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MOLLUSCA

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Originally appearing in Volume V18, Page 673 of the 1911 Encyclopedia Britannica.
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MOLLUSCA, one of the great " phyla," or sub-kingdoms, of the animal pedigree or kingdom. The shell-bearing forms belonging to this group which were known to Linnaeus were placed by him (in 1748) in the third order of his class Vermes under the name " Testacea," whilst the Echinoderms, Hydroids and Annelids, with the naked Mollusca, formed his second order termed " Zoophyta." Ten years later he replaced the name " Zoophyta " by " Mollusca," which was thus in the first instance applied, not to the Mollusca at present so termed, but to a group consisting chiefly of other organisms. Gradually, however, the term Mollusca became used to include those Mollusca formerly placed among the " Testacea," as well as the naked Mollusca. It is important to observe that the term gaXaraa, of which Mollusca is merely a latinized form, was used by Aristotle to indicate a group consisting of the ctittle-fishes only. As now classified, the Mollusca consist of the following subdivisions:- Grade A.—Isopleura. Class I.—Amphineura (see CHIToN). Grade B.—Prorhipidoglossomorpha. Class II.—Gastropoda (q.v.). Class III.—Scaphopoda (q.v.). Class IV.—Lamellibranchia (q.v.). Grade C.—Siphonopoda. Class V.—Cephalopoda (q.v.). History of Classification.—The definite erection of the Mollusca into the position of one of the great primary groups of the animal kingdom is due to George Cuvier (1788-1800), who largely occupied himself with the dissection of representatives of this type.l* An independent anatomical investigation of the Mollusca had been carried on by the remarkable Neapolitan naturalist Poli (1791), whose researches° were not published until after his death (1817), and were followed by the beautiful works of another Neapolitan zoologist, the illustrious Delle Chiaje.° The embranchement or sub-kingdom Mollusca, as defined by Cuvier, included the following classes of shellfish: (I) the cuttles or poulps, under the name CEPHALOPODA; (2) the snails. whelks and slugs, both terrestrial and marine, under the name GASTROPODA; (3) the sea-butterflies or winged-snails, under the name PTEROPODA; (4) the clams, mussels and oysters, under the name ACEPIALA; (5) the lamp shells, under the name BRACHICPODA; (6) the sea-squirts or ascidians, under the name NUDA; and (7) the barnacles and sea-acorns, under the name CIRRIOPODA. The main limitations of the sub-kingdom or phylum Mollusca. as laid down by Cuvier, and the chief divisions thus recognized within its limits by him, hold good to the present day. At the same time, three of the classes considered by him as Mollusca have been one by one removed from that association in consequence of improved knowledge, and one additional class, incorporated since his day with the Mollusca with general approval, has, after more than forty years, been again detached and assigned an independent position owing to newly acquired knowledge. The first ofCuvier's classes to be removed from the Mollusca was that of the Cirrhopoda. Their affinities with the lower Crustacea were recognized by Cuvier and his contemporaries, but it was one of the brilliant discoveries of that remarkable and too-little-honoured naturalist, J. Vaughan Thompson, of Cork, which decided their position as Crustacea. The metamorphoses of the Cirrhopoda were described and figured by him in 183o in a very complete manner, and the legitimate conclusion as to their affinities was formulated by him.° Thus it is to Thompson (1830), and not to Burmeister (1834), as erroneously stated by Keferstein, that the merit of this discovery belongs. The next class to be removed from Cuvier's * These figures refer to the Bibliography at the end of the article. 670 Mollusca was that of the Nuda, better known as Tunicata. In 1866 the Russian embryologist Kowalewsky startled the zoological world with a minute account of the developmental changes of Ascidia, one of the Tunicata,6 and it became evident that the affinities of that class were with the Vertebrata, whilst their structural agreements with Mollusca were only superficial. The last class which has been removed from the Cuvierian Mollusca is that of the Lamp-shells or Brachiopoda. The history of its dissociation is connected with that of the class, viz. the Polyzoa or Bryozoa, which has been both added to and again removed from the Mollusca between Cuvier's date and the present day. The name of J. Vaughan Thompson is again that which is primarily connected with the history of a Molluscan class. In 183o he pointed out that among the numerous kinds of "polyps" at that time associated by