THE SPECIAL SENSES 



385 



The simplest eyes, if we may call them eyes, are not 

 capable of forming an image or picture of external objects. 

 They only make the animal better capable of distinguish- 

 ing between light and darkness or shadow. Many lowly 

 organized animals, as some polyps, and worms, have certain 

 cells of the skin specially provided with pigment. These 

 cells grouped together form what is called a pigment fleck, 

 which can, because of the presence of the pigment, absorb 

 more light than the skin cells, and are more sensitive to 

 the light. By such pigment-flecks, or eye-spots, the animal 

 can detect, by their shadows, the passing near them of mov- 

 ing 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 to 

 changes in the in- 

 tensity of light 

 (Fig. 236). 



Most of the 

 many - celled ani- 

 mals possess eyes 

 by means of which 

 a picture of exter- 

 nal objects more or less nearly complete 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 the 

 retina, which is stimulated 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. These are the constant essential parts 01 an 

 image - forming and image -perceiving eye. In most eyes 

 there are other accessory parts which may make the whole 



FIG. 236. The simple eye of a jelly-fish (Lizzia 

 koellikeri). After O. and R. HERTWIO. 



