658 



ZOOLOGY 



SECT. 



XVI 



nerve actually springs from the cerebral ganglion. Patches of 

 sensory epithelium, very similar to the osphradia, and called 

 the abdominal sense-organs, occur, one on each side of the anus, 

 in Area and other asiphoniate forms, and a similar organ has 



been described beside the retractor 

 muscles of the siphons in several 

 Siphoniata. 



In a few instances eyes are present, 

 but never in what we are accustomed 

 to consider as the normal position for 

 such organs, at the anterior or head 

 end of the body. They occur, in fact, 

 in the only situation where they can be 

 of any use, namely, along the edge of 

 the mantle. The best known form in 

 which they occur is the common Scallop 

 (Pecten), which has a single row (Fig. 

 537, vii) all round the mantle border. 

 Each has a cornea (Fig. 555 1\ a cel- 

 lular (not cuticular) lens (#), a retina (5), 

 formed of cells, the inner ends of which 

 are modified into visual rods, and an 

 optic nerve (7), one branch of which 

 spreads over the front of the retina and 

 sends branches backwards to the visual 

 rods. In this pecularity, as well as in 

 the cellular lens, the eye of Pecten is 

 singularly like that of Vertebrates. 

 The pallial eyes of Pelecypoda are 

 probably to be looked upon as modified 

 tentacles. 



Reproduction and Development. 

 Most Pelecypoda are dioecious, but 

 several hermaphrodite forms are known. 

 Some of these, such as the Oyster, are 

 protandrous, the gonad producing first 

 sperms and afterwards ova : in others 

 part of the gonad serves as an ovary, 

 part as a testis, the two opening into a 

 common duct : in others again there is 

 a distinct ovary and testis on each side 

 opening by separate ducts. There are never any accessory organs 

 of reproduction, such as spermatheca, penis, etc. Fertilisation 

 frequently takes place in the water after the eggs are laid. 

 Segmentation is total but unequal, and the gastrula is formed either 

 by invagination or by epiboly. A shell -gland (Fig. 556 sd.) is 

 formed as an invagination of the dorsal surface, a stomodaBum (m) 



VIII 



VI 



FIG. 554. Nervous system and 

 auditory organs of Nucula. 

 I, pleural ganglion; II, pleuro- 

 pedal-connective ; 111, com- 

 mon connective from cerebral 

 and pleural to pedal ganglia ; 

 IV, auditory nerve ; V, pedal 

 ganglion ; VI, visceral gang- 

 lion ; VII, posterior pallial 

 nerve ; VIII, osphradium ; 

 IX, visceral connective ; X, 

 otocyst ; XI, canal of otocyst ; 



XII, its external aperture ; 



XIII, cerebro-pedal connec- 

 tive; XIV, anterior pallial 

 nerve ; XV, nerve to palps ; 

 XVI, cerebral ganglion. (From 

 Pelseneer.) 



