Page:Encyclopædia Britannica, Ninth Edition, v. 16.djvu/663

Rh SCHEMATIC MOLLUSC.] MOLLUSCA 635 convenient to construct a schematic Mollusc, which shall possess in an unexaggerated form the various structural arrangements which are more or less specialized, exagger ated, or even suppressed in particular members of the group. Such a schematic Mollusc is not to be regarded as an arche- a 3f l p q.pe z.l J q.ab FIG. 1. Schematic Mollusc. A. Dorsal aspect. B. Ventral aspect. C. The heart, pericardium, gonads, and nephridia shown in position. D. The nervous system ; the reader is requested to note that the cord passing backwards from g.pe lies beneath, and does not in any way unite with the cord which passes from g.ab to g.pl. E. Diagram in which the body-wall is represented as cut in the median antero-posterior plane, so as to show organs in position, the shell-sac is seen in section, but the shell is omitted. Letters in all the figures as follows : a, cephalic tentacle ; 7&amp;gt;, head ; c, edge of the mantle-skirt or linibus pallialis ; d, dotted line indicating the line of origin of the free mantle-skirt from the sides of the visceral hump ; e, outline of the foot seen through the mantle-skirt in A, which is supposed to be trans parent, allowing the position of this and of the various parts h, i, k, I, m, to 1 seen through its substance ; /, edge of the shell-follicle ; g, the shell ; h, the osphradium, paired (Spengel s olfactory organ) ; i, the ctenidium, paired (gill-plume) ; k, aperture of the gonad, paired ; I, aperture of one of the two nephridia ; m, anus ; n, posterior region of the foot reaching farther back than the mass of viscera (dorsal hump) which it carries ; o, mouth ; p, plantar surface of the foot ; q, cut edge of the body-wall of the dorsal region ; r, coelomic space (blood-lymph space or body-cavity), mostly occupied by liver, but to some extent retained as blood-channels and lacunse ; s, pericardia! cavity ; t, gonad (ovary or spermary), paired ; u, nephridium, paired ; v, ven tricle of the heart receiving the right and the left auricles at its sides, and sending off anteriorly a large vessel, posteriorly a small one ; w, the cephalic eye, paired ; x, dotted ring to show the position occupied by the oesophagus in relation to the nerve ganglia and cords ; y, the otocyst, paired ; z.l, the digestive gland (so-called &quot;liver&quot;) of the left side ; z.g, duct of the digestive gland of the right side ; g.c, cerebral ganglion united by the cerebral com missure to its fellow; g.pl, pleural ganglion united by the cerebro-pleural connective to the cerebral ganglion, and by the pleuro-pedal connective to the pedal ganglion ; g.pe, the pedal ganglion united to its fellow by the pedal commissure the two sending off posteriorly the long ladder-like pair of pedal nerves ; g.v, the visceral ganglion (of the left side) united by the visceral loop or commissure to the similar ganglion on the right side, and by the viscero-pleural connective to the pleural ganglion ; g.ab, abdominal ganglion developed on the course of the visceral loop ; g.olf, olfactory ganglion placed near the osphradium on a nerve taking its origin from the visceral ganglion. type, in the sense which has been attributed to that word, nor as the embodiment of an idea present to a creating mind, nor even as an epitome of developmental laws. Were know ledge sufficient, we should wish to make this schematic Mollusc the representation of the actual Molluscan ancestor from which the various living forms have sprung. To defi nitely claim for our schematic form any such significance in the present state of knowledge would be premature, but it may be taken as more or less coinciding with what we are justified, under present conditions, in picturing to ourselves as the original Mollusc or archi-Mollusc (more correctly Archimalakion). After describing this schematic form, we shall proceed to show how far it is realized or justified in each class and order of Mollusca successively. The schematic Mollusc (fig. 1, A to E) is oblong in shape, bilaterally symmetrical, with strongly differentiated dorsal and ventral surface, and has a well-marked HEAD, consisting of the prostomium (6) and the region imme diately behind the mouth. Upon the head we place a pair of short CEPHALIC TENTACLES (a). The mouth is placed in the median line anteriorly, and is overhung by the prostomium (B, o) ; the amis is placed in the median line posteriorly, well raised on the dorsal surface (A, m). The apertures of a pair of NEPHRIDIA are seen in the neighbourhood of the anus right and left (A, /). Near the nephridial apertures, and in front of them, right and left, are the pair of apertures (&) appropriate to the ducts of the GONADS (generative pores). The most permanent and distinctive Molluscan organ is the FOOT (Podium). This is formed by an excessive development of the somatic musculature along the ventral surface, distinctly ceasing at the region of the head, below which it suddenly projects as a powerful muscular mass (B, p ; E, p). It may be compared, and is probably genetic ally identical, with the muscular ventral surface of the Planarians and with the suckers of Trematoda, but is more extensively developed than are those corresponding struc tures. The muscular tissue of the foot, and of all other parts of the body of all Mollusca, is cellular and unstriated, as distinguished from the composite muscular fibre (con sisting of cell-fusions instead of separable cells) which occurs in Arthropoda and in Vertebrata, and which has the further distinction of being composed of alternating bands of substance of differing refractive power (hence &quot; striated &quot;). The appearance of cross stria tion seen in the muscular cells of some Molluscs (odontophore of Haliotis, Patella, &c.) requires further investigation. It is by no means altogether the same thing as the mark ing characteristic of striated muscular fibre. Contrasting with the ventral foot is the thin -walled dorsal region of the body, which may be termed the anti- podial region. This thin-walled region is formed by soft viscera covered in by the comparatively delicate and non- muscular body- wall (fig. 1, E). As the ventral foot is clearly separate from the projecting head, so is this dorsal region, and it is conveniently spoken of as the VISCERAL HUMP or &quot; dome &quot; (cupola). Protecting the visceral dome is a SHELL (conchylium) consisting of a horny basis impreg nated with carbonate of lime, 1 and secreted by the deric epithelium of this region of the body (g). The shell in our schematic Mollusc is single, cap-shaped, and sym metrical. It does not lie entirely naked upon the surface of the visceral dome, but is embedded all round its margin, to a large extent in the body-wall. In fact, the integu ment of the visceral dome forms an open flattened sac in which the shell lies. This is the PRIMARY SHELL- SAC, or FOLLICLE (A and E, /). The wall of the body pro jects all round the visceral dome in the form of a flap or skirt, so as to overhang and conceal to some extent the head and the sides of the foot. This skirt, really an out- 1 As to the minute structure of the shell in various classes, see Carpenter s article &quot; Shell &quot; in the Cyclop, of Anat. and Physiol. The limits of our space do not permit us to deal v.ith this or other histo- logical topics.