452 Dynamic Tfieery. 



The progressive development of the eye from the sensory ski a is 

 well seen in the G-asteropod Mollusks. Thus the eye of the Limpet, 



FIG. 186. Eye of Trochus Magus, a Gas- 

 teropod Mollusk. 



It is a little more developed than the eye 

 of the Limpet. 

 G L The Vitreous Body. 

 N Nerve. After Hilger. 



(fig, 185,) shows a mere indentation 

 or partial invagination of the skin 

 and apparently a modification of 

 the outer layer of the epidermal 

 cells into a transparent, glassy sort 

 of body, the forerunner of the 

 vitreous humor and crystalline 

 lens ; while the under layer be- 

 comes the sensitive retina. 



FIG. 186. 



FIG. 187. Eye of Murex brandarts, 



a Gasteropod Mollusk. 



( After Hilger.) 



This eye is an improved modifica- 

 tion of that of the Trochus Magus ; a 

 part of the vitreous body having 

 become set off into a lens. 



L.Lens. 



GL Vitreous Body. 



Jf.-Nerve. 



In the eye of the Trochus 

 magus, another Gasteropod, 

 (fig. 186, ) the development 

 has gone further. The in- 

 vagination is about completed 

 and the " vitreous body " 

 almost enclosed. The shape 

 of the vitreous body must 

 give it the force of an imper- 

 fect lens, in concentrating 

 the light. In the eye of the 



Murex, another Gasteropod, FIG. 187. 



we have a still further advance. Here the vitreous body is not only 

 fully enclosed, and covered in front by a transparent epidermal layer 

 grown over from the sides, but a spherical lens has become set off from 

 the rest of the vitreous humor. 



In these "invertebrate eyes," the fibres of the optic nerve, which re- 

 ceive the impressions of light, lie behind the sensitive layer of the retina 

 instead of in front of it, as in the human eye ; and so there is no hole 

 in the retina made for the passage of the nerve, and consequently no 



