142 AECA^SPONDYLUS. 



consider those of insects, where they are more highly 

 developed. 



The eyes of Pecten and Spondylus are, again, formed 

 oa a totally different plan. 



It has been already observed that there is an 



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

 Epithelium of the edge of the manile ; 2, cells of vision ; 3, lens ; 4, 5, conaectlve 

 tissue ; 6, section of one of the cells. 



essential difference between the typical vertebrate and 

 the typical invertebrate eye; in that while in the 

 fiirmer, the optic nerve (Fi"-. 77) penetrates the retina 

 and then spreads out on the anterior surface, so that 

 t:ie "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 vertebratps, 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. 



Plere, then, we get a reraarkalde approach to the 

 vertebrate eye ; but the similarity is still greater in 



* Muller's Arch., 1840. See alsn Hensen, " Ucber das Auge einiger 

 Lamellibranchiaten,'' Zeit. fiir Wiss, Zool , 1865. 



