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Originally appearing in Volume V18, Page 675 of the 1911 Encyclopedia Britannica.
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XVIJI 22coelom is formed as a single cavity, and renal and generative organs are formed from its walls. This is the primitive method, but in other cases the organs mentioned may be formed separately in the mesoderm. The renal organs are tubular outgrowths of the pericardial parts of the coelom; the reproductive cells are derived from cells lining the generative portion. The external form of the embryo meanwhile passes through highly characteristic changes, which are on the whole fairly constant (After Lankestet, i7.) dc, Directive corpuscle (outcast A, Gastrula phase (optical sec- cell). tion). ae, Arch-enteron or cavity lined B, The Gastrula has become a by the enteric cell-layer or Trochosphere by the devel- endoderm. opment of the ciliated ring bl, Blastopore. vr (optical section). vr, Velum or circlet of ciliated C, Side view of the Trochosphere, cells. formation with commencing of the foot. - dv, Velar area or cephalic dome. sin, Site of the as yet unformed D, Further advanced Trocho- sphere (optical section). - The Trochosphere passing to the Veliger stage,dorsal view showing the formation of the primitive shell-sac. Side view of the same, showing foot, shell-sac (shgl), velum (vr), mouth and anus. N.B.—In this development the blastopore is not elongated; it persists as the anus. The mouth and stomodaeum form independently of the blastopore. throughout the Mollusca. A circlet of cilia forms when the embryo is still nearly spherical in an equatorial position. As growth proceeds, one hemisphere remains relatively small, the other elongates and enlarges. Both mouth and anus are placed in the larger area ; the smaller area is the prostomium simply; the ciliated band is therefore in front of the mouth. The larval form thus produced is known as the trochosphere. It exactly agrees with the larval form of many Chaetopod worms and other Coelomata. Most remark-able is its resemblance to the adult form of the Wheel animalcules, or Rotifera, which retain the prae-oral ciliated band as their chief organ of locomotion and prehension throughout life. So far the young mollusc has not reached a definitely molluscan stage of II (After Lankester, r 5.) r, Directive corpuscle. bl, Blastopore. en, Endoderm or ec, Ectoderm or v, Velum. in, Mouth, f, Foot. t, Tentacles. fp, Pore in the foot (belonging to the pedal gland?). mf, The mantle-flap or limbus pallialis. sh, The shell. 1, The sub-pallial space, here destined to become the lung. enteric cell layer. deric cell-layer. mouth. f, Foot. mes, Rudiments of the skeletotrophic tissues. pi, The pedicle of invagination, the future rectum. F, shgl, The primitive shell-sac or shell-gland. m, Mouth. an, anus. E, development, being only in a condition common to it and other Coelomata. It now passes to the veliger phase, a definitely molluscan form, in which the disproportion between the area in front of the ciliated circlet and that behind it is very greatly increased, so that the former is now simply an emarginated region of the head fringed with cilia. It is termed the " velum," and is frequently drawn out to Mollusca generally, but acquires characters peculiar to the particular class to which its parents belong. For the later development therefore the articles on the several classes must be consulted. Relations between Me Classes.—From the preceding discussion an idea may be formed of the primitive characters of the Phylum A B A B (From Balfour, after Bobretzky.) mutabilis. A, The egg-cell has divided into two spheres, of which the lower contains more food-material, whilst the upper is again incompletely divided into two smaller spheres. Resting on the dividing upper sphere are the eight-shaped " directive corpuscles," better called " praeseminal outcast cells or apoblasts," since they are the result of a cell-division which affects the egg-cell before it is impregnated, and are mere refuse, destined to disappear. B, One of the two smaller spheres is reunited to the larger sphere. C, The single small sphere has divided into two, and the reunited mass has divided into two, of which one is oblong and practically double, as in B. D, Each of the four segment-cells gives rise by division to a small pellucid cell. E, The cap