no FIRST l.F.SSONS IN ZOOLOGY 



the light of the sun never comes to them, have no eyes at 

 all, or have eyes of so rudimentary a character that they 

 can no longer be used for seeing. But all these animals 

 have no c)'es or only rudimentary' ones because they live 

 under conditions where eyes are useless. They have lost 

 their eyes b}' degeneration. There are, however, many 

 animals that have no e\'es, nor have they or their ances- 

 tors ever had eyes. These are the simplest, most lowly 

 organized animals. Many, perhaps all eyeless animals, 

 arc, however, capable of distinguishing light from dark- 

 ness. They are sensitive to light. An investigator 

 placed several individuals of the common, tin)' fresh-water 

 polyp (Hydra) in a glass cylinder the walls of which were 

 painted black. He left a small part of the cylinder un- 

 painted, and in this part of the cylinder where the light 

 penetrated the Hydras all gathered. The e\'eless mag- 

 gots or larv:e of flies, when placed in the light will wrig- 

 gle and squirm away into dark crevices. They are con- 

 scious of light when exposed to it, and endeavor to shun 

 it. Most plants turn their leaves toward the light; the 

 sunflower turns on its stem to face the sun. Light seems 

 to stimulate organisms whether they have e\'es or not, 

 and the organisms either try to get into the light or to 

 avoid it. But this is not seeing. 



" Th.e simplest e)'es, if we may call them e3'es, 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. Man}' lowI\' 

 organized animals, as some poI\'ps, and worms, have cer- 

 tain cells of the skin specially provided with pigment. 

 These cells grouped together form what is called a j^iig- 

 ment-fleck, which can, because of the presence of the pig- 

 ment, absorb more light than the sk'in-cells, and are more 

 sensitive to the light. ]5y such pigment-flecks, or eye- 

 spots, the animals can detect, b)' their shadows, the passing 



