138 Herbert Eugene Walter 



Summary. The existence of feeling or consciousness among 

 planarians is a matter of pure conjecture. 



From their responses it may be inferred that they are able to 

 distinguish dark from light, as well as objects in motion, but it is 

 not clear that they can distinguish the forms of objects. 



The knowledge which planarians have of objects in their imme- 

 diate environment, such as food, enemies, etc., depends largely 

 upon chemical and tactile means. They are, therefore, as well 

 able to go through the entire range of their activities in the dark 

 as in the light. 



Upon repetition planarians may in some instances become accus- 

 tomed to, or acquire greater facility in, responding to stimuli, but 

 this result of experience is almost instantly lost, so that it is doubt- 

 ful whether these animals possess more than the merest rudi- 

 ments of the primary criterion of consciousness, namely, the ability 

 to learn. 



VI General Conclusions 



Probably the questions which have occupied the greatest share 

 of attention throughout the literature dealing with the reactions 

 of organisms to light, are the following: 



1 Is the direction or the intensity of light of more importance 

 in orientation ? 



2 Which theory best explains orientation and phototaxis, the 

 theory of trial and error or that of the tropisms ? 



3 How far is behavior with respect to light, adaptive ? 



I DIRECTION OR INTENSITY 



Before the part played in the behavior of planarians by either 

 the direction or the intensity of light can properly be discussed, 

 it will be necessary to present a brief historical resume of certain 

 general conclusions reached by investigators along this line. 



A Historical 



Cohn ('53), Strasburger ('78) and Loeb ('90, '93a) attributed 

 the directive effect of light to the action of the rays. In a later 



