; h, poiltlon of nBrvoui eollu' ; t, linpr««*ion of circular unbolacral vesul ; i, oriflcs In tha firtt ambalacral platg for the tcntseolar branch of the oral venal ; o, pals anKalare*. B, AMiropSjrIm, oral ikelelon leen from wtthiu (after Muller) : ■• n, periiiomial platei ; other iMtars ■• In A.
arises from two bodies, one aitoated on each side of the gullet, which are solid, Qiough it is possible that Uiej may primitively have been hollow diverticula of the archenteron. Two cellular masses become detached &om thwe bodies, applj themselves to the sides of the stomach, and are con-
566 THE ANATOMY OF INYEBTBBRATBD ANXMALS.
verted into disks, from which the parietal and Tiaceral walls of the peritoneal cavity take their origin. The rest of the solid body on the left side of the gollet aoqniree a vesicular character, opens by a dorsal pore, and grows i*ound the gullet, to give rise to the circular ambnlacral vessel. The other solid body disappears. The mouth of the Echinopeedium becomes that of the Ophiurid.
It cannot be doubted that these solid bodies take their origin, in the same way as in other Echinopsedia, from the hypoblast ; and thus the question arises, how far does the mesoblast thus formed differ from that which arises by the mere outgrowth of cells from the hypoblast, as in the Dog-fish, and how far does thia case tend to render it probable that a schizocoele is only a modification of an enterocoele P
The EoHiNiDEA. — An ordinary Sea-urchin is comparable to a Holothuri J, with the body distended into a more or less globular form, and with a skeleton in the form of regular plates arranged in meridional series; those plates which correspond with the ambulacral vessels being superficial to the latter, and consequently x>erf orated by the canals which pass from the ambulacral vessels to the pedicels.
In the Echinidea, as for instance in the ordinary Echintu or Sea<urchin, the perisoma round the mouth {perigiame) is usually strengthened for some distance by irregular oral plates. In addition, ten rounded plates are placed in pairs close to the lip ; these support as many pedicels and are perforated by the canals of the latter. A much smaller space around the anus (periproct) is similarly protected by anal plates. The rest of the body is supported by a eon- tinuous wall made up of distinct, more or less penta- gonal plates, iisually firmly united by their edges, which is called the corona. Of these plates there are twenty principal longitudinal series, constituting the great mass of the corona ; and ten single plates, which form a ring around ita aboral or apical margin. The twenty series of longitadinal ^AsA^ea «iX^ ^Y^«ft^\s^\/^^ double
THB BcanipB^ .
567
five ambalacntl, (tod fire inter-ambiilacral— alternating witK one another throt^hont tbe circn inference of the corona. Sach double eeriee of plates presents a zigzag suture in tbe middle line, formed bj the alternating arrangement of tbe triangular extremities of its component elements. The antoree between tbe reapeotiTO series of ambulacral and interambolacral plates, on the other hand, are less obvions
Fig. 1*1.
Fig. 141.— Di*gnin exhibiting tbe relBtiom of the different ijelemt of ornni in ka £cAJiH(.— a. Dtouth; b, teeth; e, lipi; d, alieoli; (, fslcet : /. Mirieate ; g, relnotor, and h, prolrkolor muules of leatem ;
*piii(
A, prolrkolor muules of leatem ; . ._.i — Ml ; i, polUn veriele J
I, tubercle to vhich It f« krliculated; t, pediecUariiE i h, , madreiiaria tubercle ; z, oculer tpot.
and more straight. Each ambulacral plate is sabdivided bj a great«r or less number of snturee, which traverse it obliquelj, into a corresponding number of minor plates; and these, inasmuch as thoj are perforated b; tbe canela
569 THB AKATOXT OT nrYBBTEBKlLTSB LVTMAUL
or poree, which give exit to the twoTesaels whereby each pedicel is placed in commnnication with ite baaal Tesielee and with the ambulacral reesel, are called pore pUttm, Throaghx>ut the greater part of the leagth of an ambnlA- crum of the common Echinus tpheera (Fig. 142, A) each ambolacral plate is thus divided into three pore plates, traversed altogether by six pores or short canals. The outer openings of these canals are arranged dose together in pairs upon little excavated shield-shaped elevations^ or umbanes, sculptured on the outer or interambulacral half of the face of the ambulacral plate; but their inner extremities are much wider apart. A pore plate, or sab- division of the ambulacral plate, thus corresponds with each pair of pores, and therefore with each pediceL Lov^n * has shown that the pore plates are the primitive ambulacral ossicles in the Echinoidea. At its apical ex- tremity, in fact, the ambulacrum is composed of only two small ossicles which meet in the middle line. Each of these primitive ambulacral ossicles is perforated by a single or double pore for the pedicel which it bears. But as, in the course of the growth of the corona, new primitive ambulacral ossicles are added between the ocular plate and those already formed, the latter shift towards the oral end of the ambulacrum and grow, in correspondence with the larger space which they have to filL But thej grow unequally ; and while all retain their primitiTe con- nexions with the adjacent interambulacral plates, some lose, while others retain their median union with the corresponding ossicles of the same ambulacrum. The former, therefore, are, as it were, pushed away from the middle line by the union of their encroaching predecessors and suc- cessors. Groups of the primitive ambulacral plates, thus modified, enter into close union, and constitute the complex ambulacral plates of the fully developed ambulacrum.
In the genus Cidaris, the primitive ambulacral plates enlarge, but do not coalesce into secondary ambulacral
* ^ Etudes sur les Eehinoidto." (* Kongl. Svenska Vetentk-Aktd. Handlingar; Bd. U , \^1^.^
THI MCBOriDIA. 869
plates; Iiaiioe tiie distinction between Bmbalaonl i^tes and pore i^tea Tanishes. The ambtUacnl pUtea are ooa- tinned on the peristome to the margins of the month, and here th^ become somewhat altered in fonn and their edges overlap.
In the living genns Agthmumoma, and in certain extinct Eehinidea {Lapidoeaninu, EchinoOmria), the plates of the corona are loosel; united and overlap one another; while, in the extinct pabeozoic Frri»ehoeehinidtB, there are more than two series of interambnlaoral plates, those in the middle of each interambidacmm being hexagonal.
Fig. 143.
Fig. Ui. (After Hfiller.)— A. Thnw Knbatecnl plitci of fcAhw ^hrra, sihibillni Ihe auiurn of the pora )>latei of which tush ■mbulaoni plate u compoted. B. fart of Uie petaloid aalnilBeruDi ■>r> CJypcutrold.
In Eekinut, the apical extremities of the ambulacra abnt npou the five smaller of the ten single plates which snrronnd the periproct. Each of these is perforated, and supports the ejeepot ; it is thence called an ocular plaia. The apical extremities of the interambnlacra, on the other hand, cor- respond with the five larger plates, which alternate with the ocular plat«B, and, like them, are perforated. The aperture is, however, larger, and constitutes the exit for the generative products. One of these five genUal plates is lai^er than the others, and presente a peculiar porous convex surface, which is the madreporic tubercla Qi rnodRi-
570 THE ANATOMY OF UfYEBTBBBATBD AKIM ALS.
porite. The latter is therefore interambulacnil in pontioiiy as in the Star-fish.
Compariaon with the elongated Echinoderma ahowa that the madreporite lies in the right anterior inter-radina of the sea-urchin, so that the anterior ambolacmiii is that which lies to the left of the madreporite, when the latter is directed forwards. In consequence of being able to dia- tingaish this odd or anterior radios, it is possible in any of the Echinidea, to separate the three anterior ambulacra, as the trivivfii, from the two posterior, the hivium ; and in the fossil genus, Dysaster, this separation of the ambulacra into trivium and bivium exists naturally. MiiUer has pointed out that in all the flattened Echinidea^ with a special ambulatory surface, the latter is formed by the bivial ambulacra and interambulacra, while, in the similarly modified Holothuridea, the animal rests upon the trivium.
Within the circle formed by the genital and ocular plates the x>eriproct presents a variable number of calcificiutionB, of which one, the anal plate, is larger than the rest. The anus lies excentrically, between this plate and the poeterior margin of the periproct.
With the exception of certain palaeozoic forms (Palachi' nus), the composition of the skeleton of the Echinidea is always essentially similar to that which has juat been de- scribed ; but the form of the body and the relative positions of the anal and oral apertures may vary very much. In the Echinoida (Cidaris, Echiniu) the body is spheroidal, and the oral and anal apertures are opposite and central, or veiy nearly so. In the Clypeastroida {Clypeader, Echinocyamui) the form of the body varies from a spheroidal to an ex- cessively flattened and even lobed shape. The mouth re- mains central, but the anus varies in position, from the apical surface to the margin, or even to the oral aurface, aa iu Echinaeyamus. In the remaining division of the Echinidea, the Spatangoida (Spatangus, Amphidotugf Anan-^ chyles), the form is usiudly a somewhat depressed oval, and both the oral and the anal apertures are excentric. The madraporite asoid tVie g<^mtal and ocular plates, on the other
.*' M
THB BCHIHIDBA. 571
hand, remain in the centre of the ahoral region in all the Echinidea,
The amhulacra present important variationB in the three divisions of the Echinidea, In the Echvnoida they are ho- mogeneofiu, presenting the same composition from their oral to close to their apical extremities, and haying the pores and pedicels similar throughout. Furthermore, the ambulacra are widest in the middle, and taper gradually to each extremity {Echinus), or are of nearly the same size from one end to the other {Cidaria),
In many Clypeasiroida, on the contrary, the oral and the apical portions of each ambulacrum differ very widely, or are heterogeneous. The apical moiety is usually very wide in the middle and tapers to a point marginally, where it joins the oral portion. Hence there is an appearance of five petals diverging from the apex ; and such ambulacra are called petaloid (Fig. 142, B). In the oral portions of the ambulacra, on the contrary, the pores are either scattered widely over the ambulacral, and sometimes over the inter- ambulacral, plates, forming |>or6-arece ; or they are arranged in bands which ramify over the inter-ambulacral as well as the ambulacral plates, giving rise to what Miiller has termed pore fascuB, In the Spatangoida (Fig. 143) the ambulacra commonly present the same heterogeneous character ; but the oral portions are not arranged in fascia ; and it not unfrequently happens that the anterior ambulacrum becomes more or less abortive, so that only four petals are obvious on the apical surface, instead of five.
The growth of the shell of the Echinidea is effected in two ways ; partly by addition to the circumference of the existing plates, partly by the interpolation of new ambu- lacral and inter-ambtdacral plates at the apical end of each series between it and the ocular or genital plate, as the case may be. New plates are never added to the oral extremity of the corona proper.
The surface of the plates of the corona in the Echinidea is covered with minute rounded elevations, or tubercles, to which are articulated the spines so charaAtAxv^^^ ^1 K^<^
572 THE AyATOMY OF IKYBBTEBRATED AKIMAIiB.
group. The tabercle may be either simple or marked by a central pit, into which and a corresponding pit on the head of the spine a ligament of attachment is inserted. Further- more, capsular muscular fibres connect the neck of the spine with the base of the tubercle, and effect the Taried movements of which the organ is capable. The spines of the Eehinidea vary very much in form and size, from the close-set yelvety pile of Souiellay or the delicate spoon- shaped blades of Amphidotus, to the long-pointed lances of Echinus and the great clubs of Cidaris, Even on the same Echinoderm the spines may, as in the two latter genera, yaiy very much in appearance ; and it becomes necessary to dis- tinguish those large ones which form a continuous series from one end of an ambulacrum or inter-ambulacrum to the other, as primary spines, from the other less complete aeeandary and tertiary series.
Lov^n * has drawn attention to the existence in all the Eehinidea, except Cidaris, of certain minute spheroidal bodies, rarely more than y^ of an inch long, which he terms ephceridea. They occur upon the ambulacral plates, and especially upon those nearest the mouth. Each contains a calcareous and more or less dense and glassy skeleton, which is articulated with a corresponding tuberde as if it were a miniature spine. In some genera, these sphoBrtdeOf to which Loven ascribes a. sensory function (probably auditory), are sunk in fosssa of the plate to which they are attached.
Scattered among their spines, the Eehinidea poasosi pedicellarice, which are usually provided with long slender stems, terminating in oval heads, divided into three jaw*like processes. The latter are strengthened by calcaz«oi» ossicles, which articulate with an ossicle contained in the basal part of the head, and a calcareous rod is iiaoally developed in the stem.
In the Spatangoida, when the skeleton is cleaxied« its surface is, in many cases {Amphidotua, Briseue, Spaiem^ru), marked by one or more symmetrical bands of doee-aet,
* ^l.\xi!ieAvaT\«&^f3t\T«ir\\^* NATS,
TBI lOHiniDKA.
minato tnberdeB (Fig. 143, e,/, 9). During life, slender Bpines Are attached to tlieM tubercles, the caloareona skeleton of
Fig. la.—AmBhiihtv cordalti.—X. Viswed flvm Bbovfl. B. Fmn beblnd. a, M. Trivium, or uiMrior and uit«roUtenl ambnlAim. cc. BiriuTD, or posMroLstenl uubulacn. d. Mkdreporio tabercla, •nrrouoded by the genitnl tperturei. t, Intr»-pet*loiu KwUta. f, Circun^aal Kmila. jr. Sub-uil KMdtd. k. Amu. t, Intn-*emltkt poretorbivlftl uabuUcra. C. Sginlta m&gulfled. a. Semilal Mber- clea. b. Ordinary taberclai. D. Semltal (pint. a. Teimliud co- ittgad, noa-dltetad porUcn. i. CUlstod tlMn.
fitfciolei. Scmitce lie oeue&u. u^_ . Bomc Renern. and iire called evb-a: otiicra aiirniiind the outer extremitii biila«ra, and are Urmed peripetahus, the inner taimiDations of their an {Ataphidotaa) (Pig. 143. A, B}.
If we tnm to the interior of the i we find in the Eehinoida, that unfa (Cidarit) intenunbnlaoral, plates of t corona are ptodoced into fire per procesaes, whiah arch over the amb the aurteula.
Besides these, processe* are dere^ lacral plates in Cidarii, which form etde of the ambnlacral canal, but d Clypeeuter, aiinilar procesees form in Uie flattened ClTpeaatroid, Scufej walla of the corona are united toget cul», eo that the cavity of the bod Bmall epace.
The Bpatangoida present neithf internal processes.
la the Eckinidea, the <S8oph^nB tuvnnd a CEecal diverticnlnm in i
middle Un«, while each half again Fig. IM.
ists of a. Ruperior
—A. Dentirf >p|Hnihu of ■ Clyptatlrid (mfler MQIIer),- a. ohu. dl Rotnlft. s. Tooth. B,C, U. t>(iitarrippantna(Am- ■ Untero) of ErJwt<a iplvrra. B. Two of the flre chief com- it p«rtj of tha lantern appoaed and Tieirad laterally. C. L»Ier»l
--■ " *--■- -■-<• of a ihigle pert. o. Prinolpal
Tiew, and D. back tIcit o
„ . «lp«l 1
_'. iSutara wilb iU fallow, b. Eplphrili. V. Satore of
flplphyiii with piineipkl piece, ir, Kotnla. a. Badioa OTOomfwHi. •. Tooth.
576 THE ANATOMY OF XNYXBTEBRATBD AJriMAL8.
epiphysis, and an inferior principal portion, united together. Each'alyeolns serves as the socket for a long tooth (e), shaped somewhat like the incisor of a Bodent, harder externally than intemallj, so as always to develope a sharp edge with wear. The tooth constantly grows from its upper extremity, while its lower half becomes united with the wall of the alveolus. The five alveoli, if fitted together, form a cone, the applied surfaces of which are united by strong trans- verse muscular fibres, while superiorly, the epiphysee of each pair of alveoli are connected by long radial piecee — the rotulcs (c) articulated with their edges. To the inner extremity of each rotula, finally, a slender arcuated rod, presenting in^cations of a division in the middle of its length, is articulated, and, running outwards parallel with the rotula, terminates in a free bifurcated extremity. This is the rekfittff {d).
Altogether, then, the Lantern consists of twenty prin- cipal pieces — five teeth, five alveoli, five rotnlsa and five radii; of which the alveoli are again divisible into four pieces each and the radii into two, making a total of forty pieces. In their normal position, it must be re- membered that the alveoli and teeth are intenunbolacral, while the radii and rotulsB are ambulacral. BesideB the inter-alveolar muscles already described, this complex ap- paratus has protractor muscles arising from the intcr- ambulacral region of the oral edge of the corona, and inserted into the upper part of the alveoli; alender oblique muscles, with a similar origin, but inserted into the radii ; transverse muscles connecting the radii together: and retractor muscles arising from the arches of the auri- culse, and inserted into the oral ends of the alveoli.
A similar, but less complex, oral skeleton exists in most Clypeastroida (Fig. 144 A), but nothing of the kind has yel been discovered in the Spatangoida,
In the Echinidea, the circular ambulacral Tessel liei between the oesophagus and the alveoli, and is nsoally pro- vided with five sacculated polian vesicles. There is a
THB BCHINIDBA. 577
in Ou2a9*M, wliicli extends nearly in the axis of the body from the circular Teasel to the madreporic tubercle. Five radial ressels ran up the middle of the inner surface of the ambulacral plates, which thej reach bj passing from the circular canal, outwards, beneath the rotuba, when these exist ; next, downwards, external to the inter-alveolar muscles; and then, outwards, through the arches of the auriculffi: these give off branches on each side to the pedicels, the bases of which open into large ambulacral vesicles. The circular ambulacral vessel of the Spaian* goida has no polian vesicles, and no vesicular appendages ; in the Clypeasters there are many vesicular appendages, but no polian vesicles. In most Echinaida, all the i>edicel8 are expanded into sucking-disks at their extremities, and are here strengthened bj a calcareous plate or plates ; but, in Echinocidaris and some other Echinoida, the pedicels of the oral portion of the ambulacra onlj, have this structure, vehile those of the apical portion are pectinated, flattened, ajid gill-like. Again, in the heterogeneous ambulacra of the Clypeastroida and Spatangoidaf the forms of the pedicels ▼ary much. Thus Miiller distinguishes four kinds of pedi- cels in the Spatangoida — simple and locomotive pedicels, without anj sucking-disk; locomotive pedicels, provided vnth terminal suckers, and containing a skeleton ; tactile pedicels, with papillose expanded extremities ; and gill-like pedicels, triangular, flattcDcd, more or less pectinated lamelke. Two or three of these kinds of feet may occur in anj given ambulacrum, and those which lie within a semita are always different from the others.
In the Clypecuftroida, the petaloid x>ortion8 of the ambu- lacra x>08ses8 branchial pedicels, interspersed with delicate locomotive pedicels, provided with a calcareous skeleton and with a terminal sucker. The latter kind alone extend on to the oral portions of the ambulacra.
The circimioral nerve of Eekinus surrounds the OBSopha- ^us near the mouth. It has a pentagonal form, and is enclosed bj the alveoli, between which the ambulacral nerves pass, over the peristome and throag,h tK^ ^ac^^^ ^V
J
ends of the ambiilacral nen'es.
The peritoneal space is fllleil bj which is kept conslantly in motion I the parietcs and the contained visi this fluid appears to be facilitated except Cuiaru, by five pairs of sp developed from the perietome; whi and Spatangoida, which poesera the monly termed ambulacra! gille, ther
In the Echinidea, a circular p«ei: branches are given off to the genit the Uina. The alimentary canal vesaela, one on the side of the meee on tlie free aide (ventral), which com network in its walla ; and besidea nuining parallel with the madrep< ing inferiorlj in a circnlar vessel circular ambnlacral vessel, around detoribed as a ' heart't
^le genital oi^ans are aa^ciilate large size in the breeding season the porea on the genital plates, t dnota are extruded. Hoffmann I doid of the males fnll of apermato
In tho Eehinidea, aa in the Ophv i» a Plvieai, and has a skeleton f<
I DBTSLOPmtHT 07 THE ICBIItlDSA, Fig. 145.
45.— Derelapumit of tn Ediiiud. (After MQIIer.)— A. Echfno- inni of KdkSim piMielhu In ths sutruU itage. B. Fnllj de- ped EgblmnwdlDDi {PlMtm) of the wne apecle* ; a, mouth ; omMh Bad IntcMliie ; c, UDi ; A P, proeeMes of (he body Into :h prolonnUoni of tbe lotenul ikelatoo eitand. C. The
which prohmgftUoni of tie lotenul ikelatoo eitand. C. The EehiDopBdimn of an Eohinld la whloh the Eohlnodenn ti n fu •dTanoedaattiMipfDWipedleali, >BdIl«dleri)]Bi■lwmVlliia^A. "%•
580 THB ANATOMY OV IKTEBTSBBATSB AVIMAIiB.
EchinofMedium of EduHus Ihndiu : a, moath ; J gnllet : 6, staOMfih ; b\ intestine ; c, rudimentary Echinoderm ; c', the mmlmiAcrmI sae ; c", the external opening of its duet \ a ti, ff,B, the ptoec«es of the body.
which support the processes into which the body, in the region of the ciliated bands and elsewhere, is prolonged.
The origin of the ambulacral system, before it has the form of a caecum with a dorsal pore, has not been made out. The blind end of this csecum lies on the left aide of the alimentary canal, and is connected with a discoidal body, which is situated on the left side of the stomach ; a similar body appears on the right side. Doubtless these discoidal bodies answer to the peritoneal diyerticula of the alimen- tary canal of the EchinopsBdium in other Echinoderma.
The blind end of the tube enlarges, and giyes rise to a rosette, whence the ambulacral vessels proceed; and a depression of the integument of the larva, forming the so-called wnbo, extends inwards to this. At the bottom of the umbo, a new mouth opens through the centre of the rosette into the gastric cavity of the larva, the primitive oesophagus being abolished. The larval skeleton undergoes resorption, but the rest of the Echinopsediimi paases into the Echinoderm.*
Loven has recently drawn attention to the fact that, in young Echinids^t the plates of the apical region are not only more conspicuous in relation to the corona, bat differ somewhat in their arrangement, from those of the adult. Thus the anus is at first wanting, and the anal plate, which occupies the centre of the apical area, is relatively large ; it is united by its edges with the five plates, which, im- perforate in the young, will become the genital plates in the adult. The five ocular plates are also imperforate, and
* See, in addition to the me- A. Agassis, 'Revision of the
moirs of MfiUer and Metschnikoff Echini,* published in the ' lUoi-
already cited, A. Agassis, ** On the trated Catalogue of the Mnseini
Embryoloffy of Echinoderms.** of Comparative Zoolofnr at Har>
O Menu Ameriean Academy of vard ('oHege,' is also mU of ia-
Scienoei.* 1864.) formation respeotinff tba joog
f The adnAitUle hmmi^^ oi itaJiei of the Eehinm.
THS CBXKOIDBA. 581
are disposed in a circle outside that formed by the genital plates, their inter-spaces being occupied bj interambu- lacral plates. The apical region of an Echinid has thus, as LoT^ points out, a most striking resemblance to the cal jx of a Crinoid ; the anal plate representing the boidUa ; the genital plates, the parahasalia ; and the ocular plates, the first rctdialia.
The Obinoidea. — This remarkable group, which
abounded in former periods of the world's history, is
represented at the present daj only by the genera Antedan
{ComahUa), AcHnometra, Comader, Fentacrinua, Bhuocrinus
and Holopus,
The first three genera are capable of locomotion, while the next two are attached by long articulated stems to submarine bodies. Holopus, which is but imperfectly known, appears to be fixed by a short thick unjointed prolongation of its base.
Bhizoorinvs lofotenaU (Fig. 14f6), which has been yery carefully and elaborately described by Sars,* is a small animal which does not attain more than three inches in length, and lires at great depths (100-^300 fathoms or more) in the sea. It consists of a relatively long, many-jointed stem, from many of the articulations of which, branched, root-like filaments, or cirri, are given off; at the summit of this is seated a cup-shaped body, the calyx, from the margins of which 5-7 arms {brachia) radiate. To each arm is attached a double series of alternating pinnuUB, The mouth is situated in the centre of that part of the perisoma which forms the surface of the calyx opposite to the stem. The oral aperture is circular, but five (or sometimes only four) triangular lobes of the perisoma, with rounded free ends, project over it, and, when shut, dose it like so many valves. From the intervals between these oral valves five (rarely four) grooves traverse the oral surface of the calyx, and extend thence throughout the whole length of each arm, giving offsets as they go to the pinnules. Thus the
* ' M^moires poor lervir Ik It eonnilttance des Crinoldeft xiWD&j^ >M(Ak.
582 TH* i.trATOMT or nrrxBTiBRiTiD asihaia.
oral anrface ctf mcb arm and of eaoh piiuitil« i> deeplj «xoavat«d.
Betweem Uie oiroolar lip and the oral Talvea, aoft flexible tontaouliform p«dioela are attached in a siiigie aeriM. Two pairs of pedioela correspond to erei; valve, each pair aiiaiiig oppoeiU the basal angle of a vtlre. Tbeae pedioela are liollow, their surface is papillose, and the onter or radial pedicel of each pair is veij contractile. Pedicels of tlie
Fij. 146.
Fig. l«.-J(*itt«riMH lofiiUiuu. (After 8»r».>
I. AAitocmiu entire ; a, enltrsed upper Joint of the stem ; i JoinU of llM ■lem ; c, cirri ; rf, br«hU,
II. Caljx and trms, wiUi tha ■uminic of the ■ten of k
having fire well-developed bnchia; n, u before; i, flnt iwluH; r>, H, Mcond and third rsdialg ; b', flnt brublftl , s, p, piiuinlea.
III. Upper part of the ■tern ud oral &oa of the oaljx, Tiewrd obliquely ; v, lower part of visceral mis ; «, r, teDlacnlai eroomi 0, oral valve* ; t, oral lentaclee ; an, aDOi.
same general character are continned througboat the braohial and piminlar grooves. The uva u ntaBtodk «X %b ood. of a conical pnanineoce
THB CBINOIDSA. 583
between two of the grooves on tlie oral face of the calyx, and is therefore interraddal in position (Fig. 146, III. an).
The skeleton consists of very numerous pieces resulting from the calcification of the perisoma. In the stem thej hare the form of elongated, sabcjlindrical, or hourglass- shaped, joints {artieuli), the opposed faces of which are united by strong elastic ligamentous fibres. The centre of each is traversed by a longitudinal axial canal, which ex- tends through the whole length of the stem and is occupied by a soft but solid substance. The distal joint of the stem is not directly fixed to the surface to which the Crinoid is attached, but is connected therewith by the branched cirri which proceed from it. Each cirrus has a skeleton com- posed of joints or artieuli, somewhat like those of the stem and traversed by a prolongation of the axial canaL Similar cirri are developed from a larger or smaller number of the artieuli of the distal portion of the stem.
