MOVEMENTS OF DAPHNIA 237 



PHOTOTAXIS IN RELATION TO TIME OF DAY 

 Daphnids exposed in the darkroom to weak horizontal light 

 from a 50 watt Edison lamp at 50 cm. distance, with observa- 

 tions taken each hour for 30 hours, gave no evidence of a change 

 in phototaxis corresponding in any way to a daily period. Neither 

 could such a change be detected in daphnids exposed for 24 

 hours to the same weak light before beginning the series of obser- 

 vations. In no experiment is there found the least evidence for 

 believing in the existence of any sort of a rhythm in phototaxis 

 independent of the direct effect of recurrent external changes. 



GEOTAXIS IN RELATION TO LIGHT INTENSITY 

 When daphnids which have been kept some time in darkness 

 are suddenly exposed to light of any intensity coming from a 

 horizontal direction there is a tendency for them to go to the 

 bottom of the dish (Exp. 7). Conversely, when daphnids which 

 have been exposed to light of any intensity are placed in com- 

 plete darkness they show a tendency to rise to the top of the 

 dish. A tendency to positive geotaxis is also found on changing 

 from weak light to stronger light (Exp. 8), or from moderately 

 strong light to the light from an electric arc (Exp. 9). There 

 is also a tendency of this kind on changing from shade to sun- 

 light (Exp. 10). Negative geotaxis is produced by the reverse 

 change in each of these cases. 



Exp. 7 



Effect of change of light intensity on geotaxis 

 March 18, 11 A.M., 14 Daphnia pulex from 17 hours in darkness placed in a 

 14 cm. high glass jar marked off into three divisions. Cooled to 13° C. 



Top Bottom 



Divisions I II III 



11.15 A.M. 8 1 5 



11.30 9 5 



11.45 8 1 5 



11.50 5 2 7 



Turned on light of a 40 watt Edison lamp 

 at a horizontal distance of one meter. 



