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XVIJI

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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 See also:

organs are formed from its walls. This is the See also: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 See also:external See also:form of the embryo meanwhile passes through highly characteristic changes, which are on the whole fairly See also:constant (After Lankestet, i7.) dc, Directive corpuscle (outcast A, Gastrula phase (See also:optical sec- See also:cell). tion). ae, See also: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 See also:ring bl, Blastopore. vr (optical See also:section). vr, Velum or circlet of ciliated C, See also:Side view of the Trochosphere, cells. formation with commencing of the See also:foot. - dv, Velar See also:area or cephalic See also:dome. See also:sin, Site of the as yet unformed D, Further advanced Trocho- See also:sphere (optical section).

- The Trochosphere passing to the Veliger See also:

stage,dorsal view showing the formation of the primitive See also:shell-See also: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 See also:Mollusca. A circlet of See also:cilia forms when the embryo is still nearly spherical in an See also: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 See also: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 See also:worms and other Coelomata. Most remark-able is its resemblance to the adult form of the See also:Wheel animalcules, or See also:Rotifera, which retain the prae-oral ciliated band as their See also:chief See also:organ of locomotion and prehension throughout See also:life. So far the See also: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, See also:

Pore in the foot (belonging to the pedal gland?). mf, The See also:mantle-flap or See also:limbus pallialis. sh, The shell. 1, The sub-pallial space, here destined to become the See also:lung. enteric cell layer. deric cell-layer. mouth. f, Foot. See also: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 See also:

condition See also: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 See also:head fringed with cilia. It is termed the " velum," and is frequently See also:drawn out to Mollusca generally, but acquires characters See also: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 See also:idea may be formed of the primitive characters of the Phylum A B A B (From See also:Balfour, after Bobretzky.) mutabilis. A, The See also:egg-cell has divided into two See also:spheres, of which the See also:lower contains more See also: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-See also:division which affects the egg-cell before it is impregnated, and are See also: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 See also:mass has divided into two, of which one is oblong and practically See also: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 See also:

body of the veliger is characterized by the development of the visceral hump on one See also:surface, and by that of the foot on the other. Growth is greater in the See also:vertical dorso-ventral See also:axis than in the See also:longitudinal oro-anal axis; consequently the foot is relatively small and projects as a See also:blunt See also:process between mouth and anus, which are not widely distant from one another, whilst the antipedal area projects in the form of a See also:great hump or dome. In the centre of this antipedal area there has appeared (often at a very See also:early See also: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 See also: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 See also:concave or may be deeply invaginated and then evaginated. In the larvae of several See also:Gastropoda and See also: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 See also:

series, and in having no communication with the coelom. The inner end of each of these organs consists of a See also:flame-cell, i.e. a cell with an See also:internal cavity containing a vibrating filament or flagellum. They are best See also: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 See also: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 See also: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 See also: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. See also: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 See also:

present views. We cannot assert that this was in all respects the condition of the common ancestor, as will be seen when we See also:attempt to derive the various sub-types from it. In the Amphineura the See also:nervous See also:system, having no 1~9 to 'at 1'g '7 1b . (From Lankester's See also:Treatise on See also:Zoology. A. and C. See also:Black.) a, Anus. pa.n, Pallial See also: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, See also:Heart. r.p.o, See also:Reno-pericardial orifice.

k, See also:

Kidney. st, See also:Stomach. la.c, Labial commissure. st.g, Stomato-gastric ganglion. m, Mouth. v.g, Visceral ganglion. pa, Mantle. See also: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 See also: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 See also:long visceral commissure passing ventral to the See also:intestine. The Lame'libranchia have markedly diverged from the See also:original type by the See also: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 See also: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 See also:round the mouth; all the ganglia, including the visceral, have been concentrated around the See also:oesophagus. Habits and See also:Distribution.—More than 28,000 See also:species of living Molluscs have been distinguished, of which more than See also: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. See also:

majority live in the See also:sea. Some, like many Cephalopods and the Pteropods, are pelagic or See also:free-See also:swimming; others creep or See also:lie on the sea bottom. Some are littoral, living between See also:tide-marks; others are found at very various depths, up to 2800 fathoms. A few species have invaded the fresh See also:waters, while the pulmonate and terrestrial Gastropods are distributed over the whole surface of the See also:land in all latitudes and to a height of 15,000 ft.

As a See also:

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 See also:habitat. Commensalism occurs in a few instances, but See also: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 See also:colour and appearance of their habitat. Entwickelungsgeschichte der einfachen Ascidien," in Mem. de l'acad. See also:des sciences de St Petersbourg (1866), and " Entwickelungsgeschichte des See also:Amphioxus lanceolatus," ibid. (1867). (6) J. See also:Vaughan See also:Thompson, Zoological Researches (See also:Cork, 1830) ; memoir v., "See also:Polyzoa, a new See also:animal discovered as an inhabitant of. some Zoophytes." (7) C. G. See also:Ehrenberg, " See also:Die Korallenthiere des Rothen Meeres " (See also:Berlin, 1834); Abhand. d. k. Akad. d. Wissenschaften in Berlin (1832).

(8) H. Milne-See also:

Edwards, Recherches sur See also:les polypiers de See also:France (See also: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), " See also:Purpura " (1859), " Haliotis (1859), " Vermetus " (186o). (11) A. See also:Kolliker, Entwickelungsgeschichte der Cephalopoden (See also:Zurich, 1844). (12) C. G. Gegenbaur, Untersuchungen fiber Pteropoden and Heteropoden, (See also:Leipzig, 1855). (13) J.

W. Spengel, " Die Geruchsorgane and das Nervensystem der Mollusken," Zeitschr. f. wiss. Zool. (1881). (14) See also:

Richard See also:Owen, Memoir on the Pearly See also:Nautilus (See also:London, 1832). (15) L. Cuenot, " See also:Excretion chez les mollusques," Arch. d. biol. xvl. (1899). (16) P. See also:Geddes, " On the Mechanism of the Odontophore in certain Mollusca." (17) T. H. See also:Huxley, " On the See also:Morphology of the Cephalous Mollusca," Phil.

Trans. (1853). (18) Von See also:

Jhering, Vergleichende Anatomie des Nervensystems and Phylogenie der Mollusken (Leipzig, 1877). (19) E. R. Lankester, " Contributions to the Developmental See also:History of the Mollusca," Phil. Trans. (1875) ; " See also:Note on the Coelom and Vascular System of Mollusca and See also:Arthropoda," Quart. Journ. Mice. Sci. xxxly. (1893).

(2o) P. Pelseneer, Introduction a l'etude des Mollusques (See also:

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) See also:Cooke, " Molluscs," See also:Cambridge Natural History, vol. iii. (1895). (22) See also:Fischer, See also:Manuel deconchyliologie (1887). (23) See also:Jeffreys, See also:British Conchology (1862-1869). (24) Simroth, " Mollusca," See also:Bronn's Klassen and Ordnungen des Thierreichs, Bd. iii. (1895), in prog.

(25) See also:

Tryon, See also:Manual of Conchology (1878), in prog. (26) See also:Woodward, A Manual of the Mollusca (188o). (E. R. L.; J. T.

End of Article: XVIJI

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