The proximal joints become gradually shorter in pro- portion to their length, until they assume a discoidal form. It appears that new artieuli are continually added at that end of the stem which lies nearest the calyx.
The summit of the stem, or the base of the calyx, is formed by an enlarged, solid, pear-shaped ossicle, which is probably formed by the coalescence of several artictUi, Upon tiiis follow five pieces (first rcicUalia) closely united together and with a central piece, which probably represents the hamilia of other Grinoids. The first radial corresponds in direction with the origin of one of the arms, and is followed by a second and third radial. With the third radial is articulated the first of the brachial ossicles, which constitute the skeletal support of the unbranched brachia. The pinnules are also supported by a series of elongated calcified joints, the basal joint being articulated with a brachial ossicle and the distal joint pointed.
The axial canal dilates in the enlarged pyrif orm ossicle above-mentioned ; and, from the dilatation, branches, which traverse the radial and the pinnular ossicles, are given off. There is a calcareous plate in the substance of eaAk cst^
584 THE ANATOMT OF INYEBTEBBATBD AVIMALS.
Talve, and minute reticulated calcifications are aoattered through the perisoma of the oral face of the disk.
The sides of the radial grooves are provided thronghout with a double series of oval calcareous plates — ^the margindl lamelloR — which are disposed transversely to the groove^ those of opposite sides alternating with one another. They can be erected or depressed ; and, in the latter case, overlap one another like tiles.
In Pentacrinus, the long stem is fixed by its distal end, and the pentagonal articuli of its skeleton give off, at intervals, whorls of unbranched cirri. No distinct basal piece is known, but the calyx appears to begin with the five first radialia. At the third radiate, the series bifurcates into two series of brcuihialia, and these again bifurcate to give rise to the palmaria, which support the free arms. There are marginal lamellae along the sides of the tentacular grooves, and a longitudinal series of calcareous ossicles occupies the fioor of each groove. The anus is situated upou an elevated interradial cone.
The body of- an adult ComcUula (Antedon) answers to the calyx, with its brachia, in other Crinoids.
The centre of the skeleton is constituted by a large centrO'dorsal ossicle, articulated with the aboral face of which are the numerous cin*i, by which the ArUedon ordi- narily grasps the bodies to which it adheres, though it ie able, on occasion, to swim freely about. This centro-dorsal ossicle appears to be the homologue of the nppermott part of the stem in the PerUcusrinus. There are five divergent series of radialia^ each containing three oeaicles. The first radials, or those nearest the centro-dorsal plate, are closely adherent to one another and to the centro-doml plate, and are not visible on the outer surface of the calyx. The space left between the apices of the five first radials is occupied by a single plate, the rosette* which is formed by the coalescence of the five hcisalia present in the larva.
The anatomy of the soft parts of the Crinoidea hsf
* CarpenteT, 'On the Struoture, Physiology, and DevelopaieBt «f
^THX CBIKOIDBA..
585
been most thoronglilj inTeetigated in tlie genos (hmattUa (ArUedon)*
The month leads, by a short, wide gullet, into a spacionB sacculated alimentary canal, which is coiled nx>on itself in such a manner as to make about one turn and a half around the axis of the body ; and then terminates in the projecting rectal cone which, as has already been seen, is situated interradially, on the oral face of the calyx. The central cavity, included by the coil of the alimentary canal, is occupied by a sort of core of connective tissue, and has received the name of eolwmella, but it must be under- stood that it is not a distinct structure. Bands of con- nective tissue connect the outer periphery of the alimentary canal with the perisoma.
The five triangular lobes of the perisoma, which surround the mouth like so many valves, contain no calcareous skeleton in the adult Antedon. Within these lobes, attached to the oral membrane, there is a circle of tentacula. From the interval between each pair of oral valves, a groove radiates outwards over the surface of the calycine perisoma and speedily bifurcates; one branch goes to the oral surface of each of the arms and runs along it to its ex- tremity, giving off alternate lateral branches to the pinnules in its course.
These grooves are the ambulacral grooves. Their sides are, as it were, fenced by small lobed processes of the peri- soma ; and, on the inner sides of these processes, groups of minute pedicels take their origin from the sides of the floor of the groove. A thickened band of the ectoderm occupies the middle of the floor, and so strikingly resembles
♦ E. Perrier, " Recherchei but rAnatomie de la Comatula rotacea** ('Aroh. de Zoologie Experimentale/ 1873). Semper, *'Kurze anatoinisohe Bemerkun- gen iiber Comatula** (* Wfinburg Arbeiten,' 1874). Ludwig. '' Ztir Anatomie der Crinoideen " ('Z«itschrift fur Wis*. ZooL' 1876). Carpenter** On the Struo-
tore, Phvsiology, and Develop- ment of Antedon" (* Proc. Royal Society,* 1876). Greef, **Ueber den Ban der Crinoideen ** (* Mar- burg SiUiingBberichte,' 1876). P. M. Carpenter, ** Remarks on the Anatomy of the Arms of the Crinoids " (* Journal of Anat. and Physiology,' 1876).
586 THB ANATOMY OF INYEBTBBRATBD AKIKALS.
the ambolacral nerve of the Star-fish, that the homology cf the two, first asserted by Lndwig,* cannot be doubted. Immediately beneath it runs a small canal, discovered bj Dr. Carpenter, and termed by him the teniaeular oonol, which gives off lateral branches to commnnicate with the cavities of the pedicels. A second much wider canal — ^the suMeniacular canal — lies beneath this, and is divided by a longitudinal septum. But the septum is incomplete at intervals, and thus the two canals communicate. A third, still larger, cceliac eanal, is interposed between the floor of the subtentacular canal and the axial skeleton of the arm.
Where the arm joins the calyx, the tentacular canals run beneath the ambulacral groove to the gullet, around which they are united by a circular canal, from which numerous short diverticula, resembling the vasa- ambuUMcraUa cavi in the Opjiiurids, described by Simrock (l.c,\ depend. The subtentacular and cceliac canals communicate with channels in the perivisceral tissue, on the oral or the abonl face of the visceral mass; and these channeki appear, eventually, to open freely into the cavities by which the columella is traversed.
In the partition between the subtentacular and the coeliac canals there lies a cellular cord, or rachis, which can be traced back into a reticulation of similar tissue in the visceral mass. The genital glands, contained in the pin- nules, are enlargements of lateral branches of this zadiit. But the rachis is apparently only an extension of tbe mesodermal tissue of the visceral mass, comparable to that in which the genitalia are lodged in the Star-fiahes ; and the multiplication of the genital glands may be regaided as a further extension of the structure which obtains in Brisinga. Thus it would seem that the position of the genital glands in the Grinoids is not so anomalous as it at first appears to be.
The centre-dorsal tubercle contains a cavity, with whii^ the canals which travei*se the osaicula of the cirri, the calyx, the brachia and the pinnules communicate. Thii
• *lA\a^\iTMlt^^W«a.'&»W 1876.
THS DBYUiOPMBHT OV THB CBIHOIDEA. 587
waa cozLBidered by Mailer to be a heart. It proTes, <er, to be largely filled by solid tissue, whicb is con- L not only into all the canals which trayerse the la» bat also into the colamella, or tissae which ies the centre of the coils of the alimentary canal. Carpenter * is of opinion that so mach of this axial as occupies the cavity of the central tubercle, and is Lued throughout the ossicula of the calyx and arms, proper central organ of the nervous system ; f ound- is opinion partly upon the fact that, when this mass bated in a living AntecUm, a sudden contraction of all nacles of the arms takes place ; and partly ux>on the )ution of the ultimate ramifications of the axial in the arms. Qreef , on the contrary,t affirms that eee tracts can be injected, and retains the name of » * for the cavity of the centro-dorsal tubercle. I perisoma of the oral surface of Comaiula exhibits a number of minute circular x>ores, with thickened ir margins. Greef has discovered that these are the lal apertures of canals, with ciliated walls, which into the body cavity, and readily allow fluids to pass >r out of, that cavity.
h mature ovary of Antedon has a distinct aperture, 3fh wbich the ova are discharged, and to which they 3 for some days like bunches of g^pes. The testis ^pes no special aperture, but the spermatozoa appear discharged by dehiscence of the integument. se the discovery by Yaughan Thompson that Comaiula \ through a Pentacrinoid larval condition, the develop- of the free Grinoids has been the subject of various igations,^ and the following results may be regarded iblished. nplete yelk-division takes place. The morula ac-
roceedings of the Royal Trans.' 1865), Metschnikoff
,' 1876. 0 Bulletin de I'Acad. Imp. des
Teber daa Hen der Crl- scienoes de Si. Petenlx>arg,'
" ('Marburg Sitzunga- 1871), and especially GOtte
«,M876). ('Archiv fOr Mikroskopiscbe
WyviUe-ThomK>n (' P^U. Anatomia,' 1876V
588 THS AKATOMT OF IHYBBTSBSATBD ANIMALS.
quires an oval form, and developes four hoop-like bands of cilia, with a tuft of cilia at the hinder end. Between the third and fourth bands of cilia, counting from the anterior end of the Echinopsddium, the blastoderm becomes in- vaginated, and gives rise to an archenteron. In the in- terspace between this blind sac, the wall of which is the hypoblast, and the epiblast, constituted by the rest of the blastoderm, a mesoblast composed of reticulated cells makes its appearance. The blastopore closes, while the archenteron detaches itself from its attachment to the posterior ventral face of the larva, and becomes connected with an oesophageal involution formed at its anterior end. The archenteron next throws out three diverticula, of which two are lateral, and one is ventral. The lateral diverticula enlarge, and apply themselves to the rest of the archenteron* now become the intestine, from which they are soon com- pletely shut off, and converted into peritoneal sacs. The left sac thus formed lies on the ventral side of the intestine^ the right sac on its dorsal side. The walls of the two sacs become applied together, and form a circular mesentery. The peritoneal sac of the aboral side sends a prooess into the hinder end of the body, which has begpin to elongate, in order to give rise to the stem of the Pentacrinoid form.
The third, or ventral, diverticulum is shut off from the alimentary canal much later than the other two. It grows round the mouth, and gives rise to the circular ambolacrdL vessel, whence the tentacular canals are given off.
Ten plates, each consisting of a calcareous network and arranged in two rows of five each, next appear in the substance of the EchinopaBdium around the alimentaiy canaL From the centre of the posterior row, eight calcareone rings extend through the length of the body of the larfii enclosing the backward prolongation of the aboral peii* toneal sac ; and the series terminates by a broad disocudil network which lies on one side of the posterior end of the larva. This discoidal plate is that which occupies the attached end of the stem of the future Crinoid ; the xingt heooanift the tieiis «DdL V2^<& \^q ^ixcles of plates the hssil
THS DETBLOPMBKT OF THE CBIirOIDEA. 589
tl ossicula of the calyx, respectiyely. As the stem tes, new rings (ariiculi) are added at the junction of m with the calyx.
larva now fixes itself by the discoidal end of its irhich becomes relatively longer and narrower ; while rt of the body which contains the basal and oral plates, to be converted into the calyx, remains thick and Its broad end becomes five-lobed, each lobe answering oral plate. These plates separate like the petals lower bnd, and discover, in the centre, the wide per- t oral aperture. Between the margins of this and al plates, tentaonliform pedicels, at first only five, entoally arranged in groups of three, between every ! oral plates, make their appearance, alimentary cavity is still a mere sac, without intestine
I radial plates next appear in the wall of the calyx m the basal and the oral plates, and alternating with and, in correspondence with them, the arms grow 1 rapidly elongating processes, in which the other 1 are successively developed. The entire zone of the which is occupied by the oiigins of the arms, at me time widens, so that the oral plates, which remain the mouth, and the basal plates, which encircle the become widely separated. The intestine grows out diverticulum of the alimentary cavity and opens interradial elevation of the calyx, in which an anal is developed, nrhe young Echinoderm has now I into the stalked Fentacrinoid stage. hmatttla, the oral and anal plates disappear altogether, lie basals coalescing into the rosette, are hidden by rst radials, on the one hand, and the centro-dorsal de, which represents coalesced joints of the stem, on her. The arms bifurcate and acquire their pinnules ; he calyx, with its appendages, eventually becomes led from its stem as a free Comaiula, In the exist- alked Crinoids, such as Penio/crinm, on the other the segments of the stem acquire whoT^ ^1 ^\srcv^^^
590 THE AKATOmr OF nfYBBTBBSATXD ASUUkLA.
intervals, and no such modification of the uppermost seg* ments into a centro-dorsal tubercle takes place.
On comparing the facts of stmcture and development which have now been ascertained in the five existing groups of the Eehinodermata, it is obvious that they are modifications of one fundamental plan. The segmented vitellus gives rise to a ciliated morula, and this, by a process of invagination, is converted into a gastrola, the blastopore of which usually becomes the anus. A mouth and gullet are added, as new formations, by invagination of the epiblast. The embryo normally becomes a free Echi- nopsedium, which has a complete alimentary canal, and is bilaterally symmetricaL The cilia of its ectoderm dispose themselves in one or more bands, which surround the body; and, while retaining a bilateral symmetry, become varioudy modified. In the Holothwridea, Aateridea, and CWnotdeo, the larva is vermiform, and has no skeleton; in the Echinidea and the Ophiuridea it becomes pluteiform, and developes a special spicular skeleton.
If an EchinopsBdium were to attain reproductive organs, and reproduce its kind, I think that it cannot be doubted that its nearest allies would be found among the TwrMiant^ the Eotifera, the Chphyrea and the Enieropneutta,* But that which characterises the Echinodertrutta is tlie ta/i that the alimentary canal of the Echinopsddinm gives rise to an enterocoele, which again is subdivided into two
** In a report upon the * Re- searches of Prof. Miiller into the anatomy and development of the Echinoderms,' published in the 'Annals of Natural History' for July 1851, I drew attention to the affinities of the Eobinoderms with the Worms ; and in a paper on Ltiuiuularia BocialU^ read be- fore the Microscopical Society in the same year, I expressed the view that the Rctifera * are the ssimmwttqnnioi Echinoderm iHrvtti tad Md \Ytt wBft i^is^^
to the Echinoderms, that tke Bydriform Polypi hold to the Medusae/ and that they * oooBset the Echinoderma wiUi the Kf" matidK and the Kematsid Worms.' When they wen pob- lished, those who did not inoit these vieiit's, ridiculed xttm. Nevertheless, thoush aoiMwIiit crudely expressed, I think it vffl be admitted they have been n^ Btantially justified by the uiognsi of knowledge durmg tM Itft ^s^oaxlcs of a centuiy.
THS AVFINITIBS OF THE ECHDrODBBMATA. 591
syBtems of cayities, one ambulacral and tlie other peritoneal, and that the mesoblaat becomee modified in accordance with the arrangement of these BystemB. The enterocoele may be formed by one diyerticalam or by three. In the former case, the first formed becomes subdivided into three, of which one is anterior, and two lateral, as in the latter case. The lateral diverticula give rise to the peritoneal cavity and its lining : the median diverticulum is converted into the circular ambulacral vessel and its dependencies ; and it is in consequence of the radiating disposition of the latter, and of the nerves and muscles which are related to it, that the Echinoderm possesses so much radial Sjrmmetry as it displays. It is clear, therefore, that the radial sym- metry of the Echinoderm results from the secondary modi- fication of an animal, which is primitively bilaterally symmetrical ; and that the apparently radiate Echinus or Star-fish is a specially modified ' Worm,' (using that term in its widest sense) in the same sense as the apparently radiate Corantda is a modified Arthropod.
Haeckel goes further than this, and supposes that each ray of a Star-fish or Ophiurid, for example, represents a Worm, and that the Echinoderm consists of coalesced vermiform buds, developed in the interior of the Echinopsedium. I must confess my inability to see that this hypothesis is supported by valid reasons. On the contrary, the more closely one compares the structure of the ray of an Echino- derm with the body of any known Annelid, the more difficult does it appear to me to be to find any real likeness between the two.
In order to find any analogy for the production of the Echinoderm within the Echinopsedium, on the contrary, it appears to me that we must look to the lower and not to the higher morphological types. Among the Hydrogaa, nothing is commoner than the distribution of the functions of life between two distinct zooids, one of which alone developes reproductive organs. In the former, the hydranih, radial symmetry is often hardly discernible (e.g, Calyeo- j^Ufridod) ; in the latter, the meduBoid^ it ia vearj xdk^^^
592 THE AKATOmr OF nrYSBTEBSATBD AXULkLB.
and especially characterises the arrangement of the gaatfo- vascnlar canals; which are offshoots of the alimentary cavitj, and if they hecame shut off therefrom, would answer to the enterocoele of the Echinoderm.
Suppose that from a hydronth such as that of a Diphfei, a medusoid were developed, and that instead of projecting from the exterior of the body, it remained hypodermic, spreading out between the ectoderm and the endoderm of the hydroid, and consequently superinducing a very marked radial symmetry upon it. The resulting form would give us a Ccelcnterate which would be a close analogue of an Echinoderm.
In a certain sense, an Actinozoon may be fairly regarded as such a combination of a hydroid with its medusoid; and, hence, it must be conceded that the parallel between the gastro-yascular system of the Ctenophora and the ambn- lacral system of the Echinoderms, instituted by the elder Agassiz, was well worthy of consideration. Shut off the gastro-yascular canals of a Cydippe from the alimentary canal, and they become an enterocoele, of which the pro- longations along the stomach may be compared with the peritoneal sacs, and those beneath the paddles with the ambulacral vessels of the Echinoderm.
But there is a long step between the admission of the force of these analogies, and the conclusion that the Echinoderms and the Ccdenteraia are so closely allied as to be properly associated in one natural assemblage of " Radiate " animals. On the contrary, the Echinoderm, hj its Echinopssdium stage, shows an advance in organisation, far beyond anything known in the CcelerUeraia ; and in the highly characteristic mode of development of its entexooode (the elucidation of which in the Star-fishes by Prof. A- Agassiz, is the most important advance in our knowledge of the Echinoderms made since the time of Miiller), the Echinoderm agrees with the hi^er, and not with the lower Metazoa,
JBbfciiiodermaia «\xrosi^ Vn ^&i& fossil state. Calcanout
THS OTSTIDSA. 598
plates, referred to tlie HohMhuridea, oconr in the meeozoio rocks, but are not known earlier. The Star-fishes are met with in the older Palffiozoio strata, nnder forms very similar to some of those which now exist. The Eehinidea abound from the Upper Silurian (PalcBchinus) onwards. The Palffio- zoio forms are spherical, and hare multiple interambnlacral plates and simple ambnlacra. Eehinidea of the modem type appear in the Mesozoio strata, the Echinoida first; while the Spatangoida and Clypecuiroida are of later date. This order of occurrence agrees with the embryonic develop* ment of the two latter groups, which ore more nearly spherical when young than subsequently.
The Orinoidea abound in the Pakeozoic and older Mesozoio rocks, gradually diminishing in number in later formations. The oldest appear to have all been stalked, and of peculiar and extinct types.
Three groups are wholly extinct, and are unknown in strata newer than the Carboniferous formation. These are the Cystidea, the Edrioasterida, and the BUuMdea,
The Ctstidea. — In their general characters the Cystidea 6ome very near the Orinoids. Crypiocrinue, the simplest form of the group, possesses a calyx supported on a stem, and composed of five baealia, five parabiualiai, and five radialia. An inter-radial aperture is surrounded by a cone of small plates, termed the pyramid. The antambulacral surface has no pores, but these were present in other genera, and sometimes are scattered irregularly {Caryocrinus) ; sometimes disposed in pairs {Sphceronites) ; while sometimes they take the form of parallel slits arranged in '* pectinated rhombs." The arms were free (Comarocystites), or recurved and closely applied to the calyx. They bore pinnules, which, in consequence of the non-development of the arms, were sometimes sessile on the radialia. In the species with recurved arms, the latter simulate calycine ambulacra. There is an aperture placed in the centre of the calyx at the point of convergence of the ambulacra; another small one on on& ^^^ ^\ ^(k^\ and, thirdly, the aperture of the p^-waxudu 'tVift %s^ ^'^
594 THE ANATOmr OF nrvXBTSB&ATBD ▲KIKAL8.
these ia commonly regarded as the mouth, the second as the anus, the third as the reprodnctive apertore.
The Cystidea would, on this interpretation, differ from all other Eehinodermaiii, except the Edrioasterida and HoUh thuridea, in the genital outlet being single ; but around the central aperture five pores are seen, in some species at least, to which a genital function has been ascribed. In any case, the Cyriidea would appear to come very close to the Orinoidea,
The Edbioastebida. — This gproup contains serend genera of extinct Echinoderms {Edrioaeter, AgelacriniUt, HemieyHiteM), which, in general form, somewhat resemble what the Asterid Chniaater would be if its angles were rounded off. Like the Cystidea, they possess an interam- bulacral pyramid, but they differ from them in that they have ambulacra perforated by canals which open directly into the cavity of the calyx, and that they possess no aims. The Edrioagierida have no stem, but seem to have been attached by the aboral face of the body.
The Blastoidea. — In PerUremites, the representatiYe of this order, the ambulacral and antambulacral regions are nearly on an equality : the body is prismatic or subcylin- drical. The pedunculated calyx is composed of three basal plates, two of which are doable. The aboral plates receiTe in their intervals five plates deeply cleft above. In the clefts lie the apices of the ambulacra, the oral portions ol which are included between the five deltoid inter^radisl pieces which surround the mouth. The cleft plates are not radials, but portions of the perisomatic skeleton of the abonl region. Surrounding the central, probably oral, aperture are four double pores, and a fifth divided into three. The median of these three seems to be anal, the others and tlie paired pores being genital. Each ambulacrum is lanceolate in form, and presents superficially a double row of ossidee, which meet in the middle line and support pinnules aft their outer extremities ; beneath them lies a mngle platob perhaps the homologue of the vertebral ossicles in the Ophm- ridea : beiie&t\i \\. ^b^^odii are parallel canals, the natoe <i which IB usLkno^m.
CHAPTER Z.
THI TDHICATA OB A^CIDIOID^.
:iB remarkable and, ia raaaj respecte, isolated groDp of Tine animala contains bstlt simple and composite, fixed 1 free organiBins. None attain a length of more than a r inchea, and some are minnte and almost ffiioroMOpio. rhe simplest members of the group, and Uiose the stmo- 'eof which is most readily comprehensible are the Appm- ulori«s ; minate pelagic organisms, which are toimd in all itades, and are propelled like tadpoles, by the flapping a long caudal appendage at the snrfaoe of the sea. ippendieuiaria flabeUtim (Fig. U7) has an OToid or flask- tped body (A), one-sixth to one-fonrlh of an inch in length. B append^^ (B) is from three to four times as long M the Ij, to one face of which it ia attached near, but not at, I posterior extremity. It is flattened, and is supported a firm central axis, which may be termed tlie uroekord g. 147, 1). The greater part of the bodj is usually in- ted by a stmctarelesa gelatinous tubetance. but, on its nded hinder extremity, this ceases to be distinguishable m the ectoderm.
>n the caudal appendage the polygonal contours of the [s of which the ectoderm is composed, are plainly dis*
The month has an orerbanging lip. It leads into a je pharyngeal sac, the walls of which are formed by the toderm. Posteriorly, thissacnarrowsintotbecwophagiw, ich bends towards the hnmal side of the body, and then ns into a epacions stomach, wfaich takes a toanneMe action and is dinded into two lobes, a rig.b.A, «jA«>^*ft^
596 THB UTATOHT OF ntVIBTIBBATID AJmULB.
From the left lobe, the mtcetine arises; uid, bntding inwards, turns abruptly forwards in the middle line, where it terminates midwaj between the oral aperture uad the attachment of the caudal appendage. The inteatjne there- Fig. 147.
n.— Sida~view of the bod;, with the caadal appendage forcibiy Ual I
A, tlie body; B, the wudii appendage; a, oral aperture; i, d* phuyni; e, an atrial opening ; d, the correipoadiDgaliema, vithiM '
-111-. . .• — . . 1 — jg. I £^ stomaoh; i, tm^;
leaf the onl end of the bob:
.; ff. ifsophagus ; /,Dii>ehord; n, celiulsr paiob at the lidi ' II, endoatyle ; /I, innglion; q, ciliated « , .,
nflFTB vim itH gacgliB, r ; m^ endoderm ; e r, ectodarm.
fore haa a hsmnJ fleiare. In the middle of its faamil aspect t^e endo^ernv lA \^ '^^iB.T^ni^aal cavity is raised into ^
▲PPBNDIOUIiASIA FIiABKLLUM.
«97
a fold, which projects into the blood cavity contained between the endoderm and ectoderm. The walls of the bottom of the fold are thicker than the rest, so that, viewed sideways, it has the aspect of a hollow cylinder. This is the endottyle^ (Fig. 147, n.)
The endoderm of the pharynx is ciliated, and the cilia are especially large over a narrow tract, or peripharyngeal band, which encircles the oral aperture at the level of the anterior end of the endostyle, and is continued back, as a hypopharyngeal band, along the middle of the neural face of the pharynx to the oesophageal opening.
On each side of the endostyle, the posterior part of the hsemal wall of the pharynx presents two oval apertures or stigmata (Fig. 147 d), encircled by cells, which are provided with very long and active cilia. Each stigma leads into a funnel-shaped oiriaZ canal, the open end of which termi- nates beside the rectum.t (Fig. 147, c.)
nrhe heart is a large sac, which exhibits rapid peristaltic contractions, and is placed transversely between the two lobes of the stomach. In the species which I observed no blood corpuscles could be seen, and the direction of the pulsations of the heart was not reversed at intervals, as
* So described and n&med in my *^ Observations upon the Anatomy and Physiolog^^ of Sal- pa and Pyrosoma, together with remarks upon Doholum and Appendicularia." (Phil. Trans. 1851.) In 1856, however, I sUted " With regard to the endostyle, I have nothing important to add to m v previous account, except that I believe it to be here, as in other ascidians, the optical expression of the thickened bottom of a fold or groove of the branchial w/6** ('Quarterly Journal of Microscopical Sdence,' April, 1856.) In my memoir on Pyro- soma (Linn. Trans. 1860, p. 205) the endostyle is stated to be ** in reality a lon^tudinal fold or diverticulum of the middle of the
hsemal wall of the pharynx, which projects as a vertical ndge into the hsemal sinus, but re- mains in free communication with the pharynx by a cleft upon its neural side."
t These stigmata were first de- scribed bv Ge^enbaur (** Bemer- kungen uber die Organixation der Appendicularien ; *^ Zeitschrift fur Wiss. Zoologie, 1855), who supposed that they communicated with canals of the interior of the body. However, l>y feeding Ap- pendkularuB with indigo, I demon- strated the communication of these stigmatic funnels with the exterior of the body. (* Quar- terly Journal of Microscopical Science,* Lc.)