naturalists with the Hydroids, there were many which had a peculiar and more elaborate type of organization, and for these he proposed the name Polyzoa. Subsequently6 they were termed Bryozoa by Ehrenberg (1831). Henri Milne-Edwards in 1844 demonstrated the affinities of the Polyzoa with the Molluscan class Brachiopoda, and proposed to associate the three classes Brachiopoda, Polyzoa and Tunicata in a large group " Molluscoidea," co-ordinate with the remaining classes of Cuvier's Mollusca, which formed a group retaining the name Mollusca. By subsequent writers the Polyzoa have in some cases been kept apart from the Mollusca and classed with the " Vermes ; whilst by others they have, together with the Brachiopoda, been regarded as true Mollusca. Increase of knowledge has now, however, established the conclusion that the agreement of structure supposed to obtain between Polyzoa and true Mollusca is delusive; and accordingly they, together with the Brachiopoda, were removed from the Molluscan phylum by Lankester in his article in the 9th edition of this work (on the which present article is based). Further details in regard to this, the last revolution in Molluscan classification, will be found in the article POLYZOA. As thus purified by successive advances of embryological research, the Mollusca were reduced to the Cuvierian classes of Cephalopoda, Pteropoda, Gastropoda and Acephala. Certain modifications in the disposition of these classes are naturally enough rendered necessary by the vast accumulation of knowledge as to the anatomy and embryology of the forms comprised in them. Foremost among those who between 1840 and 188o laboured in this field are the French zoologists Henri Milne-Edwards' and Lacaze Duthiers,10 to the latter of whom we owe the most accurate dissections and beautiful illustrations of a number of different types. To Kolliker," Gegenbaur, a and more recently Spenger,13 amongst German anatomists, we are indebted for epoch-making researches of the same kind. In England, Owen's anatomy of the pearly nautilus," Huxley's discussion of the general morphology of the Mollusca," and Lankester's embryological investigations," have aided in advancing our knowledge of the group. Two remarkable works of a systematic character dealing with the Mollusca deserve mention here-the Manual of the Mollusca, by Dr S. P. Woodward, a model of clear systematic exposition, and the exhaustive treatise on the Malacozoa or Weichthiere by Professor Keferstein of Gottingen, published as part of Bronn's Klassen and Ordnungen des Thier-Reichs. The arrangement adopted by Ray Lankester in the 9th edition, of the Ency. Brit. (art. " Mollusca "; 1883) was as follows: Of the four Cuvierian classes mentioned above, the Pteropoda were united with the Cephalopoda, on account of the apparent similarity of the cephalic tentacles in some of the former to the arms of the latter. An additional class was instituted for the reception of Dentalium and its few allies, and for this class Bronn's name Scaphopoda was used. The Chitons and their allies were placed under the Gastropoda, as a distinct branch called Isopleura, and for the Acephala de Blainville's name Lamellibranchia was substituted. The latter were regarded as forming a distinct branch, equivalent in rank to the other three classes together, the latter all possessing the radula which is wanting in Lamellibranchs. Since the 9th edition of the Ency. Brit. was published important advances have been made in our knowledge of the Mollusca, as the result of researches largely due to the interest excited in the subject by Lankester's article. Attention has been especially directed to the investigation of the most primitive forms in each group, and accordingly we can now form much more definite conceptions of the phylogeny and evolution of the various classes. The most important and extensive contributions to this progress have been made by the Belgian zoologist, Dr Paul Pelseneer, who has made the Mollusca his special study. The Chitonidae and the Aplacophora are now separated from the Gastropoda and raised to the rank of a distinct class, under the name of Amphineura. On the other hand, Boas and Pelseneer have shown that the Pteropoda have nothing to do with the Cephalopoda, but are Gastropoda modified for a pelagic life; they are therefore now united with the Gastropoda. The Lamellibranchia are no longer regarded as a distinct branch in contrast to the remaining Mollusca; according to Pelseneer they are allied to the Gastropoda and Seaphopoda, all three classes being derived from a common hypothetical ancestor, called Prorinpidoglossum. These three classes have therefore been united by Grobben into one branch or grade, the t' rorhipidoglossomorpha. General Characters of the Mollusca.