370 REACTIONS OF DAPHNIA PULEX TO LIGHT AND HEAT. 



"First Half" was shorter than that for the "Second" only by reason of the initial 

 stimulus. There was still some evidence, however, that the intense illumination 

 of the end next the light somewhat retarded the animals in that they were forced 

 by it to take a less regular course, and, although moving more rapidly if anything, 

 because of the zigzag path followed they progressed, on the whole, more slowly. 



In one instance an animal on its first trip swam to about the middle of the trough, 

 then, ceasing to move toward the light, it swam about in circles. It looked as if 

 the reaction was given to an "optimal intensity," and I at first thought that it was 

 photopathic. Further experimenting showed that the animal would swim toward 

 the end from which it had come, when the light was turned on there, but that toward 

 the other end it would not swim farther than the spot already mentioned. Search 

 for an explanation of the phenomenon revealed the presence of a number of bubbles 

 (air?) at the bottom of the trough at the point beyond which the animal would not 

 go, even when directed by strong light. The reaction therefore was not photopathic 

 but chemotropic. 



This single observation is of special interest and importance since Daphnias are 

 usually so strongly positive in their phototaxis that they will pass through harmful 

 chemical solutions, and even attempt to go through a drop of strong acid in their 

 efforts to approach a light (Yerkes, :00, p. 419). 



The question, Is there any evidence of fatigue? must also be answered in the 

 negative. Although the animals in these experiments moved almost continuously 

 for an hour, covering during that interval at least a distance of 2000 centimetres, if 

 not 2500 centimetres, there was throughout an increase in the rate of movement, 

 and no evidence whatever of fatigue. 



The fourth question formulated has already been answered. There is no evi- 

 dence of an "optimal intensity," and no clearly demonstrated photopathic reaction. 



3. The Reactions of Daphnia to Radiant Heat. — Does radiant heat have any directive 

 influence upon the movements of Daphnia? In the previously described experi- 

 ments for testing the value, for the movements of the organism, of the heat accom- 

 panying light, there was little evidence of the influence of heat; but since light is 

 itself an exceedingly strong directive agent, it may be that the effect of the heat was 

 obscured by it. To determine in a more satisfactory and conclusive manner whether 

 heat is a directive stimulus the following experiments were made. 



A. Method. — Over a V-shaped glass trough 24 centimetres long, 2 centimetres 

 wide, and 1.5 centimetres deep was swung a frame bearing five partitions, cut to fit 

 the V shape of the trough. This frame could be raised above the trough or lowered 

 so that the partition divided it into six equal portions. The trough was placed hori- 



