08 



MOLLUSCA 



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- 



d 



m. 



9P l 



\ l> I I I If 7 



* (IV 1 



y.pe z.l a y.ab 



Fio. 1. Schematic Mollusc. A. Dorsal aspect. B. Ventral aspect. C. The 

 heart, pericardium, gonacls, 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 ; i>, head ; c, edge 



of the mantle-skirt or limbus 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 

 be seen through its substance ; /, edge of the shell-follicle ; g, the shell ; h, 

 the osphradium, paired (Spengel's olfactory organ) ; i, the ctenidimn, 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 lacunee ; s, pericardia! 

 cavity ; t, gonad (ovary or spermary), paired ; 1(, 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 ; to, 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 "liver") 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 xinited 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.ati, 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 anus 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, I). Near 

 the nephridial apertures, and in front of them, right and 

 left, are the pair of apertures (k) 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 

 " striated "). The appearance of cross striation seen in 

 the muscular cells of some Molluscs (odontophore of 

 Haliotis, Patella, <fec.) 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 "dome" (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 " Shell " in the Cyclop. o/Anat. and Physiol. The 

 limits of our space do not permit us to deal with this or other histo- 

 logical topics. 



