The Reactions of Planarians to Light 6 1 



for the above statement, based upon 240 two-minute trials under 

 vaiious intensities, are 0.645 •I'^rn- P^r sec, the average during the 

 first minute, as against 0.713 mm. per sec, the average during 

 the second minute. 



Secondly, in these 240 trials, the percentage of turning under all 

 intensities is greater during the first minute than during the sec- 

 ond, being 87 per cent and 57 per cent, respectively. This result 

 may possibly be conceived to be due to a greater steadying influ- 

 ence of the light during the second minute than during the first 

 and to a consequent greater turning than during the first minute. 

 But on the other hand a similar decrease of turning, although not 

 so pronounced, took place during the second minute when the 

 worms were in the dark. It must be admitted, therefore, that the 

 fact of less turning during the second minute may have nothing 

 to do with the interval of response. 



Thirdly, on several occasions a notable piece of behavior was 

 observed, which may have a bearing on the interval of response. 

 The phenomenon in question always occurred in connection with 

 a modification of the experimental field within the light machine 

 to be more fully described later. Briefly this modification con- 

 sisted in making a field of two distinct intensities of light, the 

 latter being projected vertically from above in such a way that a 

 sharp line of demarkation formed a boundary between the two 

 areas. Ordinarily when the worms reached this boundary line 

 as they glided from one intensity to another, they responded 

 promptly to the stimulus caused by the change of intensity. Sev- 

 eral times, however, they were observed to travel indifferently 

 exactly along this dividing line for a distance of several centimeters 

 with half the body in one intensity and half in the other. This 

 curious fact lends itself to various interpretations, one of which 

 is that the response to a new intensity may not be, in all cases, 

 immediate. 



Manner of Coming to Rest. During the experiments made 

 in the non-directive light apparatus previously described, nor- 

 mal worms could never be induced to come to rest in the light. 

 If allowed to remain in the aquarium they would wander about 

 until they reached the shadow under the diaphragm (Fig. i, H), 



