REACTIONS OF DAPHNIA PULEX TO LIGHT AND HEAT. 373 



a sufficient degree to influence the animals. The averages for the series is —0.475. 

 This shows clearly that Daphnia is negatively thermotactic at a temperature of 28°. 

 It is noteworthy that all except the first experiment resulted in — movements. 



For the purpose of determining the relation of the amount of heat to the extent 

 of the animals' migration, the trough was now removed to a distance of 20 centimetres 

 from the heater (i.e., four times as far as in the experiments of Table IV). Table V 

 contains the records for five check experiments, and ten heat experiments with the 

 heater at this distance. For the check experiments the average movement was +0.65 ; 

 for the heat experiments it was —0.15. If in this case, as in the previous series, 

 we consider the + tendency, as indicated by the check series, which had to be over- 

 come before there could be any — movement, it is clear that the influence of the 

 heat should be expressed by 0.65, the positive tendency, plus 0.15, the negative result, 

 or 0.80. In the previous series it would be 0.31 plus 0.475 or 0.785. 



Examination of the experiments of Table V shows that a positive tendency grad- 

 ually gave place to a negative. This would appear, in the light of the previous series, 

 to be the result of the gradual increase in the temperature of the water, and especially 

 of the increase in the difference of the temperatures of adjacent regions. At the end 

 of the series of Table V there was only 1° difference in the temperatures of the end 

 spaces, 1 and 6. 



C. Thekmotaxis. — Now, as to the evidence for the directive influence of heat, the 

 experiments described prove conclusively that in the absence of light Daphnia moves 

 away from the source of heat at a temperature of 28° or thereabouts. Is this merely 

 a random wandering into regions of lower and more favorable temperatures, or is 

 it a definitely directed movement similar to the phototactic reaction? 



Loeb ('90, p. 43) proved that the larvae of Porthesia wander into the warmer 

 end of a dark box; and other investigators have demonstrated migration with refer- 

 ence to heat in the case of Myxomycetes, Protozoa, and certain larvae of the Metazoa; 

 but thus far no definitely directed movements in response to heat have been described. 



In the first place it seems fairly certain that Daphnia, in the experiments of this 

 paper, was affected by the heat-rays only as the temperature of the water changed; 

 for at the beginning of the series there was no evidence of a directive influence, 

 whereas just as soon as the temperature of the water began to rise at the + end the 

 animals tended to migrate toward the — end. 



As a matter of observation, the movements of Daphnia toward the — end were 

 exceedingly irregular, being zigzag and indirect, and thus sharply contrasted with 

 the usual photopathic movements. Notwithstanding this I believe that the two 

 reactions are in principle the same; in both cases difference in intensity of stimulation 



