452 Dynamic Theory. 



The progressive development of the eye from the sensory skin is 

 well seen in the Gasteropod Mollusks. Thus the eye of the Limpet, 



FIG. 186. Eye of Trochua 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 Tiochus Magus; a 

 part of the vitreous body having 

 become set off into a lens. 



//.Lens. 



;. Vitreous Body. 



JV r .-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 



