126 The Animal Mind 



43. Vision in Planarians 



In planarians, unmistakable eyes are present, yet appar- 

 ently the reactions to light are not wholly dependent upon 

 them. The general effect of photic stimulation on the plana- 

 rian is to stimulate it to movement; it comes to rest in the 

 shaded portions of a vessel (9, 239, 243, 169). To a certain 

 extent, light directs the movement of the animal away from it 

 (313). But Hesse found that one species of planarian with 

 much more highly organized eyes than another reacted to 

 light decidedly less; the strength of the light reaction does 

 not, he concludes, correspond to the development of the light 

 perception. The latter depends on the number of sensitive 

 elements in the eye, the former on the habits of the animal 

 and the feeling tone aroused by the light (169). This " feeling 

 tone" may apparently be connected with a skin sensation. 

 Decapitated and hence eyeless planarians respond to light, 

 but more slowly (243), and with less definite reference to the 

 direction of the light (313). 



44. Vision in Annelids 



The earthworm's sensitiveness to light is also dermal, 

 although Hesse believes that he has found visual organs in 

 certain structures in the skin, especially at the upper lip and 

 the tail end (168). However this may be, the effectiveness of 

 light as a stimulus is not confined absolutely to any one region 

 of the body. When the worms are in a normal condition, 

 attached to their burrows, the combined effect of light and the 

 contact stimulus at the tail produces the ordinary negative 

 reaction of withdrawal into the burrow (91, 179). The only 

 evidence that light is accompanied by a specific conscious- 

 ness is to be derived again from the fact that the reaction time 

 to light is much longer than that to mechanical stimulation. 



