SCHEMATIC MOLLUSC.] MOLLUSCA the mouth, just as the pedal ganglia are behind it. The right and left pedal ganglia are joined by transverse cords to the right and left visceral cords respectively, the point of union being marked on either side by a swelling (ff.pl) known as the pleural ganglion. The visceral nerve-cord can also be traced up on each side beyond the pleural ganglion to the cerebral ganglion. Thus we have a nearly complete double nerve-ring formed around the oeso phagus by the two pairs of nerve-cords which are in this region drawn, as it were, towards each other and away from their lateral position both behind and before the stomodseal invagination. Whilst the swollen parts of the nerve-tracts are termed ganglia, the connecting cords are conveniently distinguished either as connectives or as commissures. Commissures connect two ganglia of the same pair We have a cerebral commissure, a pedal com missure and a visceral commissure. Connectives connect ganglia of dissimilar pairs, and we speak accordingly of the cerebro- pedal connective, the cerebro- pleural con nective, the pleuro- pedal connective, and the viscero- pleural connective. An ENTERIC NERVOUS SYSTEM forming a plexus on the walls of the alimentary canal exists, but does not exhibit cords and ganglia visible to the naked eye except in the large Dibranchiate Cephalopods. Our schematic Mollusc is provided with certain ORGANS OF SPECIAL SENSE. Tactile organs occur on the head in the form of short CEPHALIC TENTACLES (a). Deeply placed are Fio. 3. Development of the Pond-Snail, Limnieus stagnalis (after Lankester, 15). dc, directive corpuscles (praseminal outcast cells) ; c/i, egg-envelope or cliorion ; or, oral end of the blastopore ; r, anal end of the blastopore. A. Formation of the Diblastula by the invagination of larger cells into the area of smaller cells (optical section). B. View of the same specimen from the surface of invagination ; the smaller cells are seen at the periphery ; by division they will multiply and extend themselves over the four larger cells. C. Fully-formed Diblastula, surface view to show the elongated form of the orifice of invagination or blastopore ; its middle portion closes up and coin cides with the region of the foot ; the extremity, or, coincides with the mouth and stomodiuum, the opposite extremity, r, with the anus. D. Optical section of an embryo a little older than A. E. Surface view of the same embryo. a pair of closed vesicles containing each a calcareous con cretion and acting as auditory organs ; these are known as OCTOCYSTS (D, y). They are situated behind the mouth in the foremost portion of the foot. At the base of each cephalic tentacle is a pigmented eye-spot the CEPHALIC EYE (D, iv). The OSPHRADIUM (A), or peculiar patch of olfactory epithelium at the base of the ctenidium, has already been mentioned. To the scheme thus exhibited of the possible organization of the ancestral Mollusc we shall now add a sketch of the mode in which this form of body and series of internal organs are developed from the egg. 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 appear to be most typical that is to say, which A f - -^ . _, C/J 637 cases which adhere to a Fio. 4. Development of the Pond-Snail, Linnxus stagnalis (after Lankester, 15). r, directive corpuscle ; bl, blastopore ; en, endoderm or enteric cell layer ; ec, ectoderm or deric cell-layer ; v, velum ; m, mouth ; /, foot ; t, ten tacles ; /p, pore in the foot (belonging to the pedal gland ?) ; m/, the mantle- flap or limbus pallialis ; sh, the shell ; I, the sub-pallial space, here destined to become the lung. A. First four cells resulting from the cleavage of the original egg-cell. B. Side view of the same. C. Diblastula stage (see fig. 3), 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, U. H, I. Veliger stage later than D. (Compare fig. 70 and fig. 72***). procedure which was probably common at one time to all then existing Mollusca, and which has been departed from A B . C Fio. 5. Early stages of division of the fertilized egg-cell in Nassa mutabilis (from Ballour, after Bobretzky). A. The egg-cell lias 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 divid ing upper sphere are the eight-shaped "directive corpuscles," better called " prspseminal 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 practi cally double, as in B. D. Each of the four segment-cells gives rise by divi sion 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, as in fig. 3, A, B.
only in later and special lines of descent show approxi-