—The forms comprised in the various groups, whilst exhibiting an extreme range of variety in shape, as may be seen on comparing an oyster, a cuttle-fish, and a sea-slug such as Doris; whilst adapted, some to life on dry land, others to the depths of the sea, others to rushing streams; whilst capable, some of swimming, others of burrowing, crawling or jumping, some, on the other hand, fixed and immobile; some amongst the most formidable of carnivores, others feeding on vegetable mud, or on the minutest of microscopic organisms—yet all agree in possessing in common a very considerable number of structural details which are not possessed in common by any other animals. The structural features which the Mollusca do possess in common with other animals belonging to other great phyla of the animal kingdom arc those characteristic of the Coelomata, one of the two great grades (the other and lower being that of the Coelentera) into which the higher animals, or Metazoa as distinguished from the Protozoa, are divided. The Metazoa all commence their individual existence as a single cell or plastid, which multiplies itself by transverse division. Unlike the cells of Protozoa, these embryonic cells of the Metazoa do not remain each like its neighbour and capable of independent life, but proceed to arrange themselves into two layers, taking the form of a sac. The cavity of the two-cell-layered sac or diblastula thus formed is the primitive gut or arch-enteron. In the Coelentera, whatever subsequent changes of shape the little sac may undergo as it grows up to be polyp or jelly-fish, the original arch-enteron remains as the one cavity pervading all regions of the body. In the Coelomata, on the other hand, there is another cavity, dividing the body-wall into two layers: an internal layer surrounding the gut, and an external layer. This cavity is excavated in a third mass of cells distinct from the cells lining the gut, forming the endoderm, and the cells covering the surface of the body, the ectoderm. This third mass of cells is the mesoderm. The Mollusca agree in being coelomate with the phyla Vertebrata, Platyhelmia (flat-worms), Echinoderma, Appendiculata (insects, ringed-worms, &c.), and others—in fact, with all the Metazoa except the sponges, corals, polyps, and medusae. In common with all other Coelomata, the Mollusca are at one period of life possessed of a prostomium or region in front of the mouth, which is the essential portion of the " head," and is connected with the property of forward locomotion in a definite direction and the steady carriage of the body (as opposed to rotation of the body on its long axis). As a result, the Coelomata, and with them the Mollusca, present (in the first instance) the general condition of body known as bilateral symmetry; the dorsal is differentiated from the ventral surface, whilst a right and a left side similarito, or rather the complements of, one another are permanently established. In common with all other Coelomata, the Mollusca have the mouth and first part of the alimentary canal which leads into the met-enteron formed by a special invagination of the outer layer of the primitive body-wall, not to be confounded with that which often, but not always, accompanies the antecedent formation of the archenteron; this invagination is termed the stomodaeum. Similarly an anal aperture is formed in connexion with a special invagination which meets the hinder part of the met-enteron, and is termed the proctodaeum. The coelom is primarily and essentially the generative cavity: the reproductive cells arise from its walls, i.e. from the coelomic epithelium. True nephridia do not primarily open into the coelom, as was formerly taught, but are intra-cellular ducts in the mesoderm. Such organs are absent in Mollusca in the adult state, but a pair of nephridia usually occurs in the larva. The coelom opens to the exterior by ducts which are primarily genital ducts by which the ova or sperms are discharged. These ducts, however, as well as the coelomic epithelium, may assume excretory functions. In Mollusca the coelom is reduced and consists of two Tarts, the pericardial cavity which surrounds the heart, and the cavity of the gonads or generative organs. There is usually one pair of coelomic ducts leading from the pericardium to the exterior, and these are the excretory organs or kidneys, formerly known as the organs of Bojanus. The walls of the pericardium are also excretory in parts, these parts forming the pericardial glands. In the majority of Mollusca the gonads are provided with a pair of ducts of their own. There are thus two pairs of coelomic ducts. This fact gives rise to the question whether the Mollusca are to be regarded as primitively segmented animals or not. In animals which exhibit typical segmentation or metamerism, such as