MOLLUSCS. 139 



regarded as a very rudimentary organ for tlie per- 

 ception of light. The skin is thrown into a pit, within 

 which the epithelial cells are elongated and pigmented. 



In the sea-ear (Haliotis), and in Trochus (Fig. 93), 

 the arrangement is similar, but the depression is 

 deeper, the mouth is very much restricted, and the 

 interior is filled by a vitreous body. 



In Murex (Fig. 94) the eye is still further developed, 

 and is entirely closed iu, a lens being present. 



^. 



Fig. 94.— Eye of Murex Irandaris (after Hilger). L, Lens; Gl, vitreous body; 



Ko, nerve. 



In the snail (Helix) the eye is still more highly 

 organized. It consists of a cornea, which lies imme- 

 diately below the skin ; a lens, behind which is the 

 retina, consisting of three layers, (1) the rods, (2) a 

 cellular layer, (3) a fibrous layer. This, indeed, appears 



