708 



ZOOLOGY 



SECT; 



like that of Vertebrates. The pallial 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 



Fio. 502.— Vortical section of eye of Fecten. 1, cornea; 2, lens; .-;, external opithelium ; 

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



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 