598 THB AVATOKT OV imrBBTSBSATSD AVIICAU.
it is in the Asoidians in g^eraL M. FoU* howerer, states that, in other AppendiaUaricB, the xeTersal of the con- tractions of the heart takes phuse. Like myself he has been unable to discover anj blood corpuscles. There are no distinct vessels, but the colourless fluid which takes the place of blood makes its way through the interspaces between the ectoderm and endoderm and the various viscera.
The nervous system consists of a ganglion (Fig. 147, p) situated nearly opposite the anterior end of the endostyle ; in front, this gives off the nerves to the sides of the mouth, while, behind, it is continued into a long cord (s), which runs back beside the oesophagus, and between the lobes ol the stomach, to the base of the appendage. It then passes along one side of the urochord to its extremity, giving off nerves at intervals. At the origins of these nerves aggre- gations of ganglionic cells are situated (Fig. 147, 1). The most anterior of these ganglia is the largestf
A rounded otocyst containing a spherical otolith is attached to the ganglion, and a small ciliated sac, whidi opens into the pharynx, is in close relation with it (Fig. 147, r, q), M. Fol describes a number of fine tactile sete situated around the oral aperture.
The urochord, which constitutes the axial skeleton of the appendage, is transparent, rounded at each end, and bounded by a delicate membrane. The remains of the oeUs of which it is composed are to be seen in it, here and there, as ramified corpuscles lodged in its periphery.
The only muscles hitherto observed in Appendieuhm are two sheets of striped fibres interposed between ths urochord and the cellular ectoderm of the appendage.
The reproductive organs occupy the rounded projection formed by the posterior part of the body behind the
* ' Etades sar les Appendiou- eularut, counts this ss the
Imiros,' 1872. ganglion of the nervous systta,
f ' Quarterly Journal of Micro- and states that a fine eaaal tia-
scopieal Science,' 1856, pp. 8, 9. verses both the ganglia aad tht
If. Fol, who filndt the same longitudinal nerve, arraagtment in othst ApfimdU
APPSHDIOUULBIA VLABBLLUX.
599
digestive canal. The testiB (Fig. 147, lb) is a large oellxilar mass which fills the greater part of the cavity of this pro- jection in the adult. When f ally formed, it is resolved into spermatozoa with rod-like heads about yi^th of an inch long and very fine filiform tails. They escape by the dehiscence of the testis.
I have never met with AppendieulariaB containing ova, nor do any other observers, except M. Fol. appear to have been more fortunate. The latter, however, states that these animals are hermaphrodite (Oikoplewra dioiea apparently is dioBcious), and that the ovary is developed later than the testis.*
Two singular rounded x>atches of a cellular structure (Fig. 147, II. m) are interposed between the ectoderm and the endoderm on each side of the anterior end of the endostyle. Similar bodies occur in other Ascidians, but their function is unknown.
One of the strangest peculiarities of the AppendicularuB is the power which they possess of excreting from the surface of the ectoderm, with extreme rapidity, a mucilag^ous cuticular investment, in the interior of which, as in a spacious case, the whole body is lodged. This is what was originally described by Mertens as the " house " of the Appendicularia. It is obviously the homolog^e of the test of other Ascidians, which is often adherent to the ectoderm by only two or three points; but no cellulose has been discovered in it. According to M. Fol, who has studied the formation of the ** house " with great care, the AppendieulatioB have no proper test, and what I have described as the structureless gelatinous investment of the anterior part of the body is the commence- ment of the *' house." It increases, assumes a peculiar
larve,ne me pamtdiff^rer en rien de oelui dee Ajoidies ; et eomme d'autre part, la petiteaie de ces ceofB et la difflonltd qu'on a de lea obtenir lea rendentpea favorablet k Tetode, je n*al pas juge kpropos d'approfondir davantage oe m- jeO^(l. c. p. 1.)
* I must confeBS that M. Fol's figures and descriptions of the ovary and ova are not satisfactory to me, and his dismissal of the suli^ jeet of their development in the following paragraph is tanta- lising :—
** Le d^veloppement, que j*al pu juivre jusqu'a la formation de la
600 THB ANATOMY OF IHYSBTBB&ATBB AITIMALB.
fibrous stmctare, and in the course of an hour, in a Tigorous animal, it is separated as an envelope in which the whole body is capable of free moYement. In front, it presents two funnel-shaped apertures supported by a filnrous treUis-work, which lead down to the cavity in which the body is con- tained. A spacious median chamber allows of the free motion of the tail. After a few hours the animal deserts its test and forms another.
In the great majority of those Timi4saia which are fixed in the adult state, the young leave the egg in an actire larval condition, and resemble Appendictdaria in being propelled by a muscular appendage in the axis of which lies an urochord. The body and appendage, however, are in- vested by a coat, or test, impregnated with cellulose, and the former presents some importuit structural differences from that of AppendictUaria, After a free existence of a certain duration, the body of the larva fixes itself, the appendage withers away, and the young animal assumes the ordinary form of a fixed Ascidian. It may remain simple, or it may develope buds and give rise to a compound organism or Ascidiariwn, consisting of many Ascidiozooids united to* gether.
All the fixed Tunicates present two, more or less closely approximated, apertures; one, oral, leads into the ali- mentary cavity, the other, atrial, opens into a chamber, the aifiwn, into which the faeces and genital products are poured. During life, when these apertures are open, a current sets into the oral and out of the atrial opening. But if the animal is irritated, the sudden contraction of the muscular walls of its body causes the water contained in the brachial and atrial cavities to squirt out in two jets, while both apertures are speedily closed.
The apertures are much further apart in some fomii than in others, and in certain of the BotryUicUB they ait almost terminal. In the pelagic genera Pyrosama (Fig. 150), Doliolwm (Fig. 151), and Salpa (Fig. 152), the atrial and oial apertuxea are at o^i^oiatb csada ^1 '^'^ longest <iii^w^^^ ^
THK TVNICATA,
the body ; and, in the two latter, locomotion is effected l^ the contraction of txansrerse mnscnlar bands, which drives
Fig- Its— PAoZ/tuu mattmla.—ttM iMt la removed, ukd bwdly more of the ■nimal repreuuted than woald be leea In ■ longiludin*! ■eedoo: a, onl apertiini ; b, nnglion ; c, elrolct of tentaelM; it, bnachUl lao, the three rowi of aperturee in Iti upper part iadieate butdoDOtrapreeent the lUgmala: «, the langueU ; /, the (Eeopbagea^ opening; g, tbe nomaoh ; A, the luteatine: i, the anw, t, 41>ft atrluin ; I, the atiU ^ertore ; wl, the endMqVv, ii,<&i«\maX.
602 THB ANATOmr OV nmBBTSBRATSD AVDIALS.
the water out of the one aperture or the other, and canaee the body to be propelled in the opposite direction.
When one of the simple fixed AscidianB, aaoh as a Phal' lugia (Fig. 148) or a Cynthia, is laid open by a section carried through the oral opening, at right angles to a transrene pkme passing through its centre, the month is found to open into a large pharyngeal dilatation, termed the bra^ ehidl sac (Fig. 148, d), A series of simple or pinnatifid ten- tacles (Fig. 148, c) is seen encircling the oral aperture, at some little distance within the margin of the lip, which is usually divided, like that of the atrial opening, into four or six lobes. Immediately behind the tentacular circlet is a ciliated peripharyngeal band.
On that side of the branchial cavity which is furthest away from the atrial opening, a pair of delicate lip-like f ddB ext€»ad, parallel with one another, from the periphaiyngeal band, along the middle line of the branchial sac, as far as the opening of the OBsophagua at the opposite end of the branchial sac. The interspace between these leads into a fold of the endoderm, lined by a thick epithelium and forming the endo- style, and, in the middle line of the peripharyngeai hcmd, on the same side as the atrial aperture, there is a tubercuhur elevation, which contains a ciliated cavity and answers to the ciliated sac of Appendicularia. The walls of this sac are variously folded, and, consequently, the surface of the tubercle presents a more or less complicated pattern. Con- tinued backwards in the middle line, as far as the obbo- phageal aperture on this side of the branchial sac, there are sometimes one, sometimes two longitudinal 1«.wip1)<p, the hypopharyngeal folds ; or there may be merely a ridge Bar- mounted by a series of tentacles, termed langueU (Fig. 148, e). The languct which is nearest the ciliated sac is often the largest of the series. Behind the peripharyngeal band, the lateral walls of the pharyngeal, or branchial, sac are perfo- rated by small elongated apertures, the stigmaia^ the edges of which are fringed with long cilia ; and, by means of thsse ftperturea, the cavity of the sac communicates with the atrinia.
THS TUVICATA. 603
The stigpnata are ammged in transyerae rowa and are usoallj very numerous. The retionlated wall of the branchial aao may be streng^ened bj longitadinal lameU», or it may be raised into few and distant, or many and close-set folds. In some oases, papilla of a compUoated form are developed from the inner surface of the sac, and its outer wall is always connected by Tascular trabecule with the parietal wall of the atrium. In some cases, {Molgula) the stigmata, instead of being elongated meshes, are coiled spirally. The atrial chamber (Fig. 148, h), into which the branchial stigmata open, is shown by laying it open from the atrial aperture, in the same way as the branchial chamber was laid open from the oral aperture. The atrial opening is thus seen to lead into a cavity, interposed between the branchial sac and the parietes, and lined ux>on all sides by a delicate membrane (the ihi/rd iwnie of Milne- Edwards) like a peritoneum. This membrane has a parietal and a visceral layer. The former is continued from the atrial aperture on to the parietes of the body, to the level of the i>eripharyngeal band in one direction, to a line par- allel with the endostyle in another, and to the alimentary and genital viscera in a third direction. From these va- rious lines, it is reflected on to the branchial sac, of which it forms the outer wall. At the margins of the stigmata it is continuous with the endoderm of the pharynx, and, at the aperture of the rectum, with the endoderm of the intestine. Thus the atrial membrane forms a bilobed sac, one lobe extending on each side of the pharynx, and opens out- wards by the atrial aperture ; it communicates by the stig- mata with the interior of the branchial sac; and, by the anal and genital openings, it receives the faces and genital products. The current which sets in at the oral and out at the atrial aperture is set in motion by the cilia of the stigmata.
The atrium of the higher Ascidians differs from that of Appendieularia, not only in extent, but in being single and not double ; and in its single aperture being placed upon the neural aspect of the body dose to the ganglion, while the atrial funnels of Appendieularick Qf[^sB^^QS!^fso^'^kiSi^\assBDa^
604 THE AKATOMT OF IHTKBTXBKATSD AVIMALS.
aspect of the body. The development of the liigiier TmmI- eaioy however, shows that the peculiarities of the atiium is them are of secondary origin ; and that, to begin with, there are two distinct atria, as in Appendieularia,
The oesophageal aperture is nsoally surrounded by a Tailed lip, and the short and wide oesophagus leads into a dilated stomach, whence a shorter or longer intestine pit)ceeds. The alimentary canal is always bent upon itself in such a iw^immj* that the anus terminatee on the neural side of the bod|f, in the atrial chamber.
In Clavelina, AmoHromeium, Didemnum, SyniefhySf and most of the compound Ascidians, the greater part of the alimentary canal lies altogether beyond the branchial sac, in a backward prolongation of the body whicb has been termed the abdomen, and is often longer than all the rest of the body ; the alimentary canal forming a long loop, and the direction of the axis of the branchial sac being continued by that of the gullet, stomach, and first half of the intestinei In the BotryllidcBt however, the stomach is bent at right angles upon the gullet, as in Appendieularia ; the intestine almost immediately turns forward, and then, turning shaiply upon itself, passes forwards parallel with the hinder paitol the branchial sac, on one side of which it opens into the atrium.
A similar arrangement obtains in Perop^oro, but the branchial sac extends backwards for a short distance on one side of the stomach. In the solitary Ascidians, the stomadi lies sometimes altogether behind the branchial sac (Peloiuns, / some Phallutice) ; but, usually, the branchial sac extends so | far back that the whole alimentary canal lies on one, usually ] the right, side of it. In Phalluna fiaonod^ns, tbe binder end of the branchial sac is recurved, and the oesophageal opening looks backwards to the fundus of the sac, instead of forwards to the mouth.
In many Ascidians, a strong fold of the endoderm ol the intestine projects into its interior, as in Liamellibraii^ and in the Earthworm, where such a fold oonBtitates the BO-cnXled lipklosoU.
THB T17KICATA.
005
In the pelagic Tunicates, Salpa, Pyroaoma, and Doliolum, I found a system of fine tubules,* which ramify over the intestine and are eventuaUj gathered together into a duct which terminates in the stomach. An apparatus of the same nature exists in Phallusia, Cynihia, Molgula, Perophora, Botryllut, BotrylUndes, Clavelinat ApUdam, and DidemnmHtf and I have little doubt that it is hepatic in its function. In some Cynthice, however, there is a follicular Uyer of the ordinary character, which opens into the stomach by several ducts.
In some Phallu8ice, the alimentary canal is coated by a very peculiar tissue, consisting of innumerable spherical sacs containing a yellow concretionary matter. In Molgula (and in the Ascidia tntrea of Yan Beneden) an oval sac containing concretions lies close to the genital gland, on one side of the body. As these concretions have been shown by Kupfer;^ to contain uric acid, the organ must be re<* garded as renal in function. M, Lacaze-Duthiers § terms this sac an "organ of Bojanus;" but, as he admits, no opening is discoverable : it would probably be more correct, therefore, to regard it as the representative of the glandular part 6f the organ of Bojanus. ||
The heart is an elongated sac open at each end, lodged near the stomach, and close to the hinder extremity of the branchial sac. After a certain number of contractions in one direction, it stops and contracts for the same number
* Savigny seems first to have observed this orgmn, as would appear from his account of Via- zona, (* Memoires sur les Aui- maux sans vertebres/ p. 176,) and the description of plate 12. Lister mentions and figures it in Perophora (Phil. Trans. 1834).
t ' Reports of the British Association,' 1852. Hancock "On the Anatomy and Physiology of the Tunicata:' (* Journal of the Linnean Society,' Vol. IX.) The development of these tubules from the stomach was trac^ by Krohn in Phtdlusia^ and by myseif
in Pyrtmoma.
X ** Zur Entwickelung der ein- fachen Ascidien." (* Archly flir Mikr. Anatomic ' 1872.)
§ **Les Ascidies simples des Cdtes de France." (' Archiyes de Zoologie ezp^rimentale,' 1874.) M. Lacaze-Uuthiers has obtained murexide by heating this sub- stance with nitric acid.
H There is a close resemblance between the cells of which this organ is composed and those wMch constitute the primitiye kidney in the PtUmonata,
606 THB ANXTOHT 01 IWrXXTOBSAIXD
of times, in the opposito direction. The ootine of the cir- culation ie thus reveraed with great regnlalitj. The blood IB a clear fluid, cofitaining colourleaa corpnaclea.
ReepiRition is effected in the nalU of the bmnchial boc through which the blood is driven. The enpply of &erat«d water is kept up bj the currents already mentioned, which BubBcrve the ingestion of food, the respiratory process and the ejection of effete matters, as well as the expulsion of tbe generative products. The test in which the body ia en- closed is somtttimes closel; adherent to the surface of the ectoderm, but sometimes is united with it only at the oral anil atrial upertures, and by prolongations of the body. In (■ODsistency it presents every variety, from soft and gela- tinous, to dense and hard like cartilage, or tough, likefibroiu tissue. In some cases the exterior of the t««t is covered with humy spinet, tubercles, or even with regularly disposed plat«3 iCkdyoemna).
In texture, the test may present merely a bomogeneotis
TBI DIVBLOPMBBT OT TBI Ftg. 1*9.
Slg. l*9.~Piaihak
of tha lu*a. (After Komlcwik;.^
i/bla. —Varionl lUgM In the diTdopinciil
I. ThP vMicQtar moral., fl.tUmf fh. blDaloFO'lc. The Urgf! I<1iwl<>in(<ri
tmalt one* the eiiililust. n. The pnsiruU with Ihe bloClnpoTe, or apvnioa nf liiv«ct>»tii>n, m
rh. Ihe hlulomerM which ooiuriiutv tliB ruiliaii<nt of tjw DradWnl;
dil, the rcmuning blutompTOB of llic hypablul. ni. A more advaiiiwl eiubiyn ; oA, iM, u bitftir*. e, the ni^blnil ,■ >,
the n»rvous Ujor of the neonU oBvlty, whioh i> no« open ©nlj la
rV. An Piabrjo n<Ih the caudal apnendagv illillnnl. Th* arm tnbf n li complete, and the muMlo-fclla a, are dUtliiKU>>h>b)*.
V. The bndy of u larva aa it euape* from tli* «gg. a, the vy*; fi. Ihe saccular anterior euA of the central mrviiioi ■pparatDi Inin which the oinliih projecta; Ro. Hm, ill InbnlU' backward prfilna- (Ration •, Chi, colli of the nrochonl ; a, moulh ; U, atrial ap^rtnivi f, oijening Bl the anterior end of the centra] nrnroiia avpantiu, bj
ment of the icsuphiigui and aiotnach ; n, blmil corpuaelf* | tf, pnnilte bj whieh (he larva attach « iiaelf.
V I. The bndv anil th(> enmmenBcmeot of the Cauda] «ppFDdag« af a free brva two Uaya i>)d ra, ondaatyle ; Ju, branohial ne ■ lli, Ui, branchial atlgmata; U. uiitrance into the blood alooa t>M«(<a them ; if, iDiostinc ; b, blood oorpuaclea i jiAa, atrial >|>(<rliir«.
reprodactiTe organs ore loilgttd i atria] cavitj, and their du«U an
, the lateral waUa of tli« dietutt from tbo maa;
THB DEYELOPMEKT OF THB T17KICATA.
609
is formed hj the perforation of a spot in wliicli the hypo- blast and the epiblast cohere (Fig. 149, YI.). The atrial cavity is formed by two involutions of the ectoderm, which extend inwards and apply themselves to the lateral and neural walls of the branchial sac (Pig. 149, VI.). Their originally separate apertures eventuaUy coalesce into one.* The atrial tunic thus formed, and the walls of the branchial sac, coalesce and become perforated, in order to give rise to the stigmata.
The test appears, at first, to be a cuticular secretion of the epiblast, and to derive its cellular elements from the wandering into its substance of cells derived from the epiblast.
In Molgtda tubulosay Kupfer and Lacaze-Duthiers have observed that the fecundated eggs are expelled from the atrial cavity and almost immediately become fixed to the surface on which they fall. Yelk-division takes place, and after four nearly equal blastomeres are formed, much smaller ones are developed from one face of these, and increase until they constitute a blastodermic layer around
* In 1852 Krohn discovered the fact that the larva of PhaUuda is provided with two distinct sym- metricallv disposed openings, by which the originally separate atria open outwards ; and that the two eventually coalesce into the single atriura of the adult. Kowa- lewsky, Fol, and later obser- vers, agree that these openings and the atrial sacs, are formed by two involutions of the ecto- derm, which apply themselves to the sides of the pharynx, and coalesce with it at the points which become perforated by the stigmata ; of which, in PhaUutiOy there are at first but two on each side. If this is a true account of the origin of the atrium, the atrial membrane is obviously part of the ectoderm, and its cavity is analogous to the pallial cavity of a mollusk.
On the other hand, Metschni- koff and Kowalewsky agree that in the buds ofJBotryttut. and other ascidians which multiply by gem- mation, the two primitively dis- tinct atrial cavities are portions of the alimentary sac, which be- come shut off from It, and sub- sequently open outwards.
Metschnikoffr*'£ntwicke1ungs- geschichtliche ^itrikge," ' Bulle- tin de r Acad. St. PetersbouiK,' ziii.) therefore compares the atrium to the enteroccele of Echi- noderms. Renewed observations specially directed to this point, which is of great morphological importance, are much needed. If the atrial cavity is really an enteroccele, it will answer to the perivisceral cavity of the Brachio' poda, the pseudo-hearts of which will correspond with the primi- tive atriflkl ai']^«tVax%.
.1
of any candiii appeuua;;*:. A-.l»^.« ^ ^
it. self with a transparent test, throws out prolonpitionB of the eetodfrni, and fmall}' adult condition. Although no tail is devel J uiasB is to l>c seen in the same position as t
the remains of this appenila^c. when it ha retrogressive mctamorphoBis, in the Ascidir larva;. The atrial aperture is single at its i and no larval sensory organs are developed.
8eepaffe6\\,
Ki«». 1 50. — Pyroaoma pigatUmm. — I. A vertical secti
Ascidiftrium near the cloacal aperture and in
The youngest condition of a bud befure the et
III. IV. v., further stages of the development of
I formed bud with a second ascidiozooid in course <
ita peduncle. ^ VII. A fcetus with the blastoderm divided into five
the cyathoxooid (I) is the largest. VIII. A firt ^ I of which half encircle the base of the cyathozof
K most advanced stage observed. The remain
cyathoxooid and ovisac are hidden by the circle The letters have tlie same signification in all the labial process ; a\ lip of the doaoal aperture ; bryonfc test ; e, oral aperture ; />^, atrial ape P, /*, branchial sac and stigmata ; r, heart ; r', - i ' ascidiarium ; H, stolons of the embryonic asc
;| . U testis; w, u\ ovum; w\ peduncle of a bw
^^r*\nn nf t.hft endoderm entering into a bu
■ I 1 1
612 TBE ANATOKT OF OITXBTSBaATED AITIHALS.
branchcB, which derelope new ucidiozooidB, ue given off at intervals: bnt, more commonly, the aacidiaritun ia maaaive, and the aecidiozooids relain no permanent connexion mth one another. In the Botryllida, the zooida are arranged in whorls around a common central cavitj, or cloaca, into which the atria of all the memhers of the whorl open. In Pyrosoma, which is a eort of floating Botryllut, the process of budding is highlj instmctlve, aa it exemplifies the majmer in which gemmation occnrs in the Taiticata in general.*
The aacidiariumoi Ptproioma (Fig. 150, 1.) has the form of a hollow cylinder, rounded and closed at one end, truncated and open at the other, formed of a firm transparent test, in which the zooida are arranged in whorls. Their oral apertures open on the exterior enrface, and their atrial apertures into the interior of the cylinder. The htemal aspect of each zooid is tamed towards the closed end of the cylinder. The brsnchial sac has the ordinary atmctare, and each 7.ooid is provided with a testisand with an ovisac,
THE BTTBDIHa AlTD 7188109 OV ASCIDIAITS.
613
a new zooid united by a narrow neok, or peduncle, with the parent (Fig. 150, lY.). The endostylic cone gives rise to the whole alimentary canal of the bud, while the ectoderm of the latter proceeds from the ectoderm, and its ovisac and testis from the mesoblastic cells, of the parent. Thns the organs of the bud are all the direct product of the corresponding parts, or of the primitive layers of the germ from which they are derived, in the parent.*
After the terminal bud is formed, a second is usually developed immediately below it (Fig. 150, YI.) by the growth of the ectoderm, endodermal axis, and mesoblastic cells of the peduncle ; and it would appear that this process is frequently repeated. The fully formed bud becomes de- tached, and takes its place among the other zooids in the test, there to repeat the process of gemmation.
The observations of Krohn, Metschnikoff, and Kowa- lewsky, have shown that two components enter into the buds of ascidians in general ; first, an outer layer consisting of the ectoderm of the region in which the budding takes place, and secondly, an inner layer derived from the endo-
• In my second memoir on Pyrotoma (Trans. Linn. Society, zxiU. p. 211) I have said :—
"Gemmation does not take place in Pyrotoma as in so many of the lower animals (e.y. the JJydrozoa and Pofyzoa, or Salpa and ClaveUnOy among the asci« dians), by the outgrowth of a proce&s of the body- wall whose primarily wholly indifferent pari- etes become differentiated into the organs of the bud ; but, from the first, several components, de- rived firom as many distinct parts of the parental organism, are distinguishable in it, and each component is the source of certain paru of the new being, and of them only. Thus the body-wall or external tunic of the parent gives rise to the external tunic of the bud : while a process of the endostylic cone of^the parent it converted into the slimentaxy
tract of the bud, and the repro- ductive organs of the latter are furnished by a part of that tissue whence the reproductive organs of the parent took their origin." As will appear further on, however, recent investigations show that the whole process of budding in the great majority of the Twucata, and at any rate the first steps of that process in Sa^po, are essentially similar to those in Pyrotoma; and it remains to be seen whether there is any differ- ence in other Ascidians. And as regards even the Hydrozoa the expression that the parietes of a bud are at first * wholly indiffer- ent' in structure is not quite accurate, inasmuch as thev are composed of an ectodermal and an endodermal layer, which aje^ continnoua n«V^ wma ^ ""^^ mrenV %ji^ ^n^ ttofe \» \»ssb>«*- loscraft otfiMDa.
614 Tirn anatokt of ntrSBTSBBATSD AnntkiA.
derm o( II10 bmncliiiil sac (furnji'if'riil : or, Bfl in Bairylbu, aooording to Metschmkoff, from the atrial tuuit.* To th^ must be added a tliird eomponeiit, deri*od fn»ii thn in- different tisaae, out of which the reprodnotive organs of di* parent bave been dereloped.
In Amoitroudmrt prol^eram, &giame. muItiplioAtion t&kw plooe when tbe laj^a has fixed itedf and grown into a aolttuiy asi'idiim. The long poat-abdumtrn (as die prok^- gation of the abdomen bejoad tUe alimeDtorj caokl it termed) Bepamtcs itacif from the body, carrying with it the heart, tuid divides iuto a number of Be.j^eatji which rise (u the snmmit of tbe teet of the parent, rongs tbemaekiw around it. and become conTerted into independeot sooidit. The parent developea a new heart and post-abdomen. The process appears to be repeated in the post'Sbdomina of tfae new zooida. The poet-abdomen is a process of tfaoectodenn,
Titr of which is divided by a septum into twal
cbambem, containing man; fatty ceUe. The septtii
THB DEYSLOPMBKT OV BOTBTLLVS, 615
loped, and then becomes divided by longitudinal partitions into three chambers, a median and two lateral. The latter give rise to the lateral chambers of the atrium, which subsequently open into one another on the neural side of the body, and finally communicate with the exterior by a median atrial opening.
