76 



THE ANIMALS AND MAN 



organized animals, as some polyps, and worms, have cer- 

 tain cells of the skin specially provided with pigment. 

 These cells grouped together form what is called a pig- 

 ment-fleck, which can, because 

 of the presence of the pig- 

 ment, absorb more light than 

 the skin-cells, and are more 

 sensitive to the light. By 

 such pigment-flecks, or eye- 

 spots, the animals can detect, 

 by their shadows, the passing 

 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 

 1--. „ f ^ I, ^ not simply with pigment but 



Diagram of vertebrate . ^ ■' ^ '^ 



with a simple sort of lens that 

 serves to concentrate rays of 

 light and make this simplest 

 sort of eye even more sensitive to changes in the intensity of 

 light (fig. 32). 



Most of the many-celled ani- 

 mals possess eyes by means of 

 which a picture of external ob- 

 jects more or less nearly com- 

 plete and perfect can be formed. 

 There is great variety in the 

 finer structure of these picture- 

 forming eyes, but each consists 

 essentially of an inner delicate 

 or sensitive nervous surface called fig. 34. Part of cornea, show- 

 the retina, which is stimulated by J^g facets, of the compound 



light, and is connected with the '^^ °l''}°''"fl- ^■^'''^"^ 

 , , . magnified; photo -microgra- 



bram by a large optic nerve, and phy by Geo. o. Mitchell.) 



Fig. 33. 



eye; c, choroid; i, iris; I, lens; 

 n, optic nerve; r, retina; s, 

 sclerotic. (From Kingsley.) 



