708 



ZOOLOGY 



SECT. 



like that of Vertebrates. The palliah eyes of Pelecypoda are 

 probably to be looked upon as modified tentacles. The only 

 cephalic eyes that occur, in this class are a pair of small but well- 

 developed organs which occur in the bases of the most anterior 



Fig. 592.— Vertical section of eye of Pecten. 1, cornea; S, lens; 8, external epithelium; 

 U, blood-sinus ; 5, retina ; 6, pigmentary layer ; 7, optic nerve. (From Korschelt and Heidcr.) 



filaments of the inner lamina of the gill in Mytilus and some other 

 genera. 



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, &c. Fertilisation 



