The Reactions of Plaiiarians to Light 49 



ference may sometimes be due to a balance of opposing stimuli, in 

 which case wandering or aimlessness is not a true measure of the 

 effect of any single stimulus. 



6 Orientation. This is a measure of behavior with reference 

 to the source of the light. It is expressed by the degree of posi- 

 tiveness or negativeness which the worm exhibits. 



7 Duration of Movement. The time it takes a worm to tire 

 out when subjected to certain stimuli or, in other words, a measure 

 of fatigue. 



8 Effect of Repetition. A measure of response is here referred 

 to which may be expressed quantitatively in units of time or quali- 

 tatively in manner of behavior. 



9 Wigwag Movements. These are waving movements of the 

 anterior end of the planarian, which appear to be a definite attempt 

 on the part of the worm to become adjusted to the stimuli acting 

 upon it. 



10 The Time Required to Leave a Unit Circle. This is a 

 rather unsatisfactory criterion because it may indicate in some 

 cases a combination of several conditions as, for instance, latency 

 of response, rate of locomotion and degree of wandering. 



11 Manner of Coming to Rest. Included under this heading 

 are such points as the position assumed, the locality selected, and 

 the abruptness of the act. 



Naturally some of the foregoing measures of behavior will be 

 seen to have more application and value than others in the follow- 

 ing study. 



V Observations 



I PHOTOKINESIS 



The term photokinesis was introduced by Engelmann ('83) to 

 denote the activities which are induced solely by the intensity of 

 light when the directive or orienting factor has been eliminated. 



In this section will be considered, (A) the behavior of planarians 

 in the absence of light; (B) their behavior in different intensities 

 of non-directive light, and (C) the effect of abrupt changes, both 

 in time and space, in the intensity of non-directive light. 



