MOVEMENTS OF DAPHNIA 



233 



Exp. 4 



Phototaxis in arc light 



January 29, 18 Daphnia pulex which had been exposed to the light of a 100 watt 

 Mazda lamp for four hours were placed in a glass dish 28 cm long marked off into 

 five divisions. Exposed them to the horizontal light from a 15 ampere electric 

 arc at a distance of 40 cm. The light was passed through 8 cm. of water to cut 

 out the heat. 



Averages 



6.3 



1.2 



2.6 



1.9 



6.0 



Exp. 5 



Phototaxis in sunlight 



April 13, 2 P.M., 13 Daphnia pulex from laboratory aquarium placed in 18 cm- 

 long glass dish marked off into five divisions. Exposed to sunlight with ultra, 

 violet rays cut off by one sheet of window glass. Temperature kept at 21° to 22° C. 



Averages 



3.7 



2.0 



2.0 



1.7 



3.6 



In these experiments it is shown that at temperatures of 

 from 17° to 22° C. these daphnids are normally positive to weak 

 electric light or to moderately strong light. They are more 

 strongly positive to moderately strong light than to very weak 

 light. The positive phototaxis to weak light persists for seem- 

 ingly indefinite periods. To light intensities stronger than weak 

 diffuse daylight these animals are indifferent and do not become 