segmented worms (Chaetopoda), each segment or metamere possesses its own coelomic cavity, a pair of coelomic ducts, and a pair of nephridia. The structure of the Mollusca in the greater number of cases agrees with the hypothesis that the primitive form was unsegmented, and therefore had but one pair of coelomic ducts and one pair of nephridia. In existing forms the latter disappear in the adult. In the most primitive forms of several classes there are no distinct genital ducts, the gonads when mature discharging into or through the kidneys. Among the Gastropoda, in the Aspidobranchia, there is no genital duct, and the gonad opens into the right kidney; in the more modified forms the left kidney alone is functional, the right has been converted into the genital duct. Among the Lamellibranchia again the kidneys serve as genital ducts in the Protobranchia and some Filibranchia. In the higher forms the opening of the gonad is shifted more and more towards the external aperture of each kidney until finally it is situated on the external surface, and thus the gonad secondarily acquires an independent aperture. In the Scaphopoda there is no distinct genital duct, the relations are as in Aspidobranchia. Among the Amphineura we find one pair of coelomic ducts in the Aplacophora, two pairs in the Chitons. In the former the genital coelom and the pericardial coelom are continuous and the reproductive cells escape by the renal ducts. In the Chitons or Polyplacophora, on the other hand, the two cavities are separate, and there are independent genital ducts. It is possible therefore to regard the latter condition as secondary, and to conclude that the separate genital ducts have been derived from the original single pair of coelomic ducts, as in Lamellibranchs. The Cephalopoda, however, do not harmonize so well with this view. The earliest forms of this class geologically are the Nautiloidea. Assuming that these ancestral forms resembled the existing Nautilus in their internal anatomy, they had two pairs of renal ducts and one pair of genital ducts, which would apparently indicate, not a single metamere or unsegmented body, but three metameres. There are however only two pairs of branchiae. The Dibranchia, with only one pair of branchiae, one pair of renal organs, and one pair of genital ducts, are much more recent, not appearing till the end of the Secondary epoch, and therefore must be regarded as descended from the Tetrabranchia. The latter are represented in the Upper Cambrian formations, together with Lamellibranchia and Gastropoda, and there are no earlier Molluscan fossils than these. Palaeontology therefore throws no light on the question whether the metameric or the unsegmented Mollusca were the earlier. The development of the Cephalopoda affords at present no better evidence that the metamerism is secondary. That of Nautilus, which would be most important in this inquiry, is unfortunately still unknown. In the Dibranchia true nephridia have not been detected in the embryo, nor has it been shown that the genital ducts are derived from the renal tubes. On the other hand, there is no evidence that the forms which show no metamerism, such as the Gastropoda, are descended from metameric ancestors. On the whole, then, the most probable conclusion is that the original ancestral form of the Mollusca was unsegmented, possessed one pair of true nephridia, and one pair of coelomic ducts whose function was to conduct the generative products to the exterior. The chief types of Mollusca were already differentiated at the beginning of the geological record, and the metamerism which occurs in the Cephalopoda has been evolved within the limits of that class. External Characters.—The characteristic organs of Mollusca are the mantle and shell, the foot, the ctenidia and the radula,of which all but the last are external. The original form was bilaterally symmetrical, and this symmetry is retained in all the classes except the Gastropoda. At the anterior end the head is differentiated; it bears the sense-organs, and contains the muscular pharynx within which is the radular apparatus. The rest of the body consists of the foot ventrally and the visceral mass dorsally. The foot is a muscular mass without cuticle or skeleton, excepting certain cuticular structures such as the byssus of Lamellibranchs and the operculum of Gastropods, which do not aid in locomotion. The foot is usually the only organ of locomotion. It corresponds to the ventral part of the body-wall in other animals. The muscular tissue of the dorsal body-wall is much reduced and the integument here is thin and a.br. lat)"\*'b' Fm. r.