PECTEN. 143 



Oncliidium (a genus of slugs, widely spread over the 

 Southern Hemisphere), in which Semper has shown * 

 that the nerve actually pierces the retina as in verte- 



■k 



Fig. 97.— Diagram of eye of Pecteu (after Hickson). a. Cornea ; 5, transparent base- 

 ment membrane supporting the epithelial cells of cornea; c, the pigmented 

 epithelium ; d, the lining epithelium of the mantle ; e, the lens ; /, the ligament 

 supporting the lens ; g, the retina ; h, the tapetum ; Tc, the pigment ; w, the 

 retinal nerve ; n, complementary nerve. 



brates. That this distinctive character should thus 

 reappear in so distant a group is very interesting, and it 

 is also remarkable that Onchidium possesses two kinds of 

 eyes : some on the head, which are constructed on the 

 same type as those of other molluscs ; while the peculiar 

 eyes just mentioned are scattered over the back, and 

 their nerves arise, not from the cephalic, but from the 

 visceral ganglion. Moreover, they differ in number, 

 not only in the different species, some having one hun- 

 dred, some as few as twelve, and others none at all, 

 but even in different individuals of the same species. 

 Indeed, they are continually growing and being re- 

 absorbed. But while thus resembling a simple verte- 

 brate eye, the dorsal eyes of Onchidium have a totally 



* " Ueber Schnecken Augen am Wirbelthier typus," Arclu fur Mic. 

 Anat, 1877. 



