ANIMAL PHYSIOLOGY 75 



take up and transmit through the auditory nerve to the 

 brain the stimuH received from the external auditory hairs. 



Not all animals have eyes. The moles, which live under- 

 ground, insects and other animals that live in caves, and the 

 deep-sea fishes which live in waters so deep that 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 eyes or 

 only rudimentary ones because they live under conditions 

 where eyes are useless. They have lost their eyes by degen- 

 eration. There are, however, , many animals that have no 

 eyes, nor have they or their ancestors 

 ever had eyes. These are the sim- 

 plest, most lowly organized animals. 

 Many, perhaps all eyeless animals, 

 are, however, capable of distin- 

 guishing light from darkness. They Fig. 32. simple eye of a 

 are sensitive to hght. An investiga- jellyfish. (Greatly magni- 

 tor placed several individuals of ^^'^' ^ffe-^ Hertwig.) 

 the common, tiny fresh-water polyp (Hydra) in a 

 glass cylinder the walls of which were painted black. 

 He left a small part of the cylinder unpainted, and 

 in this part of the cylinder where the light pene- 

 trated the Hydras all gathered. The eyeless maggots 

 or larvae of flies, when placed in the light will wriggle 

 and squirm away into dark crevices. They are conscious 

 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 organ- 

 isms whether they have eyes or not, and the organisms either 

 try to get into the light or to avoid it. But this is not seeing. 



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 



