376 REACTIONS OF DAPHNIA PULEX TO LIGHT AND HEAT. 



of light throughout the period. There was no evidence of fatigue in animals that 

 had moved at least 2000 centimetres in an hour. 



5. Heat in the absence of light has a directive influence upon the movements 

 of Daphnia. In a trough containing water of 28° C. at one end and 25° at the other 

 they migrated toward the region of lowest temperature. 



6. The movement is not direct, but irregularly .wandering. It is, however, in 

 all probability due to differences in the intensity of stimulation for different regions 

 of the animal's body, and is therefore in principle the same as the photopathic 

 reaction. Daphnia may therefore be said to be negatively thermotactic at a tem- 

 perature of about 28° C. 



7. The fact that in the case of Daphnia phototactic reactions cannot be changed 

 from positive to negative or the reverse by changes in temperature indicates that 

 light does not act upon the organism in the same way as does heat. 



I am indebted to Prof. W. C. Sabine for the suggestion of the cylindrical lens 

 as a means of obtaining a uniformly graded line of light, and to Professors E. L. Mark 

 and G. H. Parker for assistance in the work. 



V. BIBLIOGRAPHY. 



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: 01. The Theory of Phototactic Response. Amer. Jour. Physiol.; vol. 4, no. 9, pp. 460-481. 



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