of small cells has increased in number by repeated formation of pellucid cells in the same way, and by division of those first formed. The cap will spread over and enclose the four segment-cells. into lobes and processes. As in the Rotifera, it serves the veliger larva as an organ of locomotion. The body of the veliger is characterized by the development of the visceral hump on one surface, and by that of the foot on the other. Growth is greater in the vertical dorso-ventral axis than in the longitudinal oro-anal axis; consequently the foot is relatively small and projects as a blunt process between mouth and anus, which are not widely distant from one another, whilst the antipedal area projects in the form of a great hump or dome. In the centre of this antipedal area there has appeared (often at a very early period) a gland-like depression or follicle of the integument. This is the primitive shell-sac discovered by Lankester in 1871, and shown by him to precede the development of the permanent shell in a variety of molluscan types. The shell-gland is bounded by a ridge of ectodermic cells. This ridge forms the edge of the shell-secreting epithelium, and therefore of the mantle, since the shell extends to the edge of the mantle. The shell-gland, as development proceeds, extends from its point of origin as an ectodermic thickening, which may be only slightly concave or may be deeply invaginated and then evaginated. In the larvae of several Gastropoda and Lamellibranchia occur excretory organs which have the characters of true nephridia. There is a single pair of these organs situated immediately behind the velum. They agree with primitive nephridia in being of ectodermic origin, in,consisting of perforated cells in linear series, and in having no communication with the coelom. The inner end of each of these organs consists of a flame-cell, i.e. a cell with an internal cavity containing a vibrating filament or flagellum. They are best developed in the Pulmonata; in some cases they are very rudimentary and may be destitute of an external opening. They invariably disappear before the adult stage is reached, but their presence in the larva is evidence that the ancestral mollusc possessed a pair of true nephridia quite distinct from the coelomic excretory organs, which are so characteristic of existing forms in the adult condition. The ctenidia, it will be observed, have not yet been mentioned, and they are indeed the last of the characteristic Molluscan organs to make their appearance. They arise as outgrowths of the sides of the body within the cavity formed by the development of the mantle. The veliger, as soon as its shell has attained some extent and begins to assume definite shape, is no longer of a form common(From Gegenbaur.) v, Velum. A, Earlier, and (B), later, Veliger c, Visceral dome with dependent of a Gastropod. mantle-skirt. C, Veliger of a Pteropod showing p, Foot. lobe-like processes of the t, Cephalic tentacles. velum and the great paired op, Operculum. !outgrowths of the foot. Mollusca, and it is possible to construct a diagrammatic mollusc, as was first done by Lankester, which will possess these primitive features. The figure here given represents such a hypothetical form according to present views. We cannot assert that this was in all respects the condition of the common ancestor, as will be seen when we attempt to derive the various sub-types from it. In the Amphineura the nervous system, having no 1~9 to 'at 1'g '7 1b . (From Lankester's Treatise on Zoology. A. and C. Black.) a, Anus. pa.n, Pallial nerve. cg, Cerebral ganglion. pe, Pericardium. f, Foot. p.g, Pedal ganglion. g, Gill, in the pallial cavity. pl.g, Pleural ganglion. go, Gonad. ra, Radula. h, Heart. r.p.o, Reno-pericardial orifice. k, Kidney. st, Stomach. la.c, Labial commissure. st.g, Stomato-gastric ganglion. m, Mouth. v.g, Visceral ganglion. pa, Mantle. separate ganglia and-no–ventral visceral commissure, may be still more primitive. The metameric repetition of the shell-plates and of the ctenidia are probably special modifications, but it is difficult to explain the spicules of the dorsal integument except as a condition more primitive than the shell itself. The Prorhipidoglossomorpha are distinguished by the separation of the genital coelom from the pericardium, and by the long visceral commissure passing ventral to the intestine. The Lame'libranchia