100 



MOLLUSCA 



tie 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 swoUen parts of the 

 nerve-tracts are termed ganglia, the connecting cords 

 are conveniently distinguished either as coiinectives 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 ENTEEic NEEVOUS 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 oegans 

 OF SPECIAL SENSE. Tactile organs occur on the head in the 

 form of short cephalic tentacles (a). Deeply placed are 



Fig. 3. — Development of the Pond-Snail, Limnieus stagnalis (after Lankester, 

 15). . dCj directive corpuscles (prseseminal outcast cells); cfe, egg-envelope 

 or chorion ; 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 c^s. 

 0. 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 month 

 and stomodseum, the opposite exiremity, r, witti 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, w). The OSPHEADITJM (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-ceU of Mollusca is either free from food material 

 —a simple protoplasmic corpuscle — or charged with food 



material to a greater or 

 appear to be most typical- 



less extent. 

 — that is to Si 



Those 

 Y, which 



cases which 

 adhere to a 



FiQ. 4— Development of the Pond-Snail, Limnseus stagncUia (after Lankester, 

 15). r, directive corpuscle ; U, blastopore ; en, endodeiTO or enteric ceU 

 layer ; ec, ectodeiTn or deric cell-layer ; v, velum ; m, mouth ; /, foot ; t, ten- 

 tacles ; f]>, pore in the foot (belonging to the pedal,gland ?) ; w/, the mantle- 

 flap or limbus pallialis ; sft, 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. 0. Diblastula stage (see fig. 3), 

 showing the two cell-layers and the blastopore. D, B, F. Trochosphere 

 stage, D older than E or P. 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 /f-rx 



Fig. 6. — Early stages of division of the fertilized egg-cell in Nasm mittahiUs 

 (from Balfour, after Bobretzky). 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. Besting on the divid- 

 ing 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 afiects 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. 0. 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. B. 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- 