G^egenbaur * has described the detachment of the era of a species of Didemnwn into the substance of the common test, where they are developed into caudate larvae provided with an eye. Before the development of the larva is nearly complete, a zooid is formed from it, so that, at one time, the embryo appears to have two branchial sacs.
Metflclmikoff t and Krohni^ have shown that the caudate larvae of Boirylhis are not composite, as Savigny and Sars supposed, but that the bodies imagined by these observers to be buds are simply diverticula of the ectoderm, and become converted into the vascular processes, which ramify through the conmion test, and commonly end in dilatations. In the adult, the buds are developed, one, or sometimes two, at a time, at the sides of the body, and consist of an outer layer, derived from the ectoderm, and an inner layer, which, according to Metschnikoff, proceeds from the atrial tunic. From the inner layer the alimentary canal of the bud pro- ceeds, and between the inner and the outer layers the rudi- ments of the genitalia appear. The ovaria advance towards their development much more rapidly than the testes. The zooids thus developed, as they enlarge, rise to the surface, taking the place of those from which they proceed and which die away. The ova are impregnated from without, and undergo their development in the atrium of the parent. Subsequently the testes attain their full development ; and, at the same time, the buds are formed which will give rise to a third generation, supplanting the second.
* "Ueber Didemnum geUdimo- % <'Ueber die Fortpflansungs-
satm." C Archiv fiir Anat.,' 1862.) verhaltnisse bei den Botrylliden.'*
t ** EntwickelungigeschichtU- (*Archiv fur Naturgeschichte,'
ohe Beitrage." (* Bulletin de I'A- (1869.) <' Ueber die friiheste
cademie des Sciences de St Pe- Bildung der BotrYlleoia^iMft^:-
ienbourg,' xUi. 1868.) (^Iblvi.)
616 THE ANATOMT OF nTYXBTBBKATBD AjriMALS.
After the larva (which may be called A) has attached itself, the first sets of zooids which are developed are sexlesB. The first bud arises on the right side of the bodj of the larva (A) in the neighbourhood of the heart ; as it increaaes in size, the parent withers away, and the zooid (B) thus de- veloped takes its place. Two buds, a right and a left, are developed from (B) and become zooids (C,C), B disappearing. The two zooids (C, C) are so disposed that their atrial ends* are close together, and their oral ends turned away from one another. These each develope two lateral buds, which be- come four zooids (D, D, D, D). The zooida C, G disap- pear as before, and their successors arrange themselves in a circle. Each of these developes two, or sometimes three, lateral buds ; these grow into zooids, which supplant their predecessors and are themselves, in turn, supplanted.
Every new system of the later successions is, at first, devoid of a common cloaca ; and the zooids which compose it may arrange themselves into one or several circles, each of which then acquires its cloaca.
It thus appears that, in BotryUua, the ascidiozooid which results from the metamorphosis of the caudate larva serves merely as a kind of stock, from whence the other zooids which build up the ascidiarium proceed ; and this leads to the still more singular process of development in Pyrosoma, in which the first formed embryo attains only an imper- fect development, and disappears after having given rise to four ascidiozooids.
In Pyrosonia, the ovisac is attached by a short oviduct to the wall of the atrium, into which it eventually opens, and thus aUows of the entrance of the spermatozoa.
Of the process of yelk division I could see nothing in my specimen, which was preserved in spirit, but it has since been traced in fresh specimens by Kowalewsky,* who compares it to that which takes places in osseous fishes. The result is the formation of an elongated flattened blastoderm which occupies one pole of the egg, and is
* ** VJcberdie'Eiitwic.VLQluii^^eftchichte der Pyrosoma." (*Archiv flir Mikr. A.nalom\«; \^1^.^.
PTS060MA AND DOLIOLUM. 617
converted into what I tenned the eyathozooidf which is shown by Kowalewskj to be a sort of radimentarj ascidian (Fig. 150, Ylll.). From this, a prolongation or stolon is given off, which becomes divided by lateral constrictions into four portions, each of which gives rise to a complete ascidiozooid. As these increase in size, thej coil themselves ronnd the cyathozooid, with their oral openings outwards and their cloacal openings inwards* and thus lay the foundation of a new ascidiarium (Fig. 150, VIII.). The cyathozooid eventually disappears, and its place is occupied by the central cloacal cavity (Fig. 150, IX.). Thus, in Pifrosoma, the usual first stage of an Ascidian — ^the caudate larva — is abortive and serves only to found the colony by the buds which are developed from it.
In the pelagic genus Doliolwm * the cycle of life of the species is represented by distinct sexual and sexless forms. The egg produced by the sexual form (A) f gives rise to a caudate larva which passes into the first sexless form (B) ; this gives off from the neural side of the body an out- growth or stolon, from which buds are developed. These buds are arranged in three rows, two lateral and one median, and grow into zooids of two different forms, of which the median may be indicated by C m, the lateral by G I. All these zooids are detached, and swim about as independent organisms. What becomes of the lateral zooids (C I) is un- known. But the median zooids give off a stolon from the hsemal side of the body on which buds are developed, which pass into the sexual form (A).
The sexual zooid (A) (Fig. 151) is shaped like a cask with an opening at each end^ these are the oral and cloacal apertures. According to Keferstein and Ehlers there is no test, the outer wall of the body being formed, as in most AppendicularicB, by the ectoderm. Eight muscular bands
* Huxlev, '* Remarks upon Entwickelane von DoUohany
Appendicu/aria and DoUolum." (*Zeit8ch. f& Wiss. Zoologie,*
(*PM1. Trans.,' 1851.) Krohn, 1853.)
«« Ueber die Gattung Dolhlumr f Keferstein and Ehlen, * Zoo-
('Arcliiv fur Naturgeschiciite,' logisclie 6eitxt^< V^\. 1852.) Gegenbsur, "* Ueber di«
does not extend further forwardB th the wide pharynx, and thb is i)crtor of stigmata, faiir or five in each. In (Fig. 151), on the other hand, the forwards at the aides of the pharjii and the nenral side, and the stigma Terticallj elongated. An opening in the middle line of
Fig. lai.
Fig. \i\.—Ihllol<m dniiailatam.—a, gui|tli< opening ; g, (PHiphagiii ; i, Momseb ; ^ i
phaiTSX leads, bj a short gullet, int whence the slender intestine proceed atrial cavity. The nerrone ganglioi
f*'.VJ
DOLIOLUM ASO 8ALPA. 619
The first sexless zooid (B) resembles A in general form, bat has nine muscle-rings. The long stolon, which trails in the water, is attached in the seventh intermuscular space to the middle of the neural face of the body. The stigmata are arranged as in the form A, of Doliolum MiiUeri, and one of the antero-lateral nerves terminates in an otolithic sac. It is spherical and contains a single otolith.
The zooids produced by the lateral buds of the stolon (G I) have wide oral apertures, and the body is shaped somewhat like the bowl of a spoon. They possess neither auditory organs nor genital organs, nor have they been observed to develope buds. The median zooids (G m) closely resemble the sexual zooids. The stalk by which each is attached, and the insertion of which is in the middle line of the hsemal face in the sixth intermuscular space, remains as a pro- minence after the animal is set free ; and, from the base of this prominence, buds are developed, which take on the sexual form (A).
In the ScilpcB, the divergence from the ordinary Tunicctta reaches its maximum. The oral and atrial openings are situated at opposite extremities of the body, as in Pyrosoma and Doliolvm ; and the branchial stigmata are represented by wide vacuities at the sides of the branchial sac, the walls of which are thus represented only by the epipharyngeal folds on the one side, and a narrow trabecula, which occupies the region of the hypopharyngeal band, on the other side. The relatively small alimentary and reproductive viscera are sometimes aggregated into a mass, the so-called niuHeus, at the posterior end of the hssmal side of the body. The chief muscular bands, by the contraction of which the water is driven out of the branchial and atrial apertures, and the propulsion of the animal is effected, are transverse, but do not form complete hoops, as in Doliolwfn,
In aU the 8alp€B, each species is represented by two sets of zooids, the one sexual and the other sexless. The sexual zooids are produced by budding from a stolon, which is given off from the body of the sexless form in the immediate neighbourhood of the heart. When the sexual "loov^ ^}K:«^s^
620 THE AKATOMY OF INYBBTBB&ATBD AKOKALS.
formed are detached, they are at first connected into chains of yarions forms, but these eventually break up and the constituent zooids are set free. Fig. 152 shows the two zooids of the species 8alpa denwcratuxt-mucronaia, viz. the sexless zooid, Scdpa demooratiea (Fig. 152, 1.) and the free sexual zooid, 8alpa mucranaia (Fig. 152, II.).
The recent investigations of Dr. Todaro,* in accordance with those of Kowalewsky, show that the stolon is formed, as in Pyrosoma, by the conjunction of a process of the endoderm which forms the extremity of the endostyle, with an outgrowth of the ectoderm, and with certain cells of the mesoblast. But, according to Todaro, there is this essential difference : the young Salpa, which make their appearance in double series along the stolon, are developed altogether from the mesoblastic cells. These cells, in fact, become aggregated into masses, of which four are arranged in the circumference of each segment into which the stolon is divided ; and two of these masses, one on each side of each segment, are converted into young Salpcs, by a process analogous to that by which a morula becomes an embryo. If this account of the matter be correct, the agamic deve- lopment of the 8alp<B would rather resemble that of the germ masses of tiie sporocysts of Tremaioda, or the pseudova of insects, than ordinary budding.
Each sexual zooid possesses a testis and a single OTum. The latter is contained in an ovarian follicle, the slender duct of which is attached to the waU of the atrium and opens into the atrial cavity. The testis attains its full growth and functional perfection only after the ovum has undergone development. It follows, therefore, that impregnation must be effected by the spermatozoa of some other zooid. The sexless form which is developed from the egg, goes through the early stages of its development in the atrial cavity of the parent, to the walls of which it is attached by a peduncle (Fig. 152, III.), the centre of which is occupied by a diver- ticulum of the vascular canals of the parent, enclosed within a cup-shaped cavity in free communication with the blood * ' So]^ \o ^vViu^f^ «\ KsnXATiiia. ^<(&&a Sftlpe,' 1875.
8ALPA DUCOCKATICA-XirOBOHATA.
ng. isa.
F»rtol
> lo-eBlled ''br»nchl« ; ■ /. l«r„ , ,, , . „
■tolon ; t, viiecnl mast or noolmi ; Ajmoacnlu-buidi; M-plx ■, blood tinoi ; 9, oviuo *nd OTum ; I, atomftch ; v, dll&M a, celeoblut; ■, •clodaim knd teit^ p, anAodMtllu
022 THE AKATOICY OF INYBBTBBBATBD AKIMALS.
Binnses of the f oetuB. It is, in fact, a tme placenta ; and, during life, the independence of the foDtal and maternal circnlations is readily observed, as the blood corpuscles of the two organisms coarse through their respectiye channels.
The early stages of the development of the embryo Salpa have been investigated by numerous observers, most recently by Kowalewsky,* Todajro, Brooks f and Salensky.^ The observatioDs of the last-named author relate chiefly to Salpa democraHea-mueronata, and his account of the process appears to me to be the most satisfactory.
The egg is impregnated in the ovarian follicle, as in Pffrosoma ; and the oviduct, shortening, gradually draws the ovarian follicle, with its contents, into a sort of incubatory pouch, which is a diverticulum of the wall of the atrium, and projects into the atrial cavity.
For distinction's sake the incubatory pouch may be termed the otjicyti. As the oviduct shortens, it widens and constitutes, together with the ovarian follicle, a single uterine sac, the outer or oviducal half of which applies itself to the wall of the ovicyst, while the inner half con- tains the ovum. The vitellus undergoes complete division, and the superficial layer of blastomeres constitutes itself into an epiblast, investing the solid mass formed by the other blastomeres, which represent the hypoblast A mesoblastic layer subsequently appears between the two. The nervous ^^anglion results from an involution of the epiblast, while the branchial sac, the alimentaiy canal and the atrium are the product of the subdivision of a cavity which appears in the midst of the hypoblast. The maternal and the foetal parts of the placenta arise, respectively, from the wall of the ovarian sac and &om certain large blastomeres on the adjacent hsemal face of the embryo.
Todaro agrees with other observers in stating that the viteUus undergoes division, and that a small celled blasto-
* * Nachrichten der K(inigli- Comparative Zoology,' No. U.
Chen Geselltchaft xa Gdtdngen,* t ^Zeitachrift fur Wiss. Zoo-
1868. logie,' 1876.
t ' Bulletin of \3\« 14uMnm «A
THS DSVBLOPXSNT OF THB BALPM.
623
derm invests the large remaining cells which he terms the germinal mass. But his account of the farther stages of development is very different. A circular thickening of the blastoderm separates the hemisphere which is directed outwards from that which is turned inwards, and gives rise to a lamellar outgrowth. It is, at first, directed towards the inner end of the ovisac, having reached the bottom of which, it becomes reflected; and the reflected portions lining the inner wall of the ovisac, and meeting over the outer hemisphere, form a sort of amniotiG investment of the embryo. It is the cavity left between this ' amnion ' and the inner hemisphere of the blastoderm which becomes the parental blood sinus. An involution of the outer hemi- sphere of the blastoderm gives rise to the alimentary canal, which becomes shut off*, as the endoderm, from the remain- ing blastoderm, which constitutes the ectoderm. A mass of cells which appears in the middle of the outer half of the embryo, between the alimentary sac and the ectoderm, and which has only a transitoiy existence, is regarded by Todaro as the representative of the urochord.
^.\
CHAPTER X
THE PEEIPATIDEA, THE
PHEEJSTA, TUB CH^;T0ONATHA, T PHTBEUAKIA, THE ACANTHOCEP: HJDA.
I HAVE reserved for discnaaion in paiidta, which hare heretofore b authors to the Annelida ; and certai Mekuoa, the precise morphologic: are as jet nnoertoin, although it sereral of them are allied with th Sol^era, and the Twbtllaria. The; totally devoid of segmentation; wbi the ilytostomata alone present ttnj limbs, though the natore of these i the nerroos eystem ia clearlj made chain of poat-oral ganglia aa charact< The Pe&ifatidea. — At p. 257, group to the Arthropoda, Mr. Moselej having left no doubt npon my mi faotorilj proved the justice of the afflnitiee originally made by Qerva
hnwrnrar tlmt. T Tiavfl hnan ftWe. tl
THS PSBIPATIDSA. 625
from the West Indies, South America, the Gax>e of Good Hope and New Zealand, where they are found among the decaying wood of damp and warm localities. They have the curious habit of throwing out a web of yisoid filaments when handled or otherwise irritated.
The head is distinct, and is provided with a pair of many- jointed antenna-like tentacula and two simple eyes. The mouth, situated upon the und^ surface of the head, is sur- rounded by a prominent lip, which encloses a pair of jaws, each of which is terminated by two curred chitinous claws, similar to those of the feet. On each side of the mouth, the head supports a short obscurely jointed ' oral papiUa,' which is somewhat like one of the feet, but is devoid of claws and perforated at its extremity. The head is followed by an unsegmented body produced laterally into paired appen- dages, which vary in number from fourteen to more than thirty, according to the species ; and each of these appen- dages is indistinctly articulated, the terminal joint being provided with two small curved daws.
The anus is terminal and the genital aperture is situated on a papilla, a little distance in front of the anus, on the neural or ventral face of the body.
The alimentary canal commences by an ovoid muscular pharynx. The OBSophagus, continued from this, gradually dilates into a wide and long stomach, from which a very short intestine is continued to the anus, situated at the posterior end of the body. There are no Malpighian ceeca. Two very large ramified tubular glands, which secrete the viscid matter of which the web is c<miposed, lie at the sides of the alimentary canal, and open outwards by the perforations of the oral papillsB. A vessel occupies the middle line of the dorsal body- wall, and is probably a heart.
The respiratory organs are the trachesB discovered by Mr. Moseley. The numerous pores, or HigrmUa, from which the trachesB take their origin, are scattered all over the surface of the body, one row being median and ventral Each stigma is the outward termination of a short mvl^ tube, which^ at ite opposite end, \>T«iic^e» qtq^VdXx^ ^^^le&ss^
026 THB AKATOICY OF nTfBBTBBBATKD AJTIMAUB.
of fine traches, which rarely diiide, and are distribnted in great abundance to the viscera. They are very delicate tobes, which often take an nndnlating course, and are rarely more than yi^th of an inch in diameter. In optical section, their walls have a finely beaded appearance, as if from the presence of transrerse thickenings, thongh distinct transrerae marking^ are rarely to be seen.
The nervous system, as Milne-Edwards discovered, consists of two ganglia in the head, closely united above the (Bsophagus. From each of these a relatively stout longitudinal cord proceeds, overlying the bases of the feet (and hence widely separated from its fellow) to the posterior extremity of the body. As Ghnbe has stated, there are no distinct ganglia on this cord. On the contrary, ganglionic cells appear to be pretty evenly distributed along its ventral face, throughout its length ; and nerves, which pass transversely outwards and inwards, are given off from opposite sides of it at short intervals. Grube has shown that many of the branches that take the latter direction are commissures between the two cords.
The muscles of Peripahu are not striated, which is a curious exception to its g^erally well-marked arthropod characteristics.
Mr. Moseley has proved that the sexes are distinct. The ovary is small, divided by a median septum into two lobes, and lies beneath the alimentary canaL The oviduct, at first single, divides into two branches, which are long, and, posteriorly, present uterine dilatations. They then unite, and terminate by a short vagina on the ventral aspect of the rectum. The testes are ovate bodies, each with a cflBcal appendage. The long and coiled vasa defeientia unite into a common duct, which opens in the same position as in the female. The ova are developed within the uterine dilatations of the oviducts.*
* One of the fpeeimena whieli 1 of the spirit is which it hsd been
aamined wea a pregnant female, preserved, in such a manner, that
tetths vteoeca wex« tAaed to- Uttleeoiild be made of their strae-
gttti0r,a(Vtnnid;f'V>l m aftXlin^ N3(n»«t^\haitorths«oibfyos.
THS XTZ08TOMATA. 627
Mr. Moseley baa made out the chief points in the develop* mental history of Peripatus.
In an early condition, the embryo is Teiy like that of a Scorpion, but is folded upon itself, so ihai the yentral aspects of the anterior and posterior halyes of the body are turned towards one another. As in the Scorpion, there is a pair of large procephalic lobes, succeeded by a series of segments, from the sides of which, processes — ^the rudiments of the limbs — ^bud out. The procephalic lobes give rise to a kind of hood, the lateral angles of which extend over the bases of the first pair of limbs, and join with those of the second pair, which are the oral papillse of the adult. The first pair of limbs thus become enclosed within the hood (the margins of which form the suctorial lip of the adult), and developing two chitinous daws upon their extremities, like those of the other limbs, they are converted into the jaws of the adult animaL It is remark- able that the antennsd are developed from the anterior part of the procephalic lobes; while the chelicersB of the Scorpion appear at the posterior margin of these lobes, in a position corresponding with that of the first pair of limbs, or jaws, of Peripatus.
It is obvious that whether we consider the appendages, the respiratoiy and reproductive systems, or the develop* ment of the embryo, Perip<xtu8 is a true Arthropod* ap* X>arently nearly allied to the suctorial Mkfriapoda,
The Mtzostohata. — ^The genus Myzostomwn^ compre- hends certain small animals, the largest species not exceed* ing ith of an inch in length, which are parasitic upon the Feather-stars. The body has the form of a flattened oval disk, the surface of which is ciliated, while its margins may be produced into as many as twenty short filamen* tons processes or cirri. Within the margin of the ventral face are eight suckers, four on each side, and, internal to these again, are ten short conical 'feet,' five on each side; each of these lodges two strong setsa^ "whidL <sksd^
i: I
tbe middle line on tbe poeterior of the alimentary cosal long ram
No Teseela or organs of circulai All that is known of the nervoi ganglionic mass, from which bran Bide, ritnated in the middle line of i
The Bexea are combined in tl acini of the generative glands i body. IhoHe of the testes pour which unite together and open hjr each side of the bodj, about the The two oriducta conTej tbe ora
The derelopment of Xytotiomu Semper and bj Metacbnikoff.* complete diviBion, and tbe embiyi morula, covered with vibratile obwrved, the embiyo ie cjtindro a month at one end and an anni mencement of the etraigbt and m the form of a moacnlar bulb or ] pairs of mdimentaij appendag sete. The number of the se oreaaea np to five pairs, and the
THB ENTSROPNSnSTA. 629
The presence of cilia on the surface of the body and of protractile sets in the parapodia excludes Mya>$to7n/um from the Atihropoda ; while Metschnikoff has justly com- pared its larval state with that of SyUis. Sufficient doubt, however, still adheres to the determination of the true place of Myxoatomum, to lead me to discuss it apart from the Annelids.
The Entbrophteusta. — The Teiy singular animal BaJanO" glo88U8f which is the only known example of this group, is an elongated, apodal, soft-bodied worm, with the mouth at one end of the body and the anus at the other (Fig. 153, III.) The mouth is surrounded by a sort of collar or prominent lip, within the margin of which springs a long proboscidif orm median appendage, which is hollow within and has a terminal pore. On the same side as that from which the proboscis springs, the anterior region of the body presents an elon- gated, somewhat flattened area, bounded by raised longi- tudinal folds. On each side of this area is a longitudinal series of apertures — the branchial apertures. The latter communicate with saccular dilatations of the anterior part of the alimentary canal, and these branehidl aaca are sup- ported by a peculiar skeleton.
No nervous system, nor any organs of sense, have yet been certainly made out.
According to Kowalewsky,* who was the first to elucidate the true nature of Balanogloeeue, the vascular system consists of a dorsal and a ventral Tessel. At the posterior end of the branchial region, the former divides into a superior and an inferior dorsal, and two lateral, trunks. The superior trunk passes forwards ; and, at the anterior end of the body, divides into two descending branches, which unite with the ventral trunk. The inferior dorsal trunk supplies the branchisB, of which the lateral trunks are the efferent vessels.
For the pharyngeal branchiee of Bakmoglasiua, the only
* < AnAtomie dei Balsnogossus.' (* Mem. de I'Acad. Imp. de 8t« P^tenboorg/ 1866.)
observers, until its true nature waa
derw-larva, on account of its eitr
the larTiE of some Star-fisliea (Fig.
It is an elongated ovoid bodj, p
Fig. la.
, . view (abont - D, YCHela IcadiDB to tha dorul pore (d) tuodIu ■yitam ; v' prolongBtiDn of U >, ttomach : a, teMidiairiii : m. mauth ;
THS DSYSLOPMBNT OF BAULHOOLOBSUS. 631
extremitj, while posteriorly it occupies nearly the middle of the body. On the yentral face, a deep groove separates it from the prsB-oral ciliated band, and in this groove the mouth is situated. The margins of the prs-oral and post- oral ciliated bands are deeply sinuated, and they come into contact in the median dorsal line. A wide gullet leads from the mouth, and opens into the gastro-intestinal portion of the alimentary canal, which passes backwards in the middle line, to terminate in the anus, at the hinder end of the body. About the middle of the dorsal face of the body there is a circular pore (Fig. 153, 1 d), whence a canal leads to a rounded sac, which lies on the junction between the gullet and the stomach. The sac g^ves off two lateral short diverticula, which embrace the oesophagus. A delicate band, apparently of a muscular nature, connects the summit of the water-sac with that part of the dorsal aspect of the body at which the prsB-oral and post-oral ciliated bands unite. Here two eye* spots are developed. A constriction separates a rounded gastric, from a tubular intestinal, division of the alimentary canal. Diverticula of the gastro-intestinal part of the alimentary canal give rise to two pairs of discoidal bodies, from which, apparently, the mesoblast and the perivisceral cavity of the Balanoglosaus are developed.
From the sides of the (esophagus, a series of diverticula are given off, which unite with the ectoderm, open extern- ally, and become the gill-pouches. When only two of these branchial apertures are formed, they are said by Metschni- koff to have a striking resemblance to those of Appendieu' laria. A pulsating vesicle — ^the so-called '^ heart," — ^makes its appearance close to the water-sac. The anterior end of the body, in front of the mouth, now elongates, and is con- verted into the proboscis ; while the poet-oral region loses its ciliated bands, and lengthening, becomes the long body of the adult worm.* (Fig. 153, 11. III.)
* See Asassiz, ** The History gen fiber die Metamorphoso eini-
of Balanogtotsui 9Jid TomariaJ* ger SeetUere." (* Zeitichrift fBr
O Memoirs of the American Aca- Wissensohaftliohe Zooli^e,' zz.
demy of Arti and Sdenoea,' 1873) ; 1870.) and Metsobnikofi^ ** Untennohon-
settled. Anatomically, it appmacl and the oligocba'toua Annelids Ie derelopracDt presents peculiaritiei known among these animaU, whi Braehiopoda and the Eekinodermati The hoAj of Sagiila (Fig. 154) r long, ia elongated, anbcylindrical ai Urged at one end into a ronnded It tApen to a point. There are nopa the chitinoaB cuticle ia prodaoed inl fin, on each side of the body and ta On each side of the head there e cnrred, claw-like chitinons procesi allj diTaricat«d and approiimab Between them ia the moatii ; and, a are four Bets of abort but atrong a] into B simple and straight inteel anna sitoated on the ventral face tapering caudal region commences, meaenteno band connect the intest bodj, and divide the perivisceral a Beneath the ectoderm liee a layer ( moBColar fibres. The nervons syH oval ganglion, which lies in the mi of the bod;, and aenda off ant^orl] which nnite with a ■
TBI OUnOOHATHA.
attached to the pnietw of Uie body. Theii- ciliated dncta open close to the T«it and on provided with dilatations
Fig. 154. — Smitta fty—rtirfa. — a, the bead trlth Iti «Jt» and appan- dagei ; b, the anui ; c, the OTacj ; f^ the teatlcuUr ehamban.
