The Reactions of Planarians to Light 97 



Hungry planarians in the presence of food have their photo- 

 taxis entirely obscured. 



3 KINDS OF BEHAVIOR 



In the two preceding sections, treating of Photokinesis and Pho- 

 totaxis, respectively, animal behavior, as illustrated by the effect 

 of light upon planarians, has been taken up from the point of 

 view of the stimulus. In the two following sections, on the other 

 hand, the reactions of planarians will be dealt with from the stand- 

 point of the animal rather than from that of the stimulus. 



To this end a classification of the behavior of planarians in light 

 is here presented based upon (A) generic and specific differences, 

 and (B) individual differences. 



That there are morphological differences which fall naturally 

 within the lines of this classification has long been recognized, 

 indeed, the criteria used in classification by systematists are based 

 almost exclusively upon such differences, while relatively little 

 importance has been attached to differences in the behavior of 

 animals. 



As already mentioned in the historical review, Loeb ('94), in 

 dealing with the differences of behavior which characterize the 

 two genera, Planaria and Thysanozoon, pointed out that decided 

 physiological variation may appear in forms closely related mor- 

 phologically. The same fact had been previously emphasized 

 for the case of the pulmonates by Willem ('91). Obviously such 

 physiological variations do not furnish reliable criteria for the 

 systematist, since they are so largely dependent upon environ- 

 mental causes, and furthermore the work of the systematist is 

 usually done upon dead animals. Nevertheless some interesting 

 relations between behavior and systematic position await the stu- 

 dent who approaches the study of animal behavior from this direc- 

 tion. 



Strictly speaking, all behavior is individual behavior. In this 

 sense it is manifestly incorrect to speak of the behavior of a genus 

 or of a species per se. 



The behavior of individuals may, nevertheless, be classified into 

 responses which are characteristic of all the members of a genus, 