—Ctenidia of various Mollusca (original). A, Of Chiton: ft., fibrous tissue; a.b.v., afferent blood-vessel; e.b.v., efferent blood-vessel; g.l., laterally paired lamellae. B, Of Sepia: letters as in A. C, Of Fissurella: letters as in A. D, Of Nucula: d, position of axis with blood-vessels; a, inner: b and c, outer row of lamellae. E, Of Paludina: i, intestine running parallel with the axis of the ctenidium and ending in the anus a; be., rows of elongate processes corresponding to the two series of lamellae of the upper figures. soft. The external epithelium of the dorsal region secretes the shell. Between the edge of the shell and the foot there is a groove or cavity, chiefly developed laterally and posteriorly. The dorsal border of this groove is extended outwards and downwards as a fold of the integument. There is some confusion of terms here: some writers call the free fold the mantle or pallium, and this is the proper use of the term; but others apply the term to the whole of the dorsal integument, including both the projecting fold and the part covering the viscera. The shell extends to the edge of the mantle-fold, and the cavity between the mantle and the side of the body is the pallial chamber. This chamber serves two purposes: it is primarily the respiratory cavity containing the gills, but it also serves to enclose the body so that the latter is surrounded by the shell, from which the head and foot can be protruded at the will of the animal. The shell consists of an organic basis the substance of which is called conchiolin, impregnated with carbonate of lime, with a small proportion, 1–2 %, of phosphate of lime. On the outside of the shell is a non-calcified layer of conchiolin called the periostracum, secreted by the thickened edge of the mantle. The zone of the external surface of the mantle within the edge secretes a layer formed of prisms of calcite; the rest of the epithelium from this zone to the apex secretes the inner layer of the shell, composed of successive laminae; this is the nacreous layer, and in certain species has a commercial value as nacre or mother-of-pearl. Thus the growth of the shell in extent is due to additions to the prismatic layer at the edge, its growth in thickness to new layers of nacre deposited on its inner surface. In many cases in various classes the mantle is reflected over the edges of the shell, so as to cover more or less completely its outer surface. When this covering is complete the shell is contained in a closed sac and is said to be " internal," but the sac is lined by ectoderm and the shell is always morphologically external. In one or two cases the epithelium of the foot secretes a calcified shell, which is either free as in Argonaula or adherent as in Hipponyx. The ctenidia (fig. r) are the branchial organs of the Mollusca. In the primitive condition there is one on each side in the mantle cavity, towards the posterior end of the body. Each is an outgrowth of the body-wall at the side of the body, and consists of an axis containing two main vessels, an afferent and efferent, and bearing on either side a series of transverse plates whose blood-sinuses communicate with the vessels of the axis. The afferent vessel of the ctenidium receives blood from the vena cava or principal blood-sinus of the body, the efferent vessel opens into the auricle of its own side. Near the base of the ctenidium is a patch of sensory epithelium innervated from the branchial nerve, forming a sense-organ called the osphradium, whose function is to test the water entering the branchial cavity. The branchial current is maintained by the cilia which cover the surface of the ctenidia, except in Cephalopoda, in which cilia are absent and the current is due to muscular action. Thus in the primitive mollusc the mantle-cavity contains a symmetrical group of structures at the posterior end of the body, and this group of structures is called the pallial complex. It consists of the anus in the middle, a renal organ and renal aperture on each side of this, and a ctenidium outside or anterior to the renal organ, an osphradium being situated at the base of the ctenidium. Internal Anatomy: Digestive Tube.—In primitive Mollusca the mouth and anus are the two extremities of the body, but the anus may be brought to an anterior position by a ventral flexure, complicated in Gastropoda by a lateral torsion. The alimentary tube consists of three regions: firstly, the anterior buccal mass with the oesophagus, of ectodermic origin, and therefore bearing cuticular structures, namely the jaws and radula; secondly, the mid-gut, of endodermic origin and including the stomach and liver; and, thirdly, the hind-gut or intestine. The radula consists of a chitinous band bearing teeth, secreted by a ventral caecum of the pharnyx and moved by an apparatus of cartilage and muscles. It was present in the ancestral mollusc, occurs in nearly all archaic types, and is only absent in the most specialized