634 THE AKATOKT OF IHTXBTBB&^TBD AKIMALS.
which divides the cavity of the caudal part of the body into two chambers. On the lateral walls of these, cellular masses are developed, which become detached, and, floating freely in the perivisceral fluid, are developed into spermatozoa. The latter escape by spout-like lateral ducts, the dilated bases of which may be r^^arded as vesiculsB seminales.
Thus far, although the organisation of Sagiita is very peculiar, it presents analogies both with the NemcUaidea and with the Annelida. But its development, as described by Kowalewsky,* is, in some respects, unlike anything at present known in either of these groups. Yelk-division takes place, as usual, and converts the egg into a vesicular morula, with a large cleavage cavity, or blastocode. One face of the vesicle thus constituted now becomes invagi- nated, with the effect of gradually obliterating the blasto- coele, and converting the spherical single- walled sac into a hemispherical, double-walled, cup-shaped gastrula. The cavity of the cup is the future digestive cavity ; the layer of invaginated blastodermic cells which lines this cavity is the hypoblast, which will become the endoderm; and the outer layer of cells is the epiblast, and will become the ectoderm. In this condition, the embryo resembles that of the Leech in its early state. The embryo elongates, and the aperture of invagination, or blastopore, eventually ceases to be discernible. Whether it becomes the anus, or whether the anal aperture is formed anew, is not certain. The nervous ganglia result from the modification of cells of the ectoderm. The anterior end of the primitive alimentary cavity, or archenteron, is at first closed. It soon sends out an enlargement on each side, so that the archen- teron is divided into a central and two lateral divisions. The central division opens externally and anteriorly by the development of the oral aperture; and, as the body elongates, it becomes the tubular intestine. The lateral diverticula at first communicate with it, but they are eventually shut off, and constitute the right and left
• * Mimo\i« d« Vlk»u^Toi<b Ink^emla dM Setonoet de St bourgy' 18il«
THS DSYBLOPMSNT OF 8AOITTA.
635
perivisceral cavities, their walls becoming converted into the cellnlar and nrascolar lining of those cavities. It results, from the mode of development of the perivisceral cavity of ScigUta, that this cavity, like the perivisceral cavity of the Brachiopods and the '' peritoneal " cavity of the Echinoderms, is an enterocoele, comparable to that of the Hydrozoa and Aetinoaoa ; but which, instead of remain- ing in communication with the alimentary cavity, is shut off from it, its wall becoming the mesoderm, and its cavity the perivisceral cavity.*
Nothing of this kind is known to occur in the TwrheUariOt Annelida, Nemaioidea, or BoHfera; but when a peri- visceral cavity exists in these animals, it appears always to result from the excavation of the, at first, solid mesoblast. The perivisceral cavity thus developed is what I have termed a schizoccele. But whether there is any funda- mental difference between an enteroecele and a Mchiaoeceh is a matter for further inquiry. I have referred above (p. 565) to the case of an Ophiurid, in which the hollow diver- ticula of the archenteron, characteristic of the Echinoderms, are represented by solid outgrowths of the hypoblast. From this condition there would appear to be an easy ti*an8ition to that presented by the embryos of those Oligoehceia and Hirudinea, in which, though the mesoblast is a product of the hypoblast, it contains no continuation of the alimentary cavity, but eventually splits into a visceral and a parietal layer, the interval between which is the perivisceral cavity ; and there is much probability in Kowalewsky's suggestion that the longitudinal bands
* Kowalewslnr't account of the development ofSaffitta has been confirmed by Bittschli i who has further determined the ori^n of the reproductive organs, which arise as outgrowths from the hypoblast; and the division of each primitive enteroecele into two sacs, one for the head, ana another for the body. It appears
1 •« Zar Entwickelang^BBSchl<&te der fiaf^Wteu* (; 2873.)
probable that the latter becomes subdivided by a transverse par- tition between the ovary and testis. Butschli suggests that the segmentation of the meso- blast, which forms the walls of the enteroecele, is a point of ap- proximation between SagUta and the Annelids.
ment of all invertebrate periviacoi otber hand, it must be remember endoderm and the ectoderm in the d the body of a Ctenophoran or Ti gelatinouB mesoderm wbich occnpit primitive blaatocoele. Now, this m probably is, a prodact of the endod which appear in it, anch, for example canala of the TWrbellaria, can have i enteroccele.
Again, in the Timicata, aa we ha a kind of " periTiaceral cavitj," whi< an invagination of the ect«derm, in termed an cptaeJe,' oreleeitisa true the former altemstiTe, for the moD onght to be adopted, what is called a maj be one of foar things : —
1. A cavity within the meaobUa Renting the primitive blaitoeadt.
2. A diverticnlom of the digeati become ahnt off from that cavity (en
3. A solid oatgrowth, representing in which the cavitj appears only lai
THE KBMATOIDBA.
637
are devoid of limbe, thougli they may occasionally be pro- vided with setiform spines or papillse. In Desmoacolex, the papillte and setsB acquire an almost Annelidan aspect, and the annulation of the body is much more distinct than in any other Nematoid Worm.
Fig. \h5.—AnffuiUuIa brevitpinus (after Claus).*
I, initio : II, female ; III, female genital organs ; IV, teminal oorpusclea in different stages of development.
a, opsophagus; a', chitinised oral capsule; e, gastric, and d, rectal portion of the alimentary canal. A, anus ; ^', anterior and posterior thickenings with tiieir commissures ; 6, sexual aperture *, JP, fatty- looking gland ; r, dilatation of the uterus, serving as a recepta- culum seminis. D, unicellular cutaneous glands at the anal ex- tremity; D', glandular mass with its excretory duct above the giszard ; oo, ovarium ; T', testis; S, seminal corpuscles.
The outermost layer of the body is a dense chitinous cuticula, usually divisible into several layers. These layers
* ''Uebereinige in Humus leb<mda Anguillulinen." (<Zdtschrift fur Wias. Zoolo^e,' xii.)
638 THE AKATOMT OW INYBBTBBSATBD ANIMAL8.
may be fibrillated, the direction of the fibrillation being dif- ferent in the saccesBive layers. Cilia are found neither on the surface, nor elsewhere, at any period of life. The month is situated at one extremity of the body, the anus at, or near, the other end. The first portion of the alimentary canal is a thick-walled pharynx, lined by a continuation of the chitinous layer of the integument, which may be raised up into ridges or tooth-like prominences. Transrerse fibres, apparently of a muscular nature, radiate from the lining of the pharynx through its thick wall, and probably serve to dilate its cavity. A straight and simple tubular ali- mentary canal, witJiout any distinction into stomach and intestine, extends through the axis of the body, a narrow oesophageal portion usually connecting it with the pharynx.
The endoderm, or wall of the alimentaiy canal, consists of a single layer of cells, disposed in few or many longitu- dinal series ; and lined, both internally and externally, by a cuticular layer. On each side, the intestine is fixed through its whole length to the " lateral area," to be described below. The cuticle, which lines the inner faces of the endodermal cells, and circumscribes the digestive cavity, appears, on vertical section, to be divided into rods, which are possibly merely the intervals of minute vertical pores. In some cases, muscular fibres invest the posterior portion of the intestine.
Beneath the layers of the chitiuous cuticle there is a proper integument, or ectoderm, internal to which again is a single layer of longitudinally-disposed muscles, which may or may not be divided into distiuct series of '* muscle-cells." The space between these and the outer face of the intestine is occupied by a spongy or fibrous substance, which must probably be regarded as a kind of connective tissue. The muscles and tiiis tissue, taken together, constitute the mesoderm.
In the typical NemaMdea, the muscular layer does not form a complete investment of the body, but is interrupted along tour eqxadiAtant longitudinal lines. One of them is termed donid, l^e o^i^qaXa ^«ii\x^ vsL^Xystli these are veiy
THS KSMATOIDXA.
639
narrow. The other two are much broader, and are termed the lateral areas. They often (Fig. 156) pre8en%two or more series of conspicuons nuclei, and each is traversed by a canal with well-defined contractile walls and clear contents. Opposite the junction of the oesophageal with the gastric portion of the alimentary canal, each of these latend canals passes inwards and towards the mid-yenti-al line, and, joining with its fellow, opens by a pore on the exterior. In some cases, continuations of the lateral canals extend forwards into the head. A ring of fibres and nerve-cells surrounds the gullet,
Fig. 156.
Fig. 156. — Oxyuru. — a Month. 6. Pliurynx. e. Commeiicement of in- testine, and dj iti termination. The intennediate portion it not figured, e. Genital aperture, f. Opening of vessels, g. Their re- ceptacle. A. One of the vessels, t. Cellular matter enveloping them. A portion of one of the oontraotile vessels is represented more highly magnified in the upper figure.
about the level of the opening of the water-yascnlar system, and gives off filaments forwards to the head, and backwards to the muscles and to the lateral area ; while two cords pass back, along the dorsal and ventral median lines, to the hinder end of the body. In the males of some species, nervous ganglia have been observed in the neighbourhood of the sac of the spicula.* Organs of sense are not certainly
* The question of the structure system in the JVcMatoMba U^'^9tt> and disposition of the nervous ha^it, not «v«ii ^«fc ^nsoK^fM^
v.\
The female apparatuB (Fig. 155, 1] with which ie connected a eingle, tubular, organ, which tapers to a tremitj, and is at once ovarium, ovi cwcal end is occupied by a nuclea Further on, thia maee becomes diffi cord of protopUfimic aubBtance — pheral maaaee, each cont&iniiig a nn a et&lk with the rhaekit, which a Still further on, in the oridncal p ova become free; while, in the ut impregnated, and acqnire a hard, ol
The teetJB is, generallj, a ein^e end of which cells are developed, as in the ovaiy : they become frc tube which playe ihe part of a vsi what happena in most aniTnnlii, thei character of cells, and may even eihi The deferential end of the teeticnl cloee to the anoB, from the dorsal v carved chitinoiiB spicnla are deve! dnced into the vnlva of the femal< place, and appear to distend it, in i passage of the seminal corposclet ii into the ntems. In tbe female c
THI DBTBLOPMBNT OF THI KBMATOIDBA.
e4i
becomes indented on one side, and the embryo, as it grows, folds itself in accordance with this indentation. In most, it would appear that the central cells of the solid momla are differentiated from the rest to form the endoderm, which thus arises bj delamination. But Btitschli* has recently shown that the morula, which results from the division of the yitelluB of Cucuilanu$ elegana, has the form of a flattened plate, composed of two layers of blastomeres, the blastocoele being reduced to a mere fissure. The lamellar blastoderm next becomes concaye on one side, convex on the other, and passes into the gastrula form. The blastopore, at first very wide, gradually narrows and appears to be converted into the oral opening of the worm. The mesoblast takes its origin from certain cells of the hypoblast^ which lie dose to the mouth, and grow thence towards the caudal extremity. The resemblance of this developmental process to that of Lumbrieus is obvious.
The female reproductive apparatus is, at first, represented by a solid cellular body which lies in the mesoderm ; though whether it originally belongs to this, or to the ectoderm, or to the endoderm, is not clear. The cellular body acquires a tubular form, and eventually opens externally by uniting with an inward process of the ectoderm, which gives rise to the vagina.
The young cast their cuticle twice — ^first, when they leave the egg, and, again, when they acquire their sexual organs.
The Nemaioidea have been divided into three principal groups t — Polymyaria, Meromyaria, and JEToIomyaria^- cha- racterised by the nature of their muscular system.
* <*ZarEntwickelangsge8chieh- te des Cucuilanuselegan»7* (' Zeit- ichrifl fur Zoolog{e,^187ft.) Halles (* Revae des Sciences Naturelles,' 1877) has observed a similar pro- cess in Anguillula aceti, but he denies that the blastopore becomet the moQth.
t Schneider, ' Monographie der Kematoden,' 1866. See alto Bastian, 'Monograph of the
Angaillnlidse ' (Trans. linnean Society^ 1865) : and < On the Ana- tomy and Physiology of the Nema- toids' (PhU. Trans. 1866); and several memoirs by Bntschli. The latter af&rms that the mnseles are as much made up of mnsole cells in the * Hohmpria^ as in the rest. ("QiebtesHolomyarier?" «Zeitsehxifl fOr Wlss. Zoolog|i< 1878.)
642 THS AKATOMY OF INVBBTBBSATBD ANIMALS.
In the Tolymyaria, the muBcles of the parietes of the body are divided into many series, each made up of many " muscle cells." In the Meromyaria there are only eight longitudinal series of such muscle cells, two between each lateral area and the dorsal and ventral lines respectively. In the Holomyaria the muscles are not divided into series of muscle cells.
The first two divisions contain only such genera as answer to the general description just given ; but, in the Holomyaria, there are included several aberrant forms. Thus, TrichO' eephalus has no lateral areas; IcMhyonenui has no anus; Mermis has no anus and the alimentary canal is rudimentary ; though it possesses the lateral areas, and the males have spicula. Oordius has no lateral areas, and only the ventral line ; the alimentary canal is reduced to a rudiment, with- out either oral or anal aperture, and the male has no spicula. In both these genera, the anterior ends of the embryos are provided with spines, which aid them to bore their way into the bodies of the insects on which they are parasitic. In Sphcertdaria the alimentary canal is similarly rudimentary, and Sir John Lubbock discovered that the small male becomes permanently adherent to the female.
Some NemcUoidea {e.g. Lepiodera, Felodera) live in water or damp earth, and are never actually parasitic ; but they require abundant nitrogenous food in order to develope their sexual organs, and hence they are found in the sezuiEd state only among putrefying vegetable or animal matters. The sexless worms, which live in moist earth, are at once attracted by nutriment, such as a few drops of milk.* Here they multiply with great rapidity as long as the store of food lasts ; but, when it is exhausted, the last hatched young wander away. In the course of their wanderings, the embryos enter into the larval condition; but, beforo doing so, they become twice as large as those which attain the larval state in putrefying substances. The embryonic cuticle becomes thickened, and its oral and anal apertures ^oaed, so that it forms a cyst for the larva. The larva, liow^Yer, ift not T<£«^x«ASi»i by this cyst from moving about
r'**j
THB NBMATOIDEA. 643
and oontintiing its wanderings, though, at length, it passes into a quiescent condition. Its inner substance, at the same time, becomes dark by transmitted light, in consequence of the accumulation of small f attj granules ; and, if this state of things lasts long, the liurva dies. If the larvae should dry up, the circumstance tends to their preservation. The embryonic cuticle is separated, and forms a protective cyst; and, when moistened, the larvee resume their vital activity.
Nematoid worms belonging to naturally free and non- parasitic genera may enter, and become encysted in, worms and slugs; but they only attain their sexual state when their host dies, and they are nourished by the products of its putrefaction.
Anguillula scandens, the Nematoid which infests and gives rise to a diseased condition of the ears of wheat, is a true parasite. The young are hatched from the eggs laid by the parent in the infected ear, and there become encysted. When the wheat dies down, the larvsB are set free, and wander on the moist earth, until they meet with young wheat plants, up which they creep, and lodge them- selves in the developing ears. Here they acquire the seroal condition, nourishing themselves at the expense of the inflorescence, which becomes modified into a kind of gall.
Most Nematoids found in the alimentary canal of «^TiiTnft.Ttt are i>arasitic in the sexual state, but have a longer or shorter period of freedom as larvsB or aa eggs. But some, as Cucullanus elegans, are parasitic both in the sexless and the sexual condition; inhabiting Cyclops, while in the former state, and sundry fresh-water fishes, particularfy the Perch, in the latter.
Trichina gpiralia^ acquires its sexual state in the ali- mentary canal of Man, of the Pig, and other Twi^TnTn^la ; but the young, set free in the alimentary canal, bore their way through its walls, and enter the fibres of the voluntary muscles, in which they become encysted in the sexless state. If the flesh thus trichinised be eaten, the Triehimm,
* Leaekart, ^Untexsttehimgeik fAMt TrkiiMML ip^raUi^ \%Mk«
j.ne ineect paraiBit«a, Uordiui long aa they are parasitic ; bu their full f^owth. they leave th semal orgajis, copniate. ajld laj proceed, which bore their way i
It has been stated that the A part, diiEcions. Schneider has, apeoiea of the non-paiasitic gen whicli alwajra have the external in the OTarian tnbee of which and impregnation takes place, donbt bj isolating embiyoe of t1 out the development of the spei the subdivision of the first cells After a time, the development the cells separated from the i are impregnated by the aln These Nematoidea are probabi necessary hermaphrodites know
A»eari4 nigrovttutta is parasiti' Toads, and attains a length of It has the characters of a femi been met with, bnt spermatozoa i ia the same manner as in the pi
The e^is of thia Aacarit are d find their way into the inteatines
THX PHT8BMABIA. 645
The free embryos, introduced into the frog's mouth, pass into the lungs, and take on the characters of the large hermaphrodite forms. It is not unHkely that the Guinea worm (FHaria medinentia), which infests the integ^ument of Man in hot climates, may answer to the hermaphrodite stage of a similarly dimorphous Nematoid, though its mul- tiplication has hitherto been supposed to take place agamOgenetically.
The many points of resemblance between the Nemaiaidea, the OligochcBia, and the PolyeJicBta have been indicated by Schneider. They differ, however, from these, no less than from the Turhellaria and BoHfera, in possessing only longitudinal parietal muscles. In this respect they agree with Rhamphogordiua and Polygordiu8 (united by Schneider into the group of Oymnotoma),* which are segmented worms, devoid of setsB, but possessing mesenteries, segmental organs, and pseud-hsBmal vessels. Polygordius has a telo- trochous larva, and in its development, as in other respects, it is extraordinarily like a polyduetous Annelid.
Butschli,t on the other hand, dweUs upon the connexion between the NemcUoidea and the Ocuieroiricha (see Chap. I Y. p. 192) and Atrieha (Echinoderes) which he includes in the group of NenuUorhyncha, on the one side, and the lower Arthropods, such as the Tardigrada, on the other.
The Physemabia. — Since the completion of the third chapter of this work, Haeckel % 1^^ published an account of certain low Meiazoa, constituting the two genera, HciU- phyaema and Oiutrophysema, which had previously been confounded, partly, with the Sponges and, partly, with the Protozoa.
These are minute marine bodies, having the form of cups with longer or shorter stalks, by which they are attached. The cavity of the cup into which the wide or narrow oral
* See Mipra, p. 186, iMte. Ludwig, *<Ueber die Ordnang
t <* Uatereuchungen iiber frei- QMtrotricha ** (tbid,),
lebende Nematoden und die ^ * BiologUche Studien,' Heft 2,
Gattang Chatomohu^* (* Zeitschrift 1877.
f. Wise. Zoologie,' 1876). Seealao
SW THE ANATOKY QF IKTIBTEBBATED iMIMALS.
up«nmg lewis, is eithOT' eimple (HcUiphysema) or divided by circalar conatrictions into two or more ogmmiioicatiiig chamberH {Giutrophyimivi}. Ttie wbU ia composed of two latere, aa ectoderm and &ii endodorm — the latter beting forme'l b; a single layer of tlagcUate cells, like those of apongea ; and a series of larger fl^?llat« celle aie diapoaed in a apinU, ua the inner face of the endoderm near tlie mouth. The ectoderm ia a sjncf tium, vliich attaches foreign bodies, sueh as sponge Bpiculu or alieletone at Fbremtinifern. to itself, and thus becomes provided with as adventitioos skeleton, tbe nature of which vaiies in different speciea, bat is constant for each. Reprodnctian is efiected bjOTa. which are said to be modified cella of the eododerm. In Oa»tro- phygema, the endoderm of the ionei-moat chamber alone gives rise to ova. The place of development of the spar- matoKoii has not been made out.
Telk diviaion is complete and re^ar, and gives rise 1<> a vesicular morula {OirchAliuittila of Haecbel) eooh oell of which is provided with a Ha^llate cilinm. A. ga^trula arises hy invagination, but the tiuiil stages of developminit have not been made out.
As Hneckel points out, the PAi/mtnuiria are obvionslf
THK ACUTTBOCBPHAI^ 647
MntsatDTich oonetricted neck (Fig. 157,/). It would appear that, eventnallj, the Eehinorhynehi completely pass out of the inteatine, as thej are foond enclosed in detached cjeta Ijing in the peritoneal cavitj. The anterior eztremilj' of the EehinorhyTuihuM ia produced into a short cylindrical proboscis, covered with manj rows of recurved hooks, and, behind this, it forms a dilatation, in which the internment and the moscnlar coat are separated by a considerable in>
Fig. 157.
of the body. /. N«ck or eouatricliou belwesn th lmr);eincnt ind the rmt of the body, d. e. Poaterior " fuDoel." p. Meniscus. A. Superior olilique InbnlBr bandi. i. Inferior muKln of tha pnboarfi. /. ■■. Geniulik. o. Penis or vuIvk. B. Lower eilremity of the >lem of the prubotcii. a. GuigliDn. b. Vuculsr *pkce. d. Ou(er coat. t. Inner wall. t. Tubular band, with the nerve. A,/. Muicular bands, g. fiuapensDrluin of the genitalia. C. Part of the female genitalia, a. Ovary, bh. Ducta leading from ovar; lo nterui, ■permiductj (?). c. Open mouth of ovidsot. d, e, ITtema and vagina.
terval. The body, behind the constricted neck, which separates it from this ant«rior dilatation, bu & ^^Jbas^V
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the vosriels lit'S a double* layer fihrils, tlie externsil of which a U'riial are lou^tudinaL* The »: with a fluid, in which the ova, at it« .anterior extremity, two elo from the parietes, and han«; fr lemni^ci ; they are traversed b} thoHe of the parietes. The i continued downwards into an stem, rounded below, which hang the cavity of the body. The exti nect'ed by broad retractor muacl ^ives attachment to the suspenso ductive apparatus (Fig. 157, B). attached a little above these, an< to the parietes ; they are not mere norily described, but contain a wit a kirge sinus, which sepanitcs the of the proboscis from itB investing -stem of the proboscis is the oval ] some small branches upwards, and which can be followed into the vesi and, in other species, have been t
TRX AOAHTHOOBPHALA. 649
the animal being probably nomiBhed bj imbibition tbrongh the walls of the body. The reprodnctiye organs are, both in the male and in the female, attached bj a snspensorj ligament to the extremity of the proboscis, and extend thence, through the axis of the body, to the posterior ex- tremity. Here they open in a papilla at the bottom of a funnel-shaped terminal dilatation of the body, which exists both in the nude and in the female, though it is much more marked and separated by a constricted neck from the body, in the former. On each side of the papilla is an organ which has much the appearance of a sucker, but which is apparently non-contractile, while the funnel itself undergoes constant and rhythmical contractions.
In the male, the testes are two oval sacs, one behind the other, connected by vasa deferentia, often provided with peculiar accessory glands, with the genital outlet, which is provided with a long penis. In the female, the ovary is a single, long, thin- walled, cylindrical tube, the anterior end ^ of which is usually empty for a short distance. Further back, clear, pale, rounded masses appear, containing cavities in which corpuscles, like the germinal spots of ova, Ue. More posteriorly still, these masses become elliptical, and are surrounded by a membranous coat, which gradually thickens, and g^ves rise at each end to a spiral filament which surrounds the enclosed egg. The ova thus constituted then pass into the cavity of the body, where they accumulate in great numbers ; but, in this species, I have not found the free floating ovarian masses described in other Echino^ rhynchi. From the lower end of the ovarium two short ovi- ducts, or rather spermiducts, arise, and almost immediately unite into a sort of uterus, which is continued into the vagina (Fig. 157, G). The uterus passes above into a short, open, funnel-shaped canal, which lies between the two oviducts (Fig. 157, 0 c), and, according to Von Siebold, takes in the ova from the perivisceral cavity by a peculiar swallowing action.
The embryos of the different species of Eckmorhynchi vary somewhat in structure. Von Bi^\M\<l \^3m^ ^^ssrsc^rj^.
previously been Been by Leuckar water containing apeciniens of E. protein were tninsferreil. A could easilj be detected in tbe i maru*, whilst numerous embrj shell, were fonnd within the app
Each ovum has two coats ; an inner, chitinous. The first ia through the alimentary canal; ruptured by the embryo, which b walla into the cavity of the body, the sit« proper for its developmei
The body of the embryo is soi and consists of a colourless, tmn t«ct6d by a cuticle. The parench an onter, homogeneooH, contract] mednllary substance. Within this mass, made up of large highly-refi grannies of the same kind maj tbronghont the soft medullar; anl end, the embryo tapers to a puint, w is obliquely truncated towards the oblique surface may be observer
THB DBYBLOPMBNT OV BCHIKOBHTVCHUS; 651
the middle line to form an arch, the central and largest spine constituting its summit. Two short, ridge-like ele- vations of the cuticle, close to the middle line, separate the spines on either side from one another. Behind^ the peripheral layer g^ves rise to a knob-like process.
At the end of fourteen days, the embryo is found to have increased much in size, but presents few changes of form. The anterior extremity displays two rounded elevations, the spines retaining their original position. The peripheral layer has become thicker and more distinct ; its knob-like process has by this time disappeared. The central mass, now much larger, has assumed a spherical figure. No longer granular, it is seen to be composed of numerous pale cells, which continue rapidly to increase.
During the third week, ntmibers of large yellow granules begin to appear within the outer layer of the embryo. No other changes, save those of growth, take place in its walls: but the central mass, still continuing to enlarge, gradually puts on the aspect of a young Echinorhynchus, This mode of deyelopment has been compared by Leuckart to that of certain Echinoderms, or to the production of the Nemertid larva within its pilidium.