have markedly diverged from the original type by the adoption of filtration as a method of feeding. This has led to the loss of the radula, and is accompanied by the division of the shell into two valves. The peculiarities of the Gastropoda are all due to the torsion of the shell and body. The Cephalopoda can be derived without much difficulty from the schematic Mollusc, if we assume that some metameric repetition of organs has occurred, as explained above in reference to the coelom. The foot has been developed into long processes which have extended in a circle round the mouth; all the ganglia, including the visceral, have been concentrated around the oesophagus. Habits and Distribution.—More than 28,000 species of living Molluscs have been distinguished, of which more than half are Gastropods. They are essentially aquatic animals, and the (From Lankester's Treatise on Zoology. A. and C. Black.) m, Mouth. pa, Palliuin or mantle. p.g, Pedal ganglion. pl.g, Pleural ganglion. ra, Radula. st, Stomach. st.g, Stomato-gastric ganglion. v.g, Visceral ganglion. majority live in the sea. Some, like many Cephalopods and the Pteropods, are pelagic or free-swimming; others creep or lie on the sea bottom. Some are littoral, living between tide-marks; others are found at very various depths, up to 2800 fathoms. A few species have invaded the fresh waters, while the pulmonate and terrestrial Gastropods are distributed over the whole surface of the land in all latitudes and to a height of 15,000 ft. As a rule Molluscs are free and more or less active, but many Lamellibranchs are sedentary, and a few of these and of Gastropods are permanently fixed to their habitat. Commensalism occurs in a few instances, but parasitism either external or internal is rare. The latter is confined to certain Gastropods which live in Echinoderms and are extremely degenerate in structure. Protective resemb'_ance is exhibited by some Nudibranch Gastropods which have assumed the colour and appearance of their habitat. Entwickelungsgeschichte der einfachen Ascidien," in Mem. de l'acad. des sciences de St Petersbourg (1866), and " Entwickelungsgeschichte des Amphioxus lanceolatus," ibid. (1867). (6) J. Vaughan Thompson, Zoological Researches (Cork, 1830) ; memoir v., "Polyzoa, a new animal discovered as an inhabitant of. some Zoophytes." (7) C. G. Ehrenberg, " Die Korallenthiere des Rothen Meeres " (Berlin, 1834); Abhand. d. k. Akad. d. Wissenschaften in Berlin (1832). (8) H. Milne-Edwards, Recherches sur les polypiers de France (Paris, 1841-1844). (9) H. Milne-Edwards, papers in the Annales des sciences naturelles (1841-186o). (To) H. de Lacaze-Duthiers, papers in the Annales des sciences naturelles, e.g. " Anomia " (1854), " Mytilus " (1856), " Dentalium " (1856-1857), " Purpura " (1859), " Haliotis (1859), " Vermetus " (186o). (11) A. Kolliker, Entwickelungsgeschichte der Cephalopoden (Zurich, 1844). (12) C. G. Gegenbaur, Untersuchungen fiber Pteropoden and Heteropoden, (Leipzig, 1855). (13) J. W. Spengel, " Die Geruchsorgane and das Nervensystem der Mollusken," Zeitschr. f. wiss. Zool. (1881). (14) Richard Owen, Memoir on the Pearly Nautilus (London, 1832). (15) L. Cuenot, " Excretion chez les mollusques," Arch. d. biol. xvl. (1899). (16) P. Geddes, " On the Mechanism of the Odontophore in certain Mollusca." (17) T. H. Huxley, " On the Morphology of the Cephalous Mollusca," Phil. Trans. (1853). (18) Von Jhering, Vergleichende Anatomie des Nervensystems and Phylogenie der Mollusken (Leipzig, 1877). (19) E. R. Lankester, " Contributions to the Developmental History of the Mollusca," Phil. Trans. (1875) ; " Note on the Coelom and Vascular System of Mollusca and Arthropoda," Quart. Journ. Mice. Sci. xxxly. (1893). (2o) P. Pelseneer, Introduction a l'etude des Mollusques (Brussels, 1894) ; " Recherches sur les Mollusques archaiques," Mem. tour. Acad. beig., evil. (1899) ; " Mollusca," Lankester's Treatise on Zoology, pt. v. (1906). I I. Conchology.-(2I) Cooke, " Molluscs," Cambridge Natural History, vol. iii. (1895). (22) Fischer, Manuel deconchyliologie (1887). (23) Jeffreys, British Conchology (1862-1869). (24) Simroth, " Mollusca," Bronn's Klassen and Ordnungen des Thierreichs, Bd. iii. (1895), in prog. (25) Tryon, Manual of Conchology (1878), in prog. (26) Woodward, A Manual of the Mollusca (188o). (E. R. L.; J. T. C.)
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