370 



THE INFLUENCE OF LIVING SHBEOUNDINGS. 



brata (see fig. 98, 5) the optic nerve penetrates the outer skin of 

 the eye, and spieads out on its inner surface between it and the 

 lens in such a way that these ends of the nerve are turned away 

 from the lens and thus have their free ends directed outwards. 



Fn;. t)s._S'ectioii^of eyes f^O of a univalve. 5 is the iaj^r of rods and cone^ enclosed in/, 

 tlie fibrous layer of the retina, b, tlio eye of a vertebrate aiiinuil at tlie spot where 

 the optic nerve enters it. The nerve traverses all the layers ami ^prearls out, forming 

 the fibrous hiyer /; tlie colnmnar layer lies outside it, and thus in the reverse 

 position to wiiat it occupies ill the eye of the mollusc. 



In the eyes found on the tentacles of snails (see fig. 98, a) these 

 rods are in the contr.ary position ; the surface of the tijss is turned 

 towards the lens. Thu.'^, in the former, the layer of rods and 

 cones it.'^elf is pierced by the opt'c nerve, and in that spot of 



