370 



THE INiaUENCE OF LIVING gUEKOUNDINGS. 



brata (see fig. 98, 6) the optic nerve penetrates the outer skin of 

 the eye, and spreads 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 outwal-ds. 



? (-' til ^:^^ 



Fig. 98.— Sectious of eyes (n) of a univalve s is the laj er of rods and cones enclosed in/, 

 the fibrous layer of the retina, b, the eye of a vertebrate animal at the spot where 

 the optic nerve enters it. The nerve traverses all the layers and spreads out, forming 

 the fibrous layer /; the columnar layer lies outside it, and tlius in the reveree 

 position to what it occupies in the eye of the mollusc. 



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

 rods are in the contrary position ; the surface of the tips is turned 

 towards the lens. Thus, in the former, the layer of rods and 

 cones itself is pierced by the opt"c nerve, and in that spot of 



