244 



HISTOLOGY 



band lying at the level of the visual cells, at the other point where these 

 give off the visual rods. 



The epithelium lining the anterior part of the sac is differentiated into 

 two regions. That on the extreme front has secreted the round lens, a 

 cell-product. A narrow band of the same cells, lying between the retina 

 and the lens-forming area, produce the vitreous body, which probably 

 further arranges the light-rays for their reception by the retina. One of 



bl 



FIG. 217. Eye of Strombus gigas. I., lens; ret., retina; vit., vitreous body (fluid); bl.sp., 

 blood spaces; bl., blood; nv., nerve; cor., cornea, x 475. 



these vitreous body gland cells is far larger than the others, and has 

 retired outside the body of the eye-sac and lies in a lateral position. 



The lowest mollusk eyes have already been described. There are an 

 infinite number which are graded from the form possessed by Solen up 

 to forms that compare in their degree of specialization with the eye of 

 Nereis among the worms. In Patella rota we find a pit such as the eye 

 of Solen would form if half invaginated. Arochus magnus shows an 

 eye which would represent that of Patella were the invagination to be 

 almost completed and the lens material formed into a round, well-shaped 

 ball. In Turbo cceniformis the structure is a complete invagination, and 

 a cornea is for the first time formed by the cutting off of the invaginated 



