MOVEMENTS OF DAPHNIA 231 



The experiments were performed between August, 1913 and 

 April, 1914 in the Zoological Laboratories of the University 

 of California at Berkeley. 



PART I. EXPERIMENTAL 

 PHOTOTAXIS IN RELATION TO LIGHT INTENSITY 



The individuals of Daphnia pulex are normally positively 

 phototactic and under conditions similar to those they would 

 meet in nature it is difficult to obtain from them a negative 

 response. In weak light or moderately strong light most of 

 them are strongly positive. To the light from a 100 watt 

 Mazda lamp at a distance of 30 centimeters they remained 

 strongly positive throughout an exposure of nearly six hours. 

 (Exp. 1.) 



Exp. 1 



Phototaxis in weak light 



January 29, 8.30 A.M., 18 Daphnia pulex from diffuse daylight placed in a glass 

 dish 28 cm. long marked off into five transverse divisions. Exposed in darkroom 

 to horizontal light from 100 watt Mazda lamp at 30 cm. Temperature 17.5° C. 



Averages 14.2 1.4 0.4 1.2 0.8 



Another experiment (Exp. 2) shows that to the weak light 

 from a 50 watt Edison lamp at 50 centimeters the animals 

 remain positive during a continuous exposure of 60 hours. 

 Probably they would remain positive indefinitely to these in- 

 tensities. It appears that they are more strongly positive in 

 the stronger light. 



* The figures refer to the number of individuals in each division. 



