368 



MOLLUSCA. 



cavity filled with a cuticular lens (Fig. 290 B) ; finally, the pit may 

 be closed and the retinal epithelium continued in front of the lens 

 as a kind of internal cornea, thus lining a complete vesicle, while 

 the outer epithelium, together with some connective tissue, is con- 

 tinued over the eyes as a cornea proper (Fig. 290 C). The optic 

 nerve enters the retina on its internal side, and the percipient 

 elements are on the side of the retina next the light. 



The cephalic eyes are vestigial, being buried in the skin or absent, 

 in most burrowing forms ; absent iif most abyssal and subterranean 

 species, and in the Chitonidae ; and, curiously enough, absent in 

 A 



FIG. 290. Eyes of Gastropoda. A, of Patella; B, 

 of Trochus; C, of Helix. No and N optic nerve; 

 R and r retina ; L and (11 lens ; Ep cornea (A and 

 C after Carriere, U after Hilger). 



N 



some pelagic forms, e.g. lanthina, many Pteropods; though the 

 Heteropoda possess the best developed eyes of the class. 



Eyes are found in considerable numbers on the dorsal surface in 

 the Onchidiidae* in addition to the cephalic eyes. These pallial 

 eyes are constructed upon the so-called Vertebrate type, i.e. the optic 

 nerve perforates the retina, spreads out on the surface turned towards 

 the lens, and then passes inwards to the percipient elements which 

 are on the side of the retina turned inwards. Moreover, they possess 



Semper C., "Forms of Animal Life.' 



