HO IV ANIMALS KNOIV THINGS 



near them of moving bodies, and thus be in some measure 



informed of the approach of enemies or of prey. Some 



of these eye-flecks are provided 



not simply with pigment but 



with a simple sort of lens that 



serves to concentrate rays of 



light and make this simplest 



sort of eye even more sensitive p,^^ 75, _ simple eye of a 



to changes in the intensity of jellyfish. (Greatly magni- 

 ,. , ^ ,p , fied; after Ilertwig.) 



light (fig. 75). 



" Most of the many-celled animals possess eyes by 

 means of which a picture of external objects more or less 

 nearly complete and perfect can be formed. There is 



Fig. 75. Fig. 77. 



Fig. 76.— Diagram of vertebrate eye; c, choroid; i, iris; /, lens; n, optic 

 nerve; r, retina; s, sclerotic. (From Kingsley.) 



Fig 77.— Part of cornea, showing facets, of the conipnuiul eye of a horse- 

 fly {Therioplcctes sp.). (Greatly magnified; photomicrograph by 

 Geo. O. Mitchell.) 



great variety in the finer structure of these picture-forming 

 eyes, but each consists essentially of an inner delicate or 

 sensitive nervous surface called the retina, which is stim- 

 ulated by light, and is connected with the brain by a large 

 optic nerve, and of a transparent light-refracting lens 

 lying outside of the retina and exposed to the light. 



