142 



ARCA— SPONDYLUS. 



consider those of insects, where they are more highly 

 developed. 



The eyes of Pecten and Spondylus are, again, formed 

 on a totally different plan. 



It has been already observed that there is an 



Fig. 96.— Perpendicular section through an eye of Area Noce. (after Carriere). 1, 

 Epithelium of the edge of the mantle ; 2, cells of vision ; 3, lens ; 4, 5, connective 

 tissue ; 6, section of one of the cells. 



essential difference between the typical vertebrate and 

 the typical invertebrate eye; in that while in the 

 former, the optic nerve (Fig. 77) penetrates the retina 

 and then spreads out on the anterior surface, so that 

 the " rods " point away from the light ; in the normal 

 invertebrate eye, on the contrary, the nerve spreads 

 out on the back of the retina, so that the rods point 

 towards the light. Krohn,* however, made the remark- 

 able discovery that in the genus Pecten the rods, like 

 those of the vertebrates, are turned away from the light. 

 In this case, however, the optic nerve does not enter 

 the retina directly from behind, but runs round it and 

 passes, so to say, over the lip of the cup. 



Here, then, we get a remarkable approach to the 

 vertebrate eye ; but the similarity is still greater in 



* Mailer's Arch., 1840. See also Hensen, " Ueber das Auge einiger 

 Lamellibranchiaten," Zeit fur Wiss. ZooL, 1865. 