forms, in which it has evidently been lost; these forms are certain Neomeniomorpha, all the Lamellibranchia, various degenerate Gastropoda, and the Cirrhoteuthidae among Cephalopods. The teeth are secreted by a small number of cells at the closed end of the caecum, the basal membrane by a trans-verse row of cells in front of these. The teeth are disposed in trans-verse rows, and in each row they are arranged symmetrically on either side of a central tooth. In Polyplacophora there are eight on each side (8.I.8) ; in Scaphopoda two on each side (2.1.2); in almost all Cephalopoda three on each side (3.I.3) ; in Gastropoda the number varies very much in different subdivisions. Beneath the anterior parts of the radula where it emerges from the caecum are a pair of cartilages, and attached to these a number of special muscles by which the radula is moved backwards and forwards to act as a rasp. The secretion of the radula at the closed end of the caecum is continuous, so that it is constantly growing forward as fast as its exposed anterior portion is worn away by use, just as a finger-nail is pushed forward by constant growth at its posterior end,and is worn away or has to be cut short from time to time at its outer end. Circulation.—The system of blood-vessels is entirely separate from the coelomic cavities. It consists of arteries, veins and sinuses, but ramified capillaries are usually absent except in the integuments of Cephalopods. The arteries and veins have proper endothelial walls; they pass abruptly into the sinuses and in some cases communication is effected by orifices in the walls of the vessels, as for example in the vena cava of Nautilus. The heart is situated in the pericardium on the dorsal side of the intestine and at the posterior end of the animal. The pericardium never contains blood, as is well shown in those forms which have red corpuscles in their blood; these corpuscles are never found in the pericardium. The heart receives blood from the gills and mantle, and pumps it through arteries to the body. It consists of a median ventricle with muscular walls and a cavity traversed by muscular strands. On either side of the ventricle, in the primitive condition, is a thin-walled auricle, opening into the ventricle by a valved opening. Each auricle forms the terminal enlargement of the efferent vein of the ctenidium of its own side. In Nautilus two pairs of auricles are present, corresponding with the two pairs of ctenidia. In the primitive form a single anterior aorta is given off from the ventricle, the two together representing the dorsal blood-vessel of Chaetopods. In more specialized forms a posterior aorta passes backwards from the ventricle, as in Gastropods and the majority of Lamellibranchs. The ramifications of the arteries convey the blood to all parts of the body, and it finally reaches the venous sinuses, the chief of which are the pedal, the pallial and the median-ventral. The last is between the pericardium and the foot; from it the blood passes through the renal organs to the ctenidia. Some blood, however, enters the auricles directly from the mantle, without passing through the ctenidia. In the majority of Gastropoda one gill and one auricle are lost. The blood is usually a colourless liquid containing amoeboid cells and sometimes other corpuscles called haematids. It may be coloured blue by haemocyanin, a respiratory compound containing copper. In a few forms the blood contains haemoglobin, either in solution or in haematids (red blood-corpuscles). In the Gastropoda the muscular tissue of the buccal mass is coloured red by haemoglobin. Nervous System.—The central nervous system may be described as consisting of a collar surrounding the oesophagus, and two pairs of cords arising from the collar and passing backwards. The two pairs of cords arise from the same point of the collar. The ventral cords are the pedal, the dorso-lateral, the pleural, the former innervating the foot, the latter the mantle. The dorsal half of the collar is the cerebral commissure, the ventral the labial commissure. The pedal cords are connected by commissures, and the pedal and pleural of each side are similarly connected. The pallial cords are united to one another posteriorly, dorsal to the rectum. This is the condition of the nervous system found in Chiton and the other Amphineura, but may not be in all respects the ancestral condition. Generally the system is differentiated into ganglia connected by nerve-cords consisting of nerve-fibres only. At the point of the collar whence the nerve-cords arise are the cerebral ganglia; from these one pair of connectives passes to a pair of pedal ganglia, and another pair of connectives to a pair of pleural ganglia. Pedal and pleural on each side are connected by a pleuro-pedal connective Each