The first part to become differentiated is the oaviiy of the future proboscis, which appears as a transparent lenticular vesicle at the anterior end of the spherical mass. Behind this are soon seen rudiments of the central axis and its contained ganglion ; and the suspensorial ligament, with the reproductive organs, are, at the same time, marked out. The muscles of the outer wall have also commenced their development. Next, the central region of the young Echinorhynchvs rapidly elongates ; its walls become thinner, and, separating from the included structures, show the first trace of the visceral cavity. About this time distinctions of sex first make themselves evident. The posterior end of the body undergoes a disproportionate increase of size, the muscles become more distinct, and the rudimentary generative organs are clearly manifest. At length the young Echinorhynchus occupies almost iJb!^ ^V<^^ \s^ii5c\ss<&
652 THE INATOMT OF INVBRTGBRATED ANrHALS. ^H
of t.Le embrjo. the wallB of whicb h&vc, me&nwhile, UOdav> gone but aliglit histolo^coi cban^. The epines, bowever. have disappeared, together, it would se«m. with the cnliol* to which the/ were attached. No niptiire of thu other cinhr/onic struetures takea place, bat, they gmdunll; nttiKih themaelvea to the body of the coat^aed .SchiHorliynchmi. becomiiig cloaety fitted to its eurfuce, and uppureDtij per- sisting throughout its entire life. The development of titt EohinorhTpich-iig now approaches completion. The lemnimii appear. Hooke arise on the surfaoe of the proboaoia. not, as might be Bnppoeed, from it« out«r cntiub. but from Bpociolly modified cells of an inner membrane, The in- ternal organs begin to a«siinie their final napoct. The external form of theadiilt organiemia rather slowly reached, and a few changes which take place a[l«r tr>tiiBfi!rpn<>e of the Echiiiorkynekag to its final host have yet to be obeerrcii. The AeanthncepluUa tindonbtedl; present certain re«eai- hlanceB to the NmnatoiiUa, and more particularly to the Oordiaeea, but the fundamental differences in the etmctnn) of the mnaciilar and nerrons systems, ajid in that of the reprodactive organs, are so great, that it ia impoaaibte to
THB DICTXMIDA.
053
cjlindrical, or more or less fusiform, awial eeU, which extends from the slightly enlarged head-end, hj which the animal is attached, to its posterior eztremiiy, and is in- yested by a single layer of relatiyely small flattened cortical
Fig. 158.
Fig. 1&8.— DtcyenM.— I. D. typut. The large papillsB of the cortical layer and the ^enoM in the interior of the axial cell are noticeable.
IT. 1>. tvptu. Different stages of the development of a Yermifonn germ.
III. Inmsoriform embryo found tree in the renal organs of £iedone wtotehatOf treated with osmic acid : /», the urn ; oa« its capsule ; «» its lid ; t, multinucleate cells in its interior. (After Van Beneden, /. e.)
cells. These are arranged, like a payement epithelimn, around the axial cell, their edges being juxtaposed ; they are nucleated, and their free surfaces are ciliated. There is no interspace between the cortical cells and the axial cell, and the organism is a simple cell-aggregate, deyoid of connectiye, muscular, or nenrous tissues.
The cortical cells which inyest the anterior or head-end of the IHeyema, haye peculiar characters, and are dis- tinguished as the polar cells. They are arranged in such a manner that the head ia bilaterally symmetricaL Some- times the polar cells constitute the whole of the cephalic enlargement, but, in others, cells of the adjacent put of the body {parapolar eelU) contribute to the inyestn^siSQiJu ^
654 THB ▲HATOmr OV INYSBTBBRATED ANIMALS.
the head. Stronglj-refracting globules and rods aconmu- late in some of the ectodermal cells, and cause them to project in the form of papUlse.
The axial cell is a mass of protoplasm. Its relatiyelj dense outer layer passes into a central reticulation, in the midst of which there is a large oval nucleus.
Reproduction takes place by the formation of germs, and the development of embryos from them, in the axial cell. The embryos are of two kinds, the one vermiform, the other infusoriform, and are not met with in the same Dieyema, but in individuals of somewhat different characters. Those which g^ve rise to the vermiform embryos are termed Nematogena, while the others are named Bhomhogena.
In the Nematogena, the germs arise in the protoplasmic reticulum of the axial cell, and, at first, are minute spherical bodies, each of which is provided with a nucleus. This germ-cell divides into two, and each of these again becom- ing bisected, four cells are produced, of which one remains undivided, while the rest go on dividing. The former enlarges, and gives rise to an axial cell, around which the other cells airange themselves, until eventually they enclose it. Before they meet, they surround an open- ing through which one end of the axial cell protrudes. This corresponds with the oral pole.
Before the young Dieyema thus developed leaves the body, which it generally does by traversing the oral pole, (though it may make its way out through the parietes,) two embryos of the same kind appear within its axial celL
Thus the nematogenous Dieyema gives rise by an agamo* genetic process to new Dieyemas.
In the Ehombigena, the germs are developed in from two to five special nucleated parent cells, the origin of which is not known. They are found imbedded in the protoplasm of the axial cell, and the germs are developed endogenously from the protoplasm of the parent cell, the nucleus of which remains unchanged. The germs undergo division, and beoome sp\iecQvdal bodies composed of two kinds of
THB DBYBLOPMBNT OF DICTXMA. 655
cells, small and large. Each of these bodies is converted into an inf usorif orm, bilaterally symmetrical embryo, which consists of an urn, a ciliated body, and two refractive bodies.
The nm, situated on the ventral side of the embryo, is composed of a capsule, a lid, and contents.
The latter are four granular masses, each of which contains many nuclei, and eventually becomes covered with cLLia. The refractive bodies take their origin in two adjacent cells. They partially cover the urn in front, and form the largest portion of the dorsal face of the embryo. The ciliated body consists of ciliated cells, and forms the caudal poiidon of the embryo.
While the vermiform embryo becomes a Dicyema in the body of the Cephalopod on which its parent is parasitic, the infusorif orm embryo is set free, and probably serves as the means by which the parasite is transmitted from one Cephalopod to another.
Professor E. van Beneden compares the cortical layer of a Dicyema to the ectoderm, and the axial cell to the endoderm of a Metazoon ; and the mode of production of the embryo to the process of epiboly in the Metatoa. But, from the complete absence of any mesoblastic layer, he proposes to establish a new division of Mesosoa, inter- mediate between the Protozoa and the Metatoa, for the Dicyemida.
THB ASATOMY 0* raVERTBBHATEl
CKAPTEB XU.
IirVEBTEDKATBD
I
Thb grouping of the various kinds of inTertobrUvil ftpim^lH which ba.s been adopted in the preceding pagu* U to be regarded merely ha a temporarj arrangeioent. Ench chapter, from the aecoud to the tenUi, ia devoted to « snnr> of formH, the morphological relationa of which are man or leea obvious, nhile Chapter XI. is resei-ved partlj tut Buch groups BM do not rcadilj find a place in any of Ac Beriea which precede them ; and, partly, for such as hxit been establlBhed since this work was commenced.
Our knowledge of the anatomy, and espeaiully <if Um developmest, of the Znvffriebrala is increneing with aucb prodigious rapidity, that the views uf Taxonomiat* regard to the proper majuier of expressing tJ '
ITATITSAL OBDBBB OV AKOfALfl, 657
It is x>068ible to divide inyertebrated aTiimalfl into a certain nnmber of groups, each of whicli will be admitted bj every morphologist to be in itself a perfectly natural assemblage. That is to say, all the forms thus associated together will resemble one another, and will differ from all other animals, in certain respects. Each such assemblage is, in fact, a " natural order " in the sense in which that word is used by botanists ; and, although the number of these natural orders may be increased by the discovery of new forms, or diminished by the ascertainment of closer bonds of union than are at present known to exist, between the orders already discriminated ; yet, the morphological types which they represent will always remain ; and there- fore the knowledge of their characters, once acquired, will be a permanent possession.
It is not needful that these natural orders should be morphologically, still less numerically, equivalent ; and, in forming them, it is more important that similarities should not be neglected, than that differences should be overlooked. Those which have been recognized in the preceding pages are enumerated in the following list, arranged in sections corresponding with the chapters in which they are discussed. Under the head of each section, I shall proceed to make such observations as have been suggested to me by new information or by further reflection, during the progress of this work.
Section I. — Monera [Foraminifera] IHeliotoa], Badio- laria, Protoplada, ChregarinidcB, CaiaUacta, InfiMoria [Opalinina, CUiaia, Flagellata, Tentaculifera],
Section II. — ForiferajHydrozoa, Coralligena [Ctenophora].
Section III. — Turbellaria, Boiifera [NefruUorhyncha], Trematoda, Cestoidea,
Section IV. — Hirvdinea, Oligochceta, Folychceta, Qephyrea.
Section V. — Crustacea, Arachnida \_Fycnog(mida, Tardi- grada, Fentagtomida], Myriapoda, Inseda.
Section YI. — Folyzoa, Brachiopoda, LameUibranehiaia, Odontophora,
668 THX ▲HATomr or nrmnriBBATKD avukals.
Beckon YIL^EMmodermaitL
Section YUL—IHmteaick
Section IX — PeripaHdea, Myaodomaia^ EnieropneudOj Ckadognaiha, NemaMdea, Phyaemaria, AeafUkoc^phala, DieyemidcL
SscnoH L — ^In the commencement of Chapter IL, I hvre expiesBed a doubt as to the yaliditj of the distinction of the groups contained in this section bj the presence or absence of a nucleus, and the recent inyestigations of Schulze* and Hertwigf haye justified mj hesitation. These obeerrers haye, in ^t, demonstrated the existence of one or more nuclei in many Foramimfera {EnlomUenia, PolysiomeUii^ Botalia, TexMaria, some Miliolidce). These nuidei may be simple or multiple ; in the latter case, thej haye no special relation to the cameration of the skeleton, and they are single in the young.
The discoyery of the nuclei was effected by treating the Foraminifera in which they were found in a special manner : and, considering the negatiye results at which the best obseryers of the Foraminifera haye hitherto arriyed, and the fact, that the other Monera haye not been inyestigated by the same methods, it will probably be wise to consider the question of the non-existence of a nucleus in them as an open one.
Hertwig proposes to include all the Bhizopods which are inyested by a coat of chitin, or by silicious or arenaceous particles, or which possess a skeleton, under the head of Tkalavwphora ; but the name of Foramintfera is now so widely accepted and so long established that I cannot but think that the better course is to retain it.
I haye included the AeHnophryida and the similar forms found in fresh water, and proyided with Badiolarian skele- tons, with the marine Sadidaria,
* " Rhisopoden-Stadien, YI.'* tion imd tjBtemstifehe Stdlimg
CArehiv fur ilikr. AnAtomie,' der FonminifeKa." OJeosuehe
1876.) ZeitMhrifl,' 1^6.)
t **BsB>eitoBiy!ikianOtfyBSaiap
THX HBUOaOA. 659
Hertwig and Lesser,* However, in their important monograph upon the Bhizopoda, have stated reasons for separating the former as a distinct gronp (the Helioeoa of Haeckel), though their conclusion that there are, at present, no grounds for assuming even a remote relation between the Heliozoa and the Baddolaria (L c. p. 159) appears to me to have no sufficient warranty.
The Heliozoa are defined bj these authors to be uni- cellular organisms, which occasionallj become multicellular, or at any rate multinucleate, by the multiplication of the nucleus. They are usually spheroidal and free, but some are fixed by means of a stalk. In most, the protoplasm of which they consist is differentiated into a cortical and a medullary substance (ectosarc and endosarc). The sharpness of demarcation of the ectosarc from the endosaro Taries. In Aetinophrye sol the two pass, imperceptibly, one into the other ; in ActinosphcBrittm, the change from the ectosarc into the endosarc takes place within a narrow zone, eyerywhere equidistant from the centre. The line of separation between the endosarc and the ectosarc is best defined in the Aean- (hocyatidcB, HeterophryidaB, &a, but it arises only from a differentiation of the protoplasm, and not from the derelop- ment of a definite membranous investment around the endosarc. The nuclei lie in the endosaro. When only one exists it is usually eccentric, and, when there are manj, they are scattered irregularly. The ectosarc contains con- tractile, and sometimes non-contractile, yacuoles, which last may also be met with in the endosarc. The pseudo- podia are thin, filiform, and radiate from the body ; some- times their surface presents moving granules. They rarely branch or anastomose. In many cases they present an axial substance which may be traced as far as the endosaro. The silicious skeleton may c<msist of separate spicula or form a continuous shelL
* "UeberKhisopodeoiindden- of the subjeet will be found Ib ■elben nahestehender Organis- this memoir and in I>r. Car- men." (* ArchivfarMikr. Anat* penter*8 •Introduction to the Bd. X. Supplement heft, 1866.) Study of the Formminiferm,' 1682.
full references to the literature
^Ti^
(ito THS ▲HATOKT OV nTYBSTSIuiLATSD UriMALS.
The JETeZiosoa propagate by siinple diyision with or without preYioQB enc jstation ; and the products of diyision may or may not become encysted. They may either pass directly into the adult state or become monadif orm actiye larfie, provided with two flagella, a nucleus and contractile yesicle, which in course of time develope into the parent form.*
' JL completely new light has been thrown upon the vexed question of the supposed sexual method of reproduction otinfusoria by the inyestigations of Engelmann^f Biitschli^ and Hertwig,§ the results of whose observations may be ■iunined up as follows : —
1. The so-called acinetiform embryos are parasites. ' 2. The rod-like bodies occasionally observed in the endo- plast are also parasites, and probably BaeUria,
3. The globular so-called g^erms in the VoHieeUidcB and the bodies termed ' ovules ' by Balbiani have nothing to do with reproduction.
* As this chapter was passiiig through the press, Hertwie*s monograph ' Zur Histologie der Badiolarien' has come into my hands. The Radiolaria are de- fined as Rhisopods with pointed, branched, usoally anastomosing and granular pseodopodia, which proceed from a protoplasmic body enclosing either numerous •mall heterogeneous nuclei, or a single larger highlydifferentiated vesicular nucleus. The protoplasm of the bod^ is further separated into a peripheral non-nndeated and a cential nucleated portion, by a membranous capsule with porous walls. The capanle is in- vested by a homogeneous gelati- nous substance ; the extracapsular protoplasm usually contains nu- merous yellow cells.
Propagation is effected (pro- bably always) by the breaking up of Um body into unicellular IB0iiadif6im tmYn^tA ^ROtVied With % ibi^t fiai^^aaD^ hA %
result of these inyestigations, Hertwig admiU that the /lo^iio- laria and the Heiiozoa are closely allied, and even suggests that the name of RadiMorim should apply to both groups, which would then form the sub- divisions of Heiiozoa and Cfto- phora. The RadiolariaCCgttjphon) are distinguished into CoUozoa (with numerous small nuclei) and CoUida with a single highly differentiated nucleus.
t *'Ueber Entwickelnng und Fortpflanzung der Infhsorien." (* Morpbologlsches Jahrboch,* 1876.)
X *^Mittheilongen uber die Coijogation der Infiisorien and die ZeUtheUung." (* Zeitschrift fiir Wiss. Zoologie,' 1875.)
§ **Ueber Podophrya gtmmi- parOy nebst Bemerkungen sum Ban und snr systematische Stel- lung der Acineten." (*Morph. Jahrboch,' 1876.)
Iv ^
THB REPBODUCTION 09 THB INFUSOfilA. ^1
• ■ ■ ' • . .' • ■ I '
4. In the VarticeUicUBt when conjugation takes place, the endqplasts of both individuals break up into a number o£ fragments. These become mixed up in the common body which results from conjugation. The endoplast of the latter results from the gradual union of many smaller particles which make their appearance in the endosarc. Whether they are identical with the fragments into which the endoplasts of the conjugated individuals have divided, is not certain.
5. When Infusoria which possess an endoplastule, as well as an endoplast, conjugate, both of these structures undergo division ; and the endoplastule, before division, acquires the striated structure and spindle shape, which has obtain^ for it the name of ' seminal capsule.*
6. The final result of coi^jugation is the appearance in each of the individuals which have undergone conjugation of the endoplast and endoplastule (either single or multiple) which characterise the species.
It does not appear that there is any positive proof that the striated endoplastule, or endoplastules, of the conjugated individuals are or are not exchanged. From Biitschli's observations on Styhnichia myiilua, he concludes that tiie endoplast divides into four fragments ; that these round themselves oS into the so-called ' ovules ' of Balbiani, and are expelled from the body; while, of the four striated endoplastules into which the endoplastules which exist before fecundation divide, one is converted into a large transparent body, and, dividing, g^ves rise to the two new endoplasts which appear in the Stylonichica, after their separation. Two of the others become the new endo- plastules ; while one, apparently undergoing retrogpressive metamorphosis, is cast out of the body.
From these facts, and from the circumstance that the en- doplastules of Infusoria^ which are merely dividing, acquire the striated structure, it must be concluded that the as- cription of a spermatozoal nature to the strise of the modified endoplastules is not warranted. And the remarkable ob-
062 THE UTATOKT OV UrTXBTBBJlATED ▲HIMALS.
•eryations of Butschli, Strassburger,* Van Beneden and Hertwig,t on the changes which take place in the anclei of both animal and vegetable cells, which are undergoing division, or are preparing for fecundation, seem to leave no doubt as to the justice of this negative conclusion. In such oeUs the nucleus becomes elongated and assumes a striated appearance, so as to resemble in a very striking manner, the so-called ' seminal capsule ' of the Infusoria. Nevertheless, it is still possible that the conjugation of the Infusajia may be a true sexual process ; and that a portion of the divided endoplastules of each may play the part of the spermatic oorpuscle; the conjugation of which with the nucleus uf the ovum, appears, from recent researches, to constitute the essence of the act of impregnation.
With the proof that the ' acinetif orm embryos ' of the Infusoria eiliata are parasites, the view of the relations of the TerUaculifera with the CUieUa, suggested at p. 109, oeases to be exactly tenable. Nevertheless the resemblance of the ciliated young AeinetcB to the simpler forms of the CUicUa is so close that they may still be said to be modifica- tions of a common type. Hertwig X has made the interest- ing observation that, in some AeinetoB, the tentacula are of two kinds; those of the one kind are the characteristic suctorial organs, while those of the other kind are simply prehensile, and have a structure very similar to that of the prehensile pseudopodia of the AeHnophryicUB. The same author shows that the ciliated germs do not arise from the endoplast alone, but that a portion of the proto- plasm of the body invests each division of the endoplast. In fact the process by which these germs are developed is altogether similar to ordinary cell-division.
The Opalinina must clearly be arranged among the
t << Ueber PodophryapemmqHtra nebst Bemerkangen cum Bau und
^ < Ueber Zellbildung und Zell- theilung,' 1876.
f "Beitragezor Kenntnissnnd cur systeroatiscLe Stellung der Blldung, B?t'nxcY^tan^ uml Thei- Aoinetea." C Morpbologfaches luns Sea l\\\etT\«ctxwi "^x^^r ^^Su&Wcbu,' 1876.) (« liorph. Ja^Wcks' \^^.^
THX PBTSIiOPMBHT OF »PONOB8. 668
Ii^iuoria. Stein regards them as simplj the lowest forms of the Holoirieha, bat it will probably be safer to consider them as a distinct gronp, standing in somewhat the same relation to the Ciliata as the QregarinicUB do to the AmoebcB.
Sectigit IL — The elncidation of the* problem of the mode of development of the Sponges has been greatly advanced by the investigations of Oscar Schmidt,* Schnlze,t and especially of Barrois4 which confirm the assertion of Metschnikoff that the vesicolar morula >vhich constitntes the early condition of the sponge-embryo consists of blasto- meres of two kinds ; those of the one half of the spheroidal or flattened embryo being elongated and flagellate ; those of the other, rounded, granular and non-ciliated. Schulze and Barrois have independently ascertained that the latter region sometimes undergoes partial invagination ; and that a cup-shaped body is produced, composed of an epiblast formed of flagellate cells and a hypoblast of spheroidal, non-ciliated c^ls. Thus the '* gastrula " stage of Haeckel may exist, though it is not formed by delami nation, as he supposed, but by invagination. But it appears that this gastrula-stage does not always occur, and that when it does, it is transitory, in so far as the hypoblaatio cells subsequently enlarge, protrude beyond the epiblaatio cells, and give rise to the free ovate embryo formed of a ciliated and non-ciliated half, which has so often been observed. According to Barrois' observations, this free swimming larva fixes itself by its non-ciliated hypoblastic half, and the hypoblastic cells are invested by those of the epiblast^ which thus constitute the whole outer covering of the young sponge. The central cavity of the sponge, which represents the archenteron, arises in the midst of the included hypo-
* *Zar Orientirung ilber die £ntwiokeliing von Sycandra
Entwickelung der SpoDffien " raphanui " {' Zcitschrift f. Wiss.
(" Zeitschrift fur Wiss. Zoologie,' Zooloffie/ 1875) ; and *« Zur Ent-
1875) ; and ** Nochmals die wickelungsgescbichte von Sycao-
Gastrula der Kalkschwamme" dra" (Ibid. 1876.)
(• Archiv f. Mikr. Anat.,' 187d). % ' Annales des ^eiences Natu-
t "• Deber den iiau und die reUea,' 1876.
^6i THX AVATOKT OT nfYSSTBBlULTBD awtmat^.
blastic oellB, while the oscnlnm is a seoondary opening, formed apparentlj by an invagination of the ectoderm, and has nothing to do with the primitiTe blastopore. Thus even the simplest sponge has passed beyond the gastmla- stage.
Schnlze has made the important discovery that, in Sy- eandra raphanus, there is a layer of flattened cells external to the syncytinm ; whence the latter may rather be regarded as the equivalent of the mesoderm than of the ectoderm of the Ckdewterata, And the observations of Barrois on other calcareous sponges tend to the same conclusion. The care- ful investigations of the last-named writer have not enabled him to discover spermatozoa in any sponge, and he finds that the ova, when they are first disoemible, are situated in the syncytium or mesoderm, and not in the endoderm. In the free larv® of the calcareous sponges an equatorial zone of rounded equal-sized blastomeres is interposed between the ciliated, or epiblastic, and the non-ciliated, or hypoblastic, hemisphere; and it appears probable that these cells re- present a mesoblast and give origin to the mesoderm. The embryo in this condition has a very interesting resemblance to that of Clepsine, in the stage in which the epiblast occupies one face of the embryo, and the hypoblast, formed of three very large blastomeres, the opposite face; while an incomplete zone of six or eight large blastomeres, which are eventually enclosed by the epiblast, surrounds the margins of the latter.
At p. 150, 1 have quoted Haeckel's account of a process of Entogcuftrie gemmation in Carmarina hasicUa of an altogether anomalous character.
F. E. Schulze* has lately investigated specimens of Oeryonia hexaphyUa provided with entogastric processes beset with budding CwiiruB, and he proves that, in this case, at any rate, the phenomenon is one of parasitism.
* ^'UeberdieCiminenKnotpeii- sohafUichen Yereines.' Grax, aehren im Mtg<ni t. QcrfOTAea.." 1875.) (* BiitthillimgMk4«i'&«toKm^n«Bk-
CT7VINA AND aSBTONIA. 665
The stem from whicli the bads proceed, in fact, is not a process of the bodj of the C^eryonia, bnt is simply attached to the wall of the gastric chamber of the latter. It is hollow, and its cavity is lined by an endodermal epithelium. The Cunina bnds are not developed from the epithelinm which covers the stem and represents its ectoderm, but commence in the ordinary way, as csBcal diverticula of the wall of the stem, the apices of which soon open to form the hydranth of a medusoid, the disk of which results from the outgrowth of the base of the hydranth. In all pro- bability the larva of the Cwnina enters the gastric cavity of the Chryonia as a planula; and, attaching itself to the wall, grows out into a stolon whence the medusoids bud.
It may be suspected that the other cases of supposed entogastric proliferation will prove to be susceptible of a similar explanation.
Although, as I have endeavoured to show, the Ctenophora are readily reducible to the general plan of the Actinozoa, yet considering their many peculiar characters, I think it is advisable to separate them from the Coralligena, as a distinct natural order.
Moreover, the Physemaria must undoubtedly be placed in this section, which will therefore consist of the following natural orders : Physemaria, Porifera, Hydrozoa, Coralligena, Ctenophora,
Section III. — I concur in the proposal of Biitschli* to establish a group, Nematorhyncha, for the genera ChoetO' notus, Echinoderes, and their allies, to which reference is made at p. 192. The Nematorhyncha are divisible into the Gastrotrichaf (Chcetonotua, Chastura, Cephalidiwn, lehthy^ dium, Turbanella, Hemidasys and Dasyditee), which are ciliated on the ventral surface of the body, and the Atricha (Echinoderes), which possess no cilia. Biitschli finds two
♦ " Untersuchungen fiber frei- f See H. Lndwig, " Ueber die
lebende Nematodeu and die Gat- Ordnung GaBtrotricha.' (* Zeit-
tung ChattoHOhur CZeitBchrift tchrift mr Wise. Zoologie,' 1876.) fur WiM. Zoologie/ 1876.),
(W6 TUB ANATOJffT or rNVKBTKBBATBD
cDnvoli)t«cl wat«r -vessels aiiologous to thoee of tbo £ol^«nt, bat apparently not ciliated, in ChjeUmola*.
Sectiok LV. — Our fenowledge of the development of tW ^t'rurfimia has received an impartimt addition in Uw ' M^moire sur le dereloppemcnt embryogenique dee HinuU- sees,' bj M. C. Robia ; irho, amoiig other importiuii ooB- tributiona to embryology, bas rectified some iinport«Bt errors of Rathke reapocting the early sta^^ of the devnlup- mcot of Chpgine. 1 hare found the di^HoriptJoii and 6i^ne* of thii various stages of clcar^e. and of the >tepe by which the blastoderm ia cuaTert«d into tlie yoong CUtptine, given in tbia memoir, to be exceeding'ly acctimtc
The whole process in Clepsine ts v«iy eimilio' to thai whieh hae been desdribed in Baaaei by Kowalewsky* md sliarea witb it the remarkable peculiarity that the fini formed portion of tfae blajstodcrro becumea the hnoial region of the body. Aa tbta blastodermic disk graws. ib miLTgins thiekeu and give riee to tno genn-hands (Emb- streifen). Thcee gradually approximate and eventually unit« upon the opposite face of the ovom. As the dwn
of m..n«lia ia thi- n«i^,.^iL nf tbp ^iff-wnHntinn nl tlM->
CKSTODBBMA, KEOMSITIA AND CHITOK.
667
Manj years ago * I directed attention to the fact that " the development of a Mollusk commences on the hsemal side and spreads round to the neural side, thus reversing the process in Articulata and Yertebrata ;" and it is very interesting, considering the many curious points of ap- proximation between the Annelida and the MoUusea which are now coming to light, to observe that certain Annelids present this especially MoUuscan peculiarity .f As Yon Baer long ago pointed out, there is a striking likeness between the foot of a Grasteropod and the suctorial disk of one of the Hirudinea, The so-called jaws of the Leeches (the ' teeth ' of which, I may observe in passing, are calcified) are curiously similar to an odontophore devoid of cartilages, the representative of the radola being supported on a muscular cushion.
