REGULATION OF REACTIONS 277 



Upon the absolute temperature at the time of the observa- 

 tion, but also upon the preceding temperature, the time rate 

 of change in temperature, and the time of exposure at a 

 given temperature. (3) That reversal in the sense of reaction 

 may take place without any change in temperature or light 

 intensity. Reversal in the sense of reaction, therefore, 

 seems to be due to the effect of constant temperature and 

 time of exposure rather than to the effect of change in 

 temperature. 



In some of the forms mentioned above the changes are 

 very indefinite, but in Chlamydomonas alboviridis they 

 are even more pronounced and striking than in Euglena. 

 In studying Chlamydomonas it was also found that only 

 under certain conditions will changes in temperature 

 cause reversal in the sense of reaction to light. Thus, for 

 example, it could not be reversed in specimens which had 

 been in total darkness for twenty-four hours. These 

 specimens were negative at 22° in light having an inten- 

 sity as low as 150 ca. m., and they remained negative 

 when the temperature was raised until they died at a little 

 over 40°. 



No change in the sense of reaction to light could be 

 induced by varying the temperature in either direction 

 between zero and the maximum in the following forms: 

 Stentor, various species of zoeae, Scapholeberis armata, 

 Daphnia, Cyclops, Cypris and a small water spider. 

 Change in temperature, however, has certain effects on 

 the reactions in Daphnia, Cyclops, Cypris and the water 

 spider which are similar to those observed in Euglena. 

 On May 16 several specimens of each of these species were 

 exposed in light having an intensity of 160 ca. m. They 

 were neutral at room temperature (22°), and swam about 

 slowly without orienting or aggregating. When the tem- 

 perature was decreased they gradually became more and 

 more quiet, and finally sank to the bottom motionless, but 

 when the temperature reached 8° they became active 

 again, and soon collected at the side of the dish nearest the 