pleural ganglion gives off a long nerve which supplies the viscera, and the two unite posteriorly below the intestine. There are usually three small ganglia on the course of this visceral dommissure, namely, the right and left visceral ganglia and the abdominal. The perioesophageal nerve-ring of Chaetopoda and Arthropoda is represented, not by the collar first mentioned in the above description, but by the commissures connecting the cerebral and pedal ganglia. The labial commissure supplies only the buccal mass and the oesophagus and stomach. The special sense-organs are a pair of eyes on the head, a pair of otocysts or statocysts, and a pair of osphradia which have already been mentioned. In certain cases accessory eyes are also present, e.g. the pallial eyes of Pecten and other Lamellibranchs, and of Chitons. The otocysts are invaginations of the epithelium of the foot, but are innervated from the cerebral ganglia, and the same innervation has been proved in some cases for the osphradia. Reproduction and Development.—Molluscs are usually of separate sexes, but sexual dimorphism is seldom highly developed. Hermaphroditism is secondary, and occurs in one sub-class of Gastropoda, in some Lamellibranchs, and in one sub-order of Amphineura. In Cephalopods and the majority of Gastropods copulation occurs. As a rule no parental care is exhibited, but incubation of the developing ova within some part of the parental body, or receptacles attached to the parent, occurs in some Lamellibranchs, some Gastropods, and in Argonauta among the Cephalopods. True viviparity, that is the development of the ova within the oviduct, is very rare, occurring only in one case among the Amphineura and in some aquatic and pulmonate Gastropoda. The egg-cell of Mollusca is either free from food-material—a simple protoplasmic corpuscle—or charged with food-material to a greater or less extent. Those cases which appear to be most typical —i.e. which adhere to a procedure which was probably common at one time to all then existing Mollusca and has been departed from only in later and special lines of descent—show approximately the following history. By division .of the egg-cell a mulberry-mass of embryonic-cells is formed (morula), which dilates, forming a one-celllayered sac (blastula). By invagination one portion of this sphere becomes tucked into the other—as in the preparation of a woven night-cap for the head. The orifice of invagination (blastopore) narrows, and we now have a two-cell-layered sac—the gastrula. The invaginated layer is the enteric cell-layer or endoderm ; the outer cell-layer is the dermic cell-layer or ectoderm. The cavity communicating with the blastopore and lined by the endoderm is the archenteron. The blastopore, together with the whole embryo, now elongates. The blastopore then closes along the middle portion of its extent, which corresponds with the later developed foot. At the same time the stomodaeum, or oral invagination, forms around the anterior remnant of the blastopore, and the proctodaeum, or anal invagination, forms around the posterior remnant of the blastopore. There are, however, variations in regard to the relation of the blastopore to the mouth and to the anus which are probably modifications of the original process described above. In eggs which contain a larger quantity of food-yolk, the process by which the endoderm is enveloped by the ectoderm is somewhat different. Segmentation in these is very unequal, and results in the formation of small cells called micromeres and large cells called megameres, as in fig. 4. As the micromeres become more numerous they gradually envelop the megameres until the latter are completely enclosed. The gastrula is in these cases said to be formed by epibole. Between ectoderm and endoderm a third intermediate cell-layer A~~t_ ,o . /2 I Pond-Snail, Limnaeus stagnalis. A, First four cells resulting from the cleavage of the original egg-cell. B, Side-view of the same. C, Diblastula stage showing the two cell-layers and the blastopore. D, E, F, Trochosphere stage, D older than E or F. G, Three-quarter view of a Diblastula, to show the orifice of invagination of the endoderm or blastopore, (bl). H, I, Veliger stage later than D. is formed, which is called the mesoderm, and gives rise to the muscular and connective tissues to the vascular system, and to the excretory and generative organs. The mesoderm arises for the most part from the endoderm. When the segmentation is unequal one of the megameres gives rise by successive divisions to two primary mesoderm cells called mesomeres; these divide to form two masses of cells called mesoblastic bands. The coelom is formed as a cavity or cavities in the interior of these cell-masses. In some cases the
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