The statement at p. 245, that " no calcareous skeleton is found in any of the Oephyrea " ceases to be true since the discovery of L. Graff, J that the minute spines of Chcsto- derma are calcified. It is a further peculiarity of this genus, that two distinct nerve cords proceed from the cerebral ganglia, parallel with one another, on each side of the body, in the place of the single median nerve cord of other members of the group.
Dr. Jhering § has directed attention to certain points of
may remark that the evidence upon which the identification of the stmctures termed ** Kiemen- gangwOlflte *' and their products with the branchial apparatus of vertebrate animals is founded, appears to me to be wholly in- adequate to bear out the con- clusions deduced from it.
^ ' On the Morphology of the Cephaious MoUusca.' (* Phil. Trans.,' 1852, p. 45 and note.)
t The mode of development of the central nervous svstem in Ewxxe* and Clepsine often many points of interest Not the least important of them is the obvious similarity (to which atteniion
has already been directed by Semper) between the germ bands of Clepnine when tiiey have united throughout the greater part of their length, but surround the blastopore behind, and the Amphibian embryo with its dorsal ridges, which have exactly similar relations (See for example Fig. 40, in Plate III. of Gdtte's work *■ Die Entwickelungsge- schichte der Unke.*)
X *'Anatomie des Chtttodenna nkidulum:* (* Zeitschrift f. Wiss. Zoologie,* 1876.)
§ * Vergleiohende Anatomic des Nervensy stems der Mollua- k«i< \«ll.
668 THB ANATOMT OV INTBBTEBRATBD ASLKA_LS.
resemblance bctveen durtodtinna, Willi tlio allied gmnt Neomenia. imd the ChitouB, cipeeinily in Uiu nrrangiMnent of the tronkB of the nervous 8/at«m j and he pt-->||>o««« to unite the three iDtoagruupof Jifip^iw^tira — thus (i<--p.-initiag the Ohitona from tho iSollutaa oltogi^tlicr.
Sbctiok V. — I regret that I hnve bpcn unnble \<t main use of Claas' recently publiehod iDi|iovt>iQt contributionf to the history of the develcipint-ut of thi- CrualacBa.*
Sbgtioh YI.'-The thorough eiaminationof the atmetan of Pediaillitia and Loxoermia by Nitechet has ehowB tk^l the differences between the ectopri>ctoiiB and the endopmct- oue Polyma are of a more fundamental character than had been suspected. In the Eetoprocla, in fact, the endoc^st coneisls of two layers, an out^v and an inner ; of which the former ia the representative of the ectoderm in other m The latter lines the wall of the ' perivisceral cavity,' «; rehccted thence, like a peritoneal tonic, over the t eht^th and into the interior of the teotacula. wheuM itH continued ou to the oliraeutAry canal, of which it fonna du external investment. The endoderm. which Uties the all-
THE BOTOPBOCTA AND THB BNDOPBOCTA.
669
Unfortiiiiatelj, our knowledge of the embryome de- Telopment of the ectoproctons Polyzoa does not enable ns to determine, with certainty, the natnre of this perivisceral cavity and of the layer which bounds it. Nitsche shows that the saccular cystid, which results from the first de- velopmental changes of the embryo in the FhylactoUBmata, is composed of two layers, which correspond with those of the endocyst in the adult ; and, further, that the polypide (alimentary canal, tentacula and ganglion) results from an ingrowth of the outer layer of the endocyst, which pushes before it an involution of the inner layer. The latter gives rise to the reflected * peritoneum.'
But I am not aware that there is any evidence which proves conclusively the manner in which these two layers of the embryonic endocyst take their origin, or with what layers of the ordinary embryo they are homologous. If we make the ordinary assumption that the inner or peri- toneal layer of the endocyst is the partial or complete homologue of the hypoblast in other animals, it foUows that the perivisceral cavity of the Ectoproda is really an enterocoele, as it is in the Brachiopoda, The only other alternative appears to be the supposition that the inner layer of the endocyst is a mesoblast, differentiated from the germ earlier than the hypoblast ; in which case the perivisceral cavity will be a schizocoele.
Dr. Jhering*s work on the nervous system of the MoUugca, to which I have already referred, contains a number of valuable anatomical details, and especially gives a better account of the structure of the nervous system of Chiton, than has hitherto existed.*^
* In addition to a great variety of surprising phylogenio specu- lations, Dr. Jhering puts forward the novel morphological views, that the respiratory sac of the Pulmonata (Nephropneutta^ Jher- ing) is morphologically a sort of urinary bladder, and that the ganglia whence the arm-nerves of Uie Cephalopoda arise
are
cerebral and not pedal. The arms are thus parts of the head, and only the funnel represents the foot of Gasteropoda.
I do not presume to rebel against the authoritative censure of my memoir on the 'Mor- phology of the Mollosea,' pub- lished now five-and-twenty yean ago, which ia igEnfonraabSM^Xi^^V^*
^^ r KK\. •« .
■ ■ 1 1
I
li!
such an a.s8eDibl:i«^o, tormcd i\n t<*ri«t(l by the sc'Ljniontatiou ci cuticiila; tbo absence of cilia i period of life ; tbo sc^niontiit system and its perforatiou by t (with tbc possible exception t - ' wbicb, almost always, are tbems
. ! The reasons for includiog the
' , . have been given in Chap. XI. ; t
I j j * must be regarded as hardly wit
i ' \ I 1 tion, 1 think that, taking into
i j j cations which are undergone bj
Artxchnida, it is not needful t I } i f practice of associating them wi
The Lamellihranchiata and
I ; another very well marked di
t ' , characters of which have been
I I ; ^ M The proposal to separate tl
J I •- Mollusca, to which I have aire
I : V to be devoid of any justificatios
certain Ocphyrea, such as Cha the Polyplaeoj)Jiora, are accom
: ! ,i .1
; li .
I ;
^
1 '
I >
i
THS HiaHSB aBOUPS. 671
and even if these resemblances are to be r^^arded as evidences of affinity, some considerations, such as the restriction of the branchis to the hinder part of the body, and the reduction of the foot in ChiionelliiB, rather l^id to the suggestion, that Ouetoderma and Neomenia may be ex- tremely modified MoUusks, allied to the Polyphicophora.
As to the supposition that the resemblances between the Nvdibranckiaia and the TwrheUaria indicate a direct affinity between these groups, it seems to be forgotten that the Nudibranchiata are all, when young, unmistakeable Guste- ropods provided with mantle and shelL Their adult struc- ture is as little evidence of any Turbellarian affinities as that of Lemcsa is proof of its being allied to the worms, rather than to the Crustacea,
The Physemaria, the Porifera, the Hydrozoa, the CoraL Ugena and the Ctenophora are obviously modifications of the same fundamental plan. I think it is convenient to retain the well-established name of Ccdenterata for the last three orders, which are much more closely related to one another than to the other two. Haeckel*s proposal to apply the old name of Zoophyta to the whole division appears to me to be well worthy of adoption. The inconvenience of using a term the connotations of which have varied somewhat widely since it was first invented, is probably less than that which would attend the invention of a new name.
The Monera, Foraminifera, Heliozaa, Badiolaria, Proto- plagta, Qregarinida, CataUada and Infiuoria, (OpaUninat CUiaia, Tentactdifera, Flagellata), again, are so closely united together that the difficulty is to distinguish the less diffe- rentiated forms of each from one another. They consti- tute the division of the Pbotozoa, the common characters of which have been given in Chapter 11.
If there were no invertebrated animals beside those in- cluded under these four divisions of Abthbopoda, Mol- LUSCA, Zoophyta, and Pbotozoa, the task of claasificatioii would be very easy, and each of the higher dSviiiiniM
672 THB ANATOMY OF
would lie ahfu-ply defineii from the othera. But a raM*' mBiduum remaiuB to be countered: luul it in witb Um att«mpt to arranife these ivaidiuU urlI(^rB into bigbrr ^>^ulM that tbe difficiiltieB of the TaxonomUt comuiMi«u.
The Polychala and the Olu/neliiEta. tlie llimdinta and the Qephyrea, rcflemble one another gienerall; in the Begmenta- tion o! the body, indicated sit temat by the amoJly mnlri- gtrngliiite nervous centres;* in the presenile of cilia luid of Begmental organa ; and in the nature of the larrv which are set free, when their emhryoB are batched in an early Btago ot development. Aud although no one of these chu- ractere is of universal occurrenee (cilia, for example. lieiDj: absent in most adult mruditiea)^ yet they are foand in Buch nEBociation, that the accepted ammgemcnt of thmi> four groups (to tvhich, though not without some faesitatioli. I add the Myeoitomata). into the division of the Aunbioiia, is undoubtedly very convenient.
The Tretnaloda, the Twbellaria, and the Bot^ra, tana another very natural asicmbliige. But it mn«t be admiUed that the highest forma of this division are seiianitcil bjae very sharp line of demarcation from the Annelida ; wliileUw ■im|ileat TurbeHaria are almoat on a level with ti
THE HIGHER GSOUPS. 673
form between the TurheUaria and the Polychcsta ; while the Botifera, in many respects, represent larval forms of the Polychceta and of the Oephyrea,
The Cestoidea are usually regarded as anenterous Trema- toda, in which case, of course, they must be associated with the latter.
I propose to establish a division of Tbichoscolices for the natural orders now enumerated, in order to dis- criminate the morphological type which they exemplify from that of the Nematoscolices, containing the Nema* toidea, which are as remarkable for the univei'sal absence of cilia as the former are for their presence ; and which are further so cleai'ly distinguished by the arrangement of their nervous and muscular systems and of their water- vessels ; and by their ecdysis.
The connexion between the two divisions by way of the Nematorhyncha and the Botifera is undoubtedly very in- timate, and there is almost as much reason to an*ange the Nemaiorhyn^sha with the Trichoscolices, as with the NemO' to8colic€8. On the whole, however, I think that, notwith- standing the cilia of the Oastrotricha, the closest afi^ties of the Nematoi'hyncha are with the NerruUoidea, and I there- fore place them among the Nematoscolices.
But I may remark, once for all, that the attempt to establish sharply defined large divisions of the animal kingdom is futile. The progress of knowledge every day rendei*s it more and more clear, that morphological groups are comparable to distributional provinces; each, however well marked may be its characteristic features, shades off at its margins into some other group ; and the object of classification is simply to bring into prominence the morphological types which embody these characteristic features.
It appears to me impossible to compare the structure and the larval conditions of a Polyzoon with those of a Brachiopod, without arriving at the conclusion that they are more closely allied with one another thaxi.tii«3^Btfe^'«>i^ia^
674 THE AHATOICY OF INYBBTBBBATED ANIMALS.
any third gronp. Nereiihelefls, the Polytoa approach the Botifera, and the Brachiqpadaj the Annelida, on the one side; while, on the other, thej present nnmistakeable affinities with the lower MoUtuea, At the same time, the weight of the resemblances between the Fclytoa and the Tunicaia, which led Milne-Edwards to the establishment of the gronp of " MoUusco'ides " (adopted by myself under the title of MoUuscoida) has been much lessened by the progress of investigation.
I conceive that we may best keep these resemblances and differences in view by associating the Polyzoa and the Brachiopoda into a division apart, for which I propose the name of Malacoscolices ; in order to indicate its rela- tions with the Worms on the one side, and with the Mollusea on the other.
The Tunicata are absolutely distinguished from all other invei'tebrated animals except Balanoglosstu, by the per- foration of the pharynx and its conversion into a respira- tory organ.*
At first sight there appears to be little ground for the approximation of groups apparently so widely different as the Tunicata and the Enteropneusta. But the extraordinary similarity in the structure of the perforated pharyngeal sac in the larvee of Tunicates and of BalanoglossuSf is a fact of great morphological weight. An ecaudate Appendicu- laria of those species which have the alimentary canal nearly straight, would be marvellously like a larval BalanO' glosstu, which is again little more than a specially modified Turbellarian. I think, therefore, that the Tunicaia and the Enteropneusta may proi)erly constitute a division of Phabtngopne usta.
The Tunicate PharyngopneuHa, with their caudate larvss may be supposed to stand in the same relation to the
* I have alluded above to the lead ne to conclude that the
structures degcribed by Semper structures in question are homo-
in some Oligochaeta and in Sabeila. logons with either Vertebrate,
I do not d«tuA)t xYi« ac^xunjc^ «»<( Enteropneustal or Tunicate
the description \ but \\. ^<>«» imi\ XsnEM^ciMt,
THE AKEKTEBOUS INYEBTEBBATA. 675
Turbellariform Pharyngopneusta, as the Trematoda, with their oercarif orm larvs, to the TtMrbeUaria,
Another very well marked diyision is that of the Echino- DEBMATA, the characteristics and relations of which have been folly discussed in Chapter IX.
Although the structure and development of Sagitta have now been as thoroughly elucidated as those of any animal, the proper Taxonomic place of the Choetognatha is still an unsolved problem. The issues, however, appear to be narrowed to these : either they belong to the Annelida^ or to the Nematoscolices, or to the Trichoseoliees ; or the Chceto- gnatha are to be regarded as an independent division, allied to all these, and perhaps to the lower Arthropoda, I am disposed to adopt the last view, chiefly on the ground of the mode of development of Sagitta, which is unlike any- thing at present known to occur in Annelida, Trichoseoliees, Kematoseolices, or Arthropoda,
The Aeanihoeephala are hardly less anomalous than the Chceiognatha, Taking into account the Oordiacea and the characters of the proboscis in the Nematorhyneha, there is undoubtedly room for the suggpestion that they are specially modified anenterous NemtUoseolices, and should be classed among the latter. But here, as in the case of the Cestoidea, there are many difficulties in the way of accounting for these anenterous forms by the supposition that they are the results of a retrogressive metamorphosis of enterate animals.
This question of the true relations of the anenterous invertebrates — ^by which I mean not only those which, like the male Rotifers, have no functional alimentary canal in the adult condition ; but those which, like the Cestoidea and the AcarUhoeephala, never exhibit a trace of an alimentary canal, even in the embryo ; which is usually dealt with so summarily by the assumption of retrogressive metamor* phosis, acquires still more importance, when we attem'Qt to determine the Taxonomic place oi \]b& Dic^wwto»
67(1 THE ANATOMY Olf IKVEKTKBHATGD ANIMALS.
Pi-ofeaaor E. van Beiieden haa proved that theec panislUe cajinot be disniiBeed, santfa^on, as rctrogreMJvel; metajnor- pboEcd ' worma ;' and ihougli I aiii not disposed t« attei^ iQucb weiglit to the ubeence of a meH>dcrm, on wUieli Tu Beoeden maiBte a« a distinctiolilwtw^en tbo Dicf/mtida uid the Metaxoa, the majULer in which tliv cuntenU uf the aiia] cell give rise to genua ia so completely tuUiko Bay- thing which ia known to obtain in the Melatoa, aa, to mj mind, to justify the aeparation of the IHeyemida trata the whole of tbia division. On the other band, the similarity td their development to the ftirmation of oaetosoic embryua hf epiboly, aa completely divides the Di^e^nida from all the Protozoa, It niuat be recollected that the chiLDgea which are undergone by the cili&t«d emWyoe aie etiU to be dis> covered; but. provisionally, I um disposed Ui agree tviib Van Beneden, that the Dieyrmido should be regarded M the repreaentatives ot n diHtinct division, th« UssoxtU, intermediate between the PTototoa uid the Metatat. Ami without distinctly pledging myself to way auch view, I jk thinli it ia worth while to throw out the suggeatiou tluU the Ceatoidfa, if not the Acanlhoevplutla, may be modiSa-
tm MlOAL ttlliAf X6tf6 61" iirtrifittfiAA^^. 6?7
lin^, none of wtdoli, aa f ar as ottf pifdftent knowledge enabled us to jtidge, can be traced, withont intermption, thronghOnt the whole length of the eetdei
If we asenme, in the absence of proof to the contrary, that the Monera have the simplicity of stmctnre ascribed to them by Haeckel ; then^ on comparing the BndoplatHca with the Monera, the different gronps of the former appear to be related to those of the latter diyision, as if thej were similar forms complicated by the addition of one or many nnclei. Proiogenes may thns be considered as the root of the Foraminiferal series, Pratama^a of the Protoplasia, MytMghrum of the Chregarinidct, VampyreUa of the Heliotoa, Prak/monaa of the FiagelMa, A Moneran, cUiated over liM whole sorfaoe, which might stand in the same relation to the OpaUrnna, CcUaUcicta, TentaoiUifera, and Ciliata, is at present unknown. The Proiozoa thus fall into the following
series : —
PnOTOZOA.
I. n.
^ Prctog9M9, Prokmaha,
I I
FoTomiin^ferfu Protoplcuta.
III. IV.
MyxasirtMn. VampyreUa,
GregarimdcB, HeUozoa,
Badiolaria.
V. 7
I
?
vn.
Protomonas.
TfffUaculifera, Caia^
Uaeta, PlagetuUa.
Opalmvna, Ciliata.
1 am nnable to trace any one of these series of modifies^ tions further ; that is to say, to find forms which actually bridge over the interval between any one of them and the Metazoa, though it is easy enough to imagine what such forms might be. The spheroidal free-swimming ih.<^i&s^ aggregates, such as Uvella and Polijionnva^ «aA lAn^tMr^^xenw
678 THE ANATOMT OT IBTEl
itself, ore. in many reepecta. comparable to Phjaenuuian or Poriforan embryos ; while an animal Volvor. wonld be » aort ot permanent vesicular mornliL So, one of the hif^her In/iitoria, if it became multiniicWt«, Uko an Opaliaa. would approach the lowest TttrbaUaria,
The axial cell of a Dw^«na. from tbo protoplwiin of -which its ciliated and noa-oiliat«d genua tire produced, i*. to a certain extent, comparable to the capnnlu of a BAdii»> larian ; while, on the other hand, n Radiolarian with a multinucleate cortical layer would approach the etructmv of Dieyema, And if what is at present known of i)(«y«mit girea a just couoeption of the essentia! points tA its eatiTv hiBtory, it nndoubtedly, as E. Tan Beueden has sngge8t«iil. repi-escnts a type intermediate between the Proiotoa aad the Mdatoa. though it can hardly be said to fill np tiM ( hialvi between them.
ill ip th«_tj
f animA^B^I
In our further search after the aei-ial rolatiot we must therefore start nfresb from the lowest MetoMiM. Here a ZooPHTrro Sekies is very W'lU marked; com- menoing with the Phy»nmtria. and thence diverging, on the one hand, to the Porifera. and, on the other, to Uw
9HB BBBIAL BBI.ATI0N8 OF IKYEBTSBRATA. 679
The Nemaiorhyneha, wHetHer they are really trandtional forms between the Nemataidea and the Arthropoda or not, at any rate, indicate the road by which the tranfiition may be effected; and I am mnch inclined to think that the ChcBtagncUha may occupy a place in this seriee. The oral armature of Sagitta may be regarded as a modification of the oral spines of Eehinoderes, and its nervons system is as mnch .Ajrthropodal as is that of the PetUoHomida. This may be called the Asthsozoic Sekibs.
A fourth series is that which I shall term the Mala- cozoio Sebiss. It includes the Mdlaeogeoliees and the Molltuea, The entoproctous Polyzoa form the lowest term of this series. The resemblances of the Polyzoa with the Motifera (e. g. with St^hanoceros) have often been remarked, and, indeed, insisted upon, with too little regard to the differ- ences which are established by the water-vessels and the peculiar pharyngeal armature of the Rotifers. Neverthe- less, these resemblances are important as far as they go, and in grade of organisation the two groups are much upon the same level. On the other hand, the comparison of a Polyzoon with a larval Lamellibranch or Gkuteropod, or with a Pteropod, leaves no doubt in my mind that the Malacoscolices have the same relation to the MoUusea, as the Triehoscoliees to the Annelida.
A fifth gradation is presented by the TSmioata and the EfUeropneusta, which constitute the Phabtnoopnbu8TAI« Sebies. I do not regard the Enieropnetuia as of distinctly lower organisation than the Tunieata, but rather as a collateral group ; and I conceive it to be probable that some lower forms, connecting the BinteropneuHa and the Tunieata with one another and with the TrickoacoliceM, will yet be found. However this may be, Appendieularia presents a grade of organisation but little higher than that of the Polyzoa.
A sixth gradation is representedb^ V^afc'¥jCHX»^TS«wa>3*.
t ISTERTKBEATID KVXXXIS. 1
Sksies. Liko th« fore^ing, this aeri«« at preooat etamda icolated.'no uiDectent forma between the Eckinodorma Bti4 liigher or lower ^Tcrnpn 1>eiiig known. On the ground ot tlie vntEormity of chaTiuTt«r of the knw of tbe £oluiu>- dermH. honercr, there cut be little doabt llwt, if «ver mch larniH are diacovercJ, tbey will proTc to be tUiod to the Qrphgrra, the TriehtiKoliwt and tbe Bnterojiaaiuta.
Thna the ntadj of the frroduticinB of atractnre Bmong the Mi^tatoa leade to the conctnBiou that they full into ax seriea, which may be anunged in the tuUowing tabular
S&RtES.
I.
n.
in.
|
ZOOPHYTIC. ECHINODEKMAL. Ccelentfral.!. Echtnodennata. E-it Parifn-a. |
pHABYSOOPSBUaTAt. |
|
|
IV. Malaoozoic, |
T. Asirntorv. |
TI. -. Arthiiosok:. ^ Artliropoda. i |
i
THE BBSULTS OF EMBBYOLOaT. 681
form ; and that common form, when the special characters of each group are eliminated, and the alimentary canal is reduced to its primitive aproctous condition, would be exceedingly similar to a Physemarian.
Hence the consideration of the gradations of structure which are presented by the various series of Invertebrated animals, irresistibly leads to the conclusion that the whole of the Metazoa may be conceived as diverse modifications of a common fundamental plan.
The Sebial Relations of the Inyebtebbata com- PABED with the besults OP Embbyoloqy. — The concep- tion of the unity of organisation of the Invertebrata thus reached, so far as it is based upon the comparison of adult structures, is purely ideal ; and the study of the development of individual animals is alone competent to decide the question whether this ideal unity has a foundation in ob- jective fact. But the history of the development of animak appertaining to every group of the Invertebrata which has been given, bears out tiie statement which is made in the Introduction that the ideal unity has such a foundation in fact ; inasmuch as all these animals commence their existence under the same form ; that namely of a simple protoplasmic body, the ovum or germ.
In the Introduction I have said that " among the lowest forms of animal life, the protoplasmic mass which represents the morphological unit may be, as in the lowest plants, devoid of a nucleus " (p. 12). However, as I have remarked at the commencement of this chapter, until the seai*ch for the nucleus has been instituted afresh, with the help of such methods as have recently proved its existence in the Foraminifera, 1 think it will be wise to entertain a doubt whether any of the Monera are really devoid of this amount of structural differentiation ; and the tendency of recent in- vestigations appears to render it very questionable whether the nucleus of the ovum ever really disappears, whatever may be the modifications undergone by the germinal vesicle and its contents. I shall, therefore, assume provisioiiaUyy
I
of the euihryo iroiu mv y,..
ovum, in iill thn Mrf.ivni. i« ita di
forui -if division rortults iu the fon
diHcoidal muss of eiiual. or Biib-oqii
blastomerca. Next, tlie nwruln.
acquires a central cavity, the hi
hollow vesicle, the blaatotphere, tht
of a ainfflc layer of bloatonierce, is
I ' The bhistomercH of the hUetod
' entiatiun into two kinds, diating
activities, if not by their outward
> set constitute the cpiblatt. the O
further changes of the cmliryo ar
tendencies towards further rood
I epiblastic uud hypoblastic blastoi
; ' of these is, as it were, a ffcrm,
I , ' tho adnlt organism will be evolTc
1 i Every series of the Invertebrata
''( of examples of the further iiiodif
\ by the process of iHvaifituition:
■^ J which is that the hypoblast be
) ' pletely enclosed within the epit
; j accompanied by the diminution
' ' blastoccele, and the formation o
THB BBSXJLT8 OF BMBBTOLOOT. 683
It very oommonly happens that the process of deyelopment is modified by an inequality in the size of the blastomeres ; which inequality may be manifest from the bisection of the OYum, or may appear later. In this case, it usually happens that the smaller and more rapidly dividing blasto- meres belong to the epiblast, and the larger and more slowly dividing to the hypoblast. Moreover, no blastoccele may arise, and the process of inclusion of the hypoblast within the epiblast may have the appearance of the growth of the latter over the former, or what is termed epiholy; while the archenteron may not be formed within the hypoblast till very late.
When, in cases of epiboly, the blastoderm is small in relation to the vitellus, the epiblast and hypoblast, at their first appearance, necessarily adapt themselves to the surface of the yelk ; and thus the gastrula, instead of having the form of a deep cup, becomes more or less flattened and discoidaL I am inclined to believe that all the various processes by which the gastrula or its equivalent are produced, are re- ducible to epiboly and emboly. Even when the epiblast and the hypoblast appear to be formed by delaminationt or the splitting into two layers of cells of a primitively single-layered blastoderm, there seems little doubt that what happens is, either the very early inclusion of the hypobiastic blastomeres within those which give rise to the epiblast ; or a very late and inconspicuous ingrowth, or invagination, of the hypobiastic region of the blastoderm.
If we employ the term gastrula in the broad sense defined above, it may be truly said that every metazoon passes through the gastrula stage in the course of its development. The question whether the mode of develop- ment of the gastrula by emboly is primitive, and that by epiboly secondary; or whether epiboly is primary and emboly secondary; or whether the two processes have originated independently, is of secondary importance, and belongs to the debateable ground of phylogeny.*
* Compare Haerkel, **Studien zur Gastrsea-theorie," in hit *Biolo- gische Studien,' 1877.
GS4 THE ANATOMY OF INVBBTEBBATBD ANIMALS.
The meauing of the differeutiatioii of the agg;regate of cjtodes, of which the bod; of a siaiple metazoon ia com- poaed, into a hypoblostic, or endodermal, and an epi- blastic, or ectodermal group ia to be sought in the pUyBiological division of labour, which is the primarj source of morphological changes. It is a separation of the aggregate of morphological units into one set with a specially nutritive, and another set, with a specially motor and protective, fnnction. It is quite possible to conceive of an adult metazoon having the structure of & sponge embryo ; moving by its ectodermal hemisphere, and feeding by its endodermal hemisphere.
