370 



PHYSIOLOGY OF THE INVERTEBRATA. 



In the Tetrahranchiata, as represented by Nautilus and its 

 allies, the eye has no cornea, lens or vitreous humour. It is 

 a mere cup or cavity lined by the retina. 



Fig. 73. — EvK of Skpia. {After Gegenbaur.) 



a = lens, d = anterior chamber, c — cornea, d = ciliary body, e = cartilage 

 of iris. /= sclerotica. ^— tapetum. // = cephalic cartilage. / = optic 

 ganglion. k = white glandular body. ;« = pigment layer. = outer and 

 inner layers of retina. / = optic nerve. 



In Onychotcutlds, Ommastre^jlics, and allied genera, the 

 crystalline lens is exposed to the sea water ; this is due to 

 the entire absence of the cornea. 



The eye of the Dihranchvda has been stated to resemble 

 the Vertebrate eye, but this resemblance is merely superficial. 

 In fact, "the rods and cones of the Vertebrate eye exactly 

 correspond with the crystalline cones, &c., of the Arthropod 

 eye ; and the reversal of the ends, which are turned towards 

 the light in the VcHehrata, is a necessary result of the extra- 

 ordinary change of position which the retinal surface under- 

 goes in them." The above is an additional fact substantiating 



