The Reactions of Planarians to Light 135 



lamp from one position to another. To changes in directive light 

 planarians are known to respond very definitely, and consequently 

 they may be said to distinguish the motions of objects. 



With regard to true seeing, however, in the sense of distinguish- 

 ing the forms of objects, it is safe to assume that planarians have 

 almost no power whatever, since their eyes are optically unable to 

 form images even if the central nervous system were highly enough 

 developed to interpret images when formed. In the case, there- 

 fore, of Planaria alpina, which, according to Collin ('91, p. 180), 

 "shuns" Planaria gonocephala when the latter has been put into 

 the same aquarium with it, seeking "strenuously to escape" from 

 its larger relative, the conclusion does not necessarily follow that 

 P. alpina sees an enemy and experiences the sensation of fear. 

 As previously pointed out (p. 95), the whole matter is probably 

 explainable on the basis of negative chemotaxis alone. To attri- 

 bute fear, therefore, or any other similar complex sensation, to 

 an organism whose responses are so plainly of a simple reflex 

 nature, is to go quite beyond the evidence. 



In the performance of the two great life processes of nutrition 

 and reproduction, light is apparently in no way a direct aid to pla- 

 narians, since they thrive in situations from which light is entirely 

 excluded, as in caves, and since they habitually frequent places 

 where this factor is reduced to a minimum. Light cannot, then, 

 be regarded as a directly essential factor in the life of planarians. 



That light is not essential to the activity of protoplasm has more 

 than once been demonstrated. Engelmann ('79), for example, 

 showed that the streaming protoplasm of plant cells occurs nor- 

 mally in darkness, while Maupas ('87) found ciliates multiplying 

 as rapidly in the dark as in the light. 



b Are Planarians Able to "Learn" ? 



With regard to the ability of these worms to acquire upon repe- 

 tition an abbreviated form of response; that is, to "learn," a few 

 suggestions may be drawn from experiments already described 

 in other connections. 



It will be remembered (p. 93) that when a small aquarium 