The next advance in orgaaisatioii of such a metazoon would doubtless consist in the more complete extension of the protective layer over the nntritive layer, with due pro- viEion for the access of the eurroimdiDg medinm to the latter. It ia obvious that this advance might he effected in either of two ways ; the one by emboly, the other by L'lAljoly. In the former, the lilastopore w(mld be left aa the npiertttre of commiuiication of the endoderm with the exterior; and the result would be the formation of an iircIui:iiel<jiiiiiloiiegasiTaia,.»\n:h aatbat which is suppoaed by
r*-r
THX BBSXTLTS OF XMBBTOLOGT. 685
ition of the epiblaet and the hypoblast, or it may id constitute the peri-enienm, or primitiye peri- iTity.
Miimalfl which, in their adult condition, most •resent simple gastmlsd with obliterated blastocoele, ^y§emaria and Hydra ; cup«shaped bodies with an ing at one end, the walls of which are made up an ectoderm and an endoderm.* preat majority of the Metatoa) a further advance cation is effected by the appearance, between the ad the hypoblast, of cytodes, either isolatedly or QuouB layer, which constitute the mesoblast, and r are conyerted into mesodermal structures. The bheee is still a matter of doubt, but in many cases 1 to be unquestionable that they are derived from •last.
rienteron, more or less interrupted and broken e constituents of the mesoblast, may give rise o the perivisceral space, or channels, of the lich thus constitute a aekizocmle. It is hardly I think, that the perivisceral cavity takes its this manner in the Botifera, the entoproctous he Echinopsedia of the Echinoderms, the Tuni- the NemaJtoidea,
other hand, in many Iwoertebraia, one or more El of the archenteron extend into the perienteron mtained mesoblast. Sometimes, as in the Coelen- »e remain connected with the alimentary cavity it life, and are termed gastrovascular canals. In !8 {Eehinodermata, Braohiopoda, ChcstogncUha) they ut off; their cavities constitute a variously modi- ^coele ; and their walls give rise, along with the mesoblastic elements, to the mesoderm. )h of these two possible sources of the mesoderm, ermal structures of the Annelida and the Arthro- Lch so very generally take on the form of two
fcibink that KIeinenberg*s fibres in Jlydra strictly represent I, though they occupy the potiUon oC ou^
686 THE ANATOMY OF INVSBTEBRATED ANIMALS.
longitudinal germ-bands in the embryo, and subsequently undergo segmentation, are to be referred, is a yery interest- ing, but, as yet, unsolved problem. It is possible that they are solid representatives of the hoUow diverticula which, in other animals, give rise to the enteroocele; in which case, the perivisceral cavity in these animals will be a virtual entero* ooele. On the other hand, they may merely represent the cells of the mesoblast of the entoproctous Polyzoa and of the Echinopsddia, and their perivisceral cavity would then be a schizocoele. But it is needless to pursue this topic further ; enough has been said to show conclusively that, however different one invertebrated animal may be from another, the study of development proves that each, when traced back through its embryonic states, approaches the earlier stages of all the rest ; er in other words, that all start from a common morphological type, and even in their eztremest divergence retain traces of their primitive unity.
It is very important to remark that these morphological generalisations, so far as they are correctly made, are simple statements of fact, and have nothing to do with any speculations respecting the manner in which the inverte- brated animals with which we are acquainted have come into existence. They will remain true, so far as th^ are true at all, even if it should be proved that every animal species has come into existence by itself and without reference to any other. On the other hand, if there are independent gi'oundif for a belief in evolution, the facts of morphology not only present no difficulty in the way of the hypothesis of the evolution of the Itwertebrata from a common origin, but readily adapt themsdvee to it.
Hence the numerous phylogenic hypotheses which have of late come into existence, and of which it may be said that all are valuable, so far as they suggest new lines of in- vestigation, and that few have any other significance, do not desire to add to the number of these hypotheses ; and I win only venture to xemark that, in the absence of any adequate -j^aS^saisfi^uc^o^^^cA^iA^^ of the IfwerMnraiaf
f~ « 1
PALiBONTOLOOY AND PHYLOGBKY.
687
any attempt to constmct their Phylogeny most be mei*e Bpecolation.
But the oldest portion of the geological record does not furnish a single example of a fossil which we have any reasonable grounds for supposing to be the representative of the earliest form of any one of the series of invertebrated animals ; nor any means of checking our imaginations of what may have been, by evidence of what has been, the early history of invertebrate life on the globe.
Already, indications are not wanting that the vast multi- tude of fossil Arthropods, Mollusks, Echinoderms, and Zoophytes now known will yield satisfactory evidence of the filiation of successive forms, when the investigations of palaeontologists are not merely actuated by the desire to discover geological time-marks and to multiply species, but are guided by that perception of the importance of mor- phological facts which can only be conferred by a large and thorough acquaintance with anatomy and embryology. But, under this aspect, the palaeontology of the hwertehrata has yet to be created.
^^'=L
AbioEenesis, 35. AbioTagicBl uiensu, 1. AcBiilliohdclU, an. Acnnthocephala, 646, B
Aciuftjc, 95, 100, 101, 107-109. AcriJiilffi, 439. Aelinin, 53, 156, 173.
ullferom, G14.
pjCDOgDDoidM
AmuioQ, 69. AnKL'bB indiogB. 91.
AmiElni. 5, 91, 92,1... Amouroucium prullferam, Amphibia. 59, 60, es, 71, i.^. AmpbldiiDiu. 117. AmpliiilotuB cordatiu, 573 Amphloxug, b». 60. AmphlpodB, 36a. Amphiihttf, 3.'i<l, Ampul laria, 61. Anitomy, 9, 10.
Anenteroiu lavettebrfttM, 6T5. Anguillula brevlapiuoB, 637.
scBndeDS, 6(3.
Animals, cbaroclen, 43 ; moipho- phyiio"
INDEX.
689
Aptychus, 536. Apus, 258, 27d-281.
caneriformis, 279-281.
gloclalii, 278.
Arachnida, 59, 253, 256, 371,
670. Araneina, 379. Area, 486. Arcella, 91. Arctisca, 387. Argonauta argo, 538, 539. Argulufl, 276.
** AristoUe'B lantern," 575. Arthrogastra, 371. Arthropoda, 15, 29, 34, 52-57, 65-
67, 218, 234, 250, 256, 371, 670,
678, 685. Arthrozoic series, 679. Articalata, 469. A scans nigrovenosa, 644. Aseetta primordialis, 112. Aseidiaus, 44, 52. Ascidioida, 595. Ascones, 115, 116, 120. Ascala, 11.5. Asellus, 363, 367. Aspergillum, 472. Aspidobranchia, 517. Aspidogaster conehlola, 194-201* Astacut, 68, 250» 303-^88.
flaviatilis, 306.
Asteridse, 543, 5.^. Astrsea calycularis, 163. AtaxB<msi,383. Athorybia, 143.
rosacea, 140-142.
Atolls, 169. Atrocha, 243. Aurelia aurita, 134. Avicolaria, 457.
Bactzria, 4-7, 36. Balanidae, 299.
BalanogkMBus, 52, 629, 674, 680. Balantidiom, 105, 106. Balanus, 291-299.
balanoides, 296.
Bees, 30, 31. Beetles, 424. Beleronitidffi, 541. Beryx, 38.
Bicosoeca, 96.
Bilbarzia, 202.
Biogenesis, 38.
Biology, principles, 1 ; diviaions,
9. Bipinnaria, 561. Blastoderm, 444. Blastoidea, 594. Blastomere, 14» 16, 23^ 29, 32, 47,
367,484. Blastosphere, 484. Blastostyle, 131. Blatta, 351, 431, 433, 438, 442,
444.
orientalis, 401, 414, 417.
Blood and circulatory apparatus,
57. Bojanus, organs of, 52, 58, 62,
68, 478. Bombus, 429, 432. Bothriocephalus, 212.
latus, 208.
BotrylUd», 600, 604, 609, 612-
616. Botrytis Bassiana, 43. Brachionus, 190. Brachiopoda, 452, 461, 486, 685. Brachvura, 324, 338^ 340. Broncneliion, 214. Brancbise, 58. Branchiogasteropoda, 494, 505,
506,509. Branchiopoda, 277. Branchipos, 283, 285. Brisinga, 560. Bryoxoa, 453. Buccinum, 506.
undatum, 489.
Bucepbalus polymorphut, 205. Bugula avicularia, 457. Butterflies, 425.
O.
CxiiCIBTOlfGIiB, ll^iaO.
Caligus, 276.
CalycophoridsB, 38, 128, 141-
145. Cambium laver, 16. Campanulana, 130. Campanularida, 128, 129. Campodea ntanbylinos, 420. CapiUlla, 227.'
|
Cnprellft, 363. |
Clionid», 119. 130. |
|
(.larclniii raaTM, 341, 349 350. Cardium. 185, 4S6. |
CocC™«*,3M. ' |
|
Cannikrina, 1^6,150. |
|
|
CaryaphyllKUs, m. |
Codonellido, 106. |
|
CnUllarto, 34, 109, 671. |
Codwig*, 96. |
|
OiiilBrpB, 47. |
|
|
CMldomyis, 448. |
lis, 120, 12&, 671, 685. |
|
CpII», 10. 15, 24, a7-2U. |
CiEnanu, 209, |
|
Cell-Ull, 12, |
Cold, aaUin of, on living matter. |
|
Ccntii)odM, 399. |
4. |
|
ColBUpIera, 424, i%, 438. |
|
|
Cephf. or-cllau, laS-iaB. |
Collombola. 252, 420. |
|
I'ermritE, 53, 204, 406. CerennUiiii, 163. |
Colpod., 6i, 103, 106. |
|
CstMulcB, 57, 176, 206, 673. |
ComatulB, 34. |
|
Ceiliacinn, 71. |
(AB;«>l«n),5=4. |
|
Cetiioeo, 72. |
Conjugation, 23, 77. |
|
Cliff lodermn, 067, |
Contractiio li.iue, 25. |
|
ChiElaKastcr, 219, Q^. |
vacBole, 76, |
|
t.'hn;toBn.Iha, 612. 67S, 6BS. |
|
|
Conlligena, 153,671. |
|
|
94, B6. |
CoralUue*, 453. |
|
ChBDfici, cyoliul, in living mkt- |
CoralliW, 155, |
|
ter, 3. |
Cora Ilium rubrum, IM. |
|
Chora, 15. |
Coral., 120. |
|
Chemical compoiiUon of living |
Conlylophora,3i,M, |
|
C-^ryne, 129. |
|
|
rhirk, 13,^^^ |
Crayfish, 3I». 30^1, 328, 330. |
|
^M |
CjnthJ
,8«.
Crpraa Europna, 409. Cjprii, -iSS-iW. Cyatic wotm, an.
CyitidcK, MS. CjLhare, 'JSd, 389.
DAUumm, 309. Dkpbnit, 383, M*. Dceapodi, 540.
Deep-iM fiDnk, 3^ 70, HI, 85. I>«adroc<Ela, ISO. DenUUdr, Ml, 503.
Detmli (endaron), 5S, DsiailduE, 9i. DvvelopmenC, 10, IS, GS. Viatnmaecn, a, 79, 84, Si, 9&. Dibranchiala, 9U-M3. Dlcena, 173. DicoryDfl con Carta, 131. Diclyocyite, 81. Dietjocjstlda, 106. Diciema,6T8. Dinjemid*, G5S, 873, 676. Dldemnum styllferum, 6U Didiuium, 105.
lerpulk, S.
DiO'eratitlMlon, 16.
DimjarU, 480.
DiphydK, 145.
Dipbya* ippcndlouUts, 138.
UiphyllidM, 21 a.
DiphruuHid, 139, 145.
DiploiDon parkdoxum, 30, 206.
Dipnoi, Bl.
Diporpa, 30, 306.
Diptars, 4:25, 4M, 443.
Ditcophora, 139, 112, 146-149.
niainlegrallop of liviog nutler, !
Di.t..m», -iUl.
Ri.iHbution, 9, 19-aa, TO, 71.
Dog-louie, 312.
Dogi, relrieviQK of, 33.
DuJlolum, 6i», 605, 616.
" Donblo oircDlfttiOD," 61. Dngon-lllei, 3&3. DjaUriB, 103.
Eaithwoih, 319. £ehenell>olhriuin, 313. EehiDidra, S6, 543, 566. EcbiDococcua, L-OJ.
1, 210. Echinodareii, Iu3. Eehiaodann4l aerin, 6T9, 680. Eobinodermatn, ii, 34, 53-56,
543, 675, 635. EchinoidK, 570. EcblDOpedlum, M, 544, 561, 590,
680. Echlnarbrnchu*, 646. Echlnua, h«7.
spli^r*. 568, 563.
Ecloderm, 56.
Ecloprocbi, 458, 668, 669.
Eetoi-arc, 78.
Edrjmilsrida, 594.
Edriopblhilmik, 368, 3.59.
ElTtron, 3;ll.
EmbryolDg]', 41, 49, 681.
£mpuw,43.
Eiiduparuites, 306.
End.-plut, 46, 77.
EDdoplutlc*, 76, 87.
Enduprocca. 668.
Eudoaarc, 78.
Endoslyle, 597.
EnteropDaasCs, 60, 629, 671,
679.
Entomoitnea, 356, S66. EnloprocU, 458. Eoioon, 75.
cuisdciue, 87.
Epiblaat, 16, 50. Epidarmis (mloderm), 56> Evigenaaia, 13. Epimerk, 3CK). EpiioL 373. EquidK, 31. EiKUllui, 376. Enaulia floreui, 43S. Errantia, 235, 237.
Ea^ariu, 335, 287. Euuiaa, 336, 6««. Euglaiu, 4, 5.
EukIbtir viridii 97, i:"i'liausin, 3sr.. Euplopielln, IM), FurjpteriJ*. isi, SS8. Eii-jpiPrut rmipw, M7. tvLlmion, 38.
Oaleodei, STB.
Ganoldi, 61.' Cuterot'O'}*- *T(i, 5(H. Oulerowomuni. 206. Gaaleroiriclii, 19a,
Giulropliv(em«, 116, 119 'iMtrula, lis.
GetRT-
7, «13.
Genu"l'7.""'
Uephyrc*, 59, 314, 245, 667, 670,
Geryonia, 665. GciyoaidiE, 138, ISO Glimtcrabi, SM.
GloOi^feriM. 39, 1*4-67. GlouocodDii, 136. Gnalhiie*. ITS. 270. GoiDphooema, 7S, 103.
GraaQkn jdlllole*, 68, «43. Graptolltes, ISl. Gregarina, 76, 92, 9S,
gimintom, 9S, 94, 103.
Gitgarinidir, flS-M, 671.
INDEX.
693
Heteromorphe, 72. Heteronereis, 245. Heteropoda, 494, 497, 512. Hetcrotricha, 102. Uexacoralla, 162, 164. liipparitidsB, 486. Hirudinea, 214. 218, 242, 243,
666, 672. Hirado medidnalis, 215, 216. Histoloffy, 9. Histriobtlella, 214-217. Holomyaria, 641. Holothuria, 171. Holothuridea, 59, 54S, 545. Holotricha, 102. Homarus, 68. Hamming of insects, 255. Hyalonema, 120. Hydatina senta, 189. Hydra, 56, 63, 65. 126, 129, 6S5. Hydractina, 67, 126. Hydranth, 120. Hydrophilus pioeos, 424. Hydrophora, 129, 130, 146. HydrophyUia, 128. Hydrosoma, 127, 128. Hy<irotheca, 128. Uydrozoa, 33, 34, 67, 116, 120,
146-152,172, 173,671,672. Hymenoptera, 428. Hypoblast, 16, 50. Hypotricha, 102.
ICBTHTOPSIDA, 58.
Idoteidse, 364.
Imperforata, 83.
Impregnation, 28.
Inarticulata, 469.
Infusoria, 5, 14, 30, 43, 46, 77, 79,
95-97, 101, 106-113, 176, 660,
671.
ciliata, 95.
flagellata, 95, 96.
— tentaculifera, 9 a.
Insecta, 15, 59, 69, 256, 366, 897,
432-449, 670. Insectivorous plants, 42. Integumentary organs, 55. Invertebrata, morphological types
among. 48. Isocardia, 473.
Isopoda, 362. lulus, 396, 397. Ixodes ricinus, 382.
J.
JAIIEU.IDJS, 514.
Jaws, 56. Jelly-fishes, 120.
I..
Labium, in insects, 230.
Lacinularia, 190, 191, 193.
Laeraodipoda, 362, 366.
Lamellibranchiata, 470, 472, 670.
Lampyris splendidula, 440.
Laomedea, 131.
Larva?, 68, 69, 186, 191, 192, 203, 243, 283, 296, 369, 385, 393, 424, 430, 4.35, 436, 448-451, 467, 469, 544, 643, 664, 680.
Leeches, 214, eS6.
Lepadidee, 299.
Lepas, 291-299.
aufltralis, 296.
Lepidoptera, 425, 436, 438, 443.
Leptoplana, 182.
Lemsea, 276.
Lemseodiscus porcellanie, 302.
Leucifer, 346.
Leucones, 115, 120.
Lice, 421.
Lieberkuhnia, 82.
Ligula, 207.
Lima, 475. *
Limaeidse, 514.
Limax, 493, 498, 499.
Limnetis, 283, 285.
braehyurus, 286.
Limpets, 511.
Limulus, 259-269, 374.
— moiuccanus, 261.
polyphemus, 264.
Linens, 186.
Linguatula, 371, 388.
Lingula, 462. 466.
Lithocysts, 126.
LituiUds, 83.
Living matter, propertieB of, 1-41.
Lobster, 303.
LocustidsB, 440.
Loxosoma, 459, 485, 668.
694
tucenmriB, 131, l4T, 153.
Liimbricui, ai9>2al. Lung,, 60, I,yinnn!iii,49T, 500. paluslri*, 493.
.MaoTurii, 3-M, 338-316. MndrDporei, 161. MaJropuHle, 670. Ktaeoiphcern, \H, 95. MntacobdellB, 314-SI7, 666, 672. '■-' colloes, 67*, 679.
' Mothi, 30. Mueor, 92. MUDDB, 3&8.
MjB.1e .
itlallDpliiigiL, 420, 411. JUnubrium, 127. Mutieouods. 76. Mcanarina, tei*. JiBduBo.-, 34, 120, iaS-li9, 67i. McEBlopa, 31!(, 350. Merumy Brill, 641. MtroiLomata. 256, 36S.
MyrlBjn>ei«, 59, 256. 399, 670. My«U,a36, 346, 350,367. Mylilm, 476, 486. Myxulram, 79, 92. Hyxodictytim, 7tf. 79,81. SljiomycBlct, 5, -U, 92. Jlyiopods, 7G, 7f, 87, 88. Myxospuugjs', 119. M>EOStuinBLB, 627, 672.
K.
Nlu, 219, 22U. Nnked-cyed nieiiiia«, 129. NBUfHiu. 36». 271, 2S3, 290, 396- 30), 317-H4S. 354, 337, 385, 3j7, NButitiu, 66, 71, 5ai-5ta. Kebalia, 177,283. 3B3. NimaloideB, 29, 636, 67d. Nematuphorea, 131. NoniBlQrhyncha, 66 j. NemBtoHolicci, 673, 678. Nemcnida. 186,
Onchidmn, 61. . Uniicidn, 366. Oniscua, 250, 361. Ophiodei, 131. Uphlolepii oiliat^ 565. (>|ihiurld«% Mr% S62. Opbnrdlda, 1(». UpiilhobrftDohlftU, 510. UpUtbomuiD, 179. OrbuUu, $4. Olden, IT.
" O^uiiMd," meaning of, 8. OrgSDi, S4 65, 67. Origin of liting matter, S3. "iriigiD of Siweiea," 35. Orthido!, 471).
Urthopwn, 422, 43G, 438, *tS. (hiicula audilui, 67. U*[ncuda, S52, 287. Uitraa, 4T 5-486. UvipaTDUi anlmata, 69. Ovovlvipaiuus aaimali, 69, Uiidallon, waala of living matter
by, 2. Oiyuria, 639. Oyiler, 475.
Pxaimirn, 346.
PalKucycluB, 171.
PalteoDloloBy. 8m Foul li.
Palinimu vulgarii, 33SI.
Paludina, 204, 4>*5, 4117-
Pangeneili, 40.
Paramceolum, 47, 104-109.
Faraaite*. 43, 6% 193, S05, S06-207, 212, 272, 276, 277, 291,301, 364, 334, 388, 421, 450, 484, 513 634.
646. Puleur'a <
an, SI
Fecten, 475-485. Feutoitraca, 290. Fedalion, 192, 193. Fedicellina, 668, 680. Pedietia, 53. Pedieulina, 420, 421. Pelagia, 146, 147. Fellogaater pigurl, 302. Pennia, 3^)1,349. Penielliinm, 43. PeonataiidK, 161.
PentAcrimu, 584. Paututomlda, 257, 368, 670. Pentaitaaium tnuioide*, 388, 389. Pentremlte^ 594. PerenolbnnchiatB, 59. Perfonta, 83, 163-167, 171, ITi Peridlntn, 81, 99,10a PgripatiO*^ S57, 624, 670. Peritriehm, 102, i03, 109. Perik nigra, 422. pMonladw, 513. Peronia vermoulata, 517. Peronnporai, 43, 44. Phaluiglde, 379. Fballuila, 601, 607. PhaijBgopDeoita, 674. PharyugopneiuUil leiies, 679. Fliolaa, 472, 475, 4S5. PboTDDJg, 248. Pbrollna, 364. Ffaryoldie, 379. PbyUetoltttoaCa, 669. Pbyllndoce, 243.
viridii, 240.
Ptayilopoda, 277. PbylloMinula 355, 356. Phylogany, 41. Fhyialia, 125, 143, 143. FhyKmarla, 845, 671, 672, 685. Pbyiiology, 1, 9, 29. PhyaophoridB, 12S, 141-145. Filidiuro gynui, 185, 186. Fiaidium, 483. Ptaceuta, 69, 109. Flagioatome Bibea, 69. Flanaria, 180^218,672.
dioica, 180.
Plant!, 27, 29, 42, 71. Pleurobranchia, 173, 174, PleuTodiclyon, 172. Plumatellk repeui, 453, 454. PlumularldK, 130. Foelllopara, 165.
PoduridK, 444. Potcilopudk, 260. Foiiao veaiolea, 546, 561. Palyarthra, 192. Pulycalia, 218.
InvlgaU, ISl.
PolychKta, 68, 214, 226, 237, 673. Polyciilina, 88.
' -■'■
1=> : :
I'orlio, )(»■ Fnwn, 517.
TriapnlM, a«. Primordial atricU, 12. ProEWch*. 177, 183. rroduoHiiB, 470. Proglc.ltl.,211.
Proteoa, G1.
BoiraalcalW, '.'2.
P™t«occu.,5,ll. . .,
ProuiiOT. 36, 4W7, 51, 62, 6.1, .«,
110,111,117,671. Protoeoia serlts, 677. ProtuK 233, 2-U.
Dystari, a3B.
ProTinees of distribution, lA Pseuri-hirmn! syileni, 57.
INBBX.
697
Scyllaros, 339.
Sea anemones, 120.
Sensitive plant, 42.
Sensory organs, 23.
Sepiadae, 520, 528, 540, 542.
Sepia officinalis, 518-5*29.
Serial relations of invertebrata,
676. Serpulidse, 235, 244. SertularidsB, 128, 129. Shrimp, 350.
Siphonophora, 129, 140, 147-149. Sipunculos nudus, 247-249. Snail, 515.
Solenhofen slates, 151. Somatopleure, 57. Somites, 227. Sounds from insects, 437. Spatangoida, 570. Spermatophores, 529. Spbsromidse, 3&4. Spbaerozoura ovodimare, 89, 90. •~-— punctatum, 89. Sphinx ligustri, 426. Spiders, 379. Spiriferidae, 470. Spirillum volutans, 4. Spirorbis, 235. Spirostomum, 105. Spirulid«e, 540. Splanchnopleure, 57. Spongida, 110, 663. Spongilla, 113-120.
lluyiatilis,lll, 116.
Sporocysts, 206.
Springs, hot, living things in, 7.
Squids, 540.
Squilla, 361, 369.
scabricauda, 368, 369.
Star-fish, 553.
Stentor, 108.
Stephanoceros, 187, 191.
Stemaspis, 245.
Stigmata, 60, 377, 394, 413, 435.
Stings of insects, 432.
Stomatopoda, 272, 367.
Stone corals, 165.
Strepsiptera, 431, 450.
Strombidium, 104.
** Struggle for existence," 26.
Stylifer, 513.
Srylonychia, 108.
Stylops aterrimus, 450.
Son-animaloule, 87. Sundew, 42.
" Survival of the fittest," 40. Sycandra raphanus, 664. Sycon, 120.
dliatum, 115.
SyUis, 244.
vittata, 240.
Synapta, 513, 545.
digitata and inhaerens, 543.
Syncytium, 113. Syrphus ribesii, 427.
T.
Tabulata, 163, 165, 166, 167. Taenia, 207-213. Tape-worms, 206. Tardigrada, 257, 387, 670. Taxonomy, 10, 16, 656. Teeth, 56.
Tegumentary system, 55. Telotrocha, 186. Temperature in relation to living
matter, 3, 37. Tentacula, 50. Tentaculifera, 100. Terebratula, 33.
psittacea, 38.
Terebratulidse, 470. Terebratulina septentrionalis,
465. Teredo, 472-486. Testacellidse. 514. Tetrabranchiata, 532. Tetraphyllidea, 212. Tetrarhynchus, 207, 212. Tetrastemma, 183, 18i. Teuthidee, 528, 540. 542. Thecosomata, .',07. 503, 510. Thysanopoda, 346. Thysanura, 252, 420. Ticks, 382. Tissues, 10. Tomopteris, 235. Torquatella, 105. Torula, 36. Tracheae, 59. Tracheo-branchise, 2.'i2. Trachynemata, 149. Tradescantia hair, 78. Trematoda, 53, 57, 176, 193, 207-
213, 217, 672.
'rrioiincjsts, ins.
TriFhodinB 'grnndlnelll, 103.
Tubicola, M5. Tubifoi. 21!', 226. Tiibipon. 163. TiibulariiJiE, 129, 146. I'unicalB, 52, ib, 60, 69, 595, 671,
fiT'J. TiirlwllBris, 43, 51, 66-fia, 67, 176,
Vibraruli, *Ti6. Vibrioniite, 95, Tilal force. 9. TiFipsroui anhnali, 63.
WMte of living ni«i Water in living mi WilldK, 131. Wolffian duet, 69.
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