MOVEMENTS OF DAPHNIA 



241 



Proof that the reversal of geotaxis by changes in light in- 

 tensity is independent of phototaxis is given in Experiment 11. 

 Here the daphnids remained at the bottom of the dish when 

 exposed to the light from an arc light, no matter whether the 

 light was falling from the side or at an angle of 45 degrees from 

 above or below. When exposed to weak light following the 

 exposure to the arclight the daphnids showed a strong upward 

 tendency indicating that the effect on geotaxis in this case was 

 undoubtedly due to the light intensity. 



Experiment 12 is presented to show that the vertical dis- 

 tribution remains altered for a long time after reduction of 

 light intensity. 



Exp. 12 



Persistence oj negative geotaxis in darkness 



March 28, 8.30 A.M., 10 Daphnia pitlex from laboratory aquarium at a tempera- 

 ture of 18.5° C. placed in a 29 cm. high jar marked off into five divisions. Exposed 

 to diffuse light of laboratory. Cooled to 13° C. 



We have shown that in Daphnia pulex increase of light in- 

 tensity causes a tendency toward positive geotaxis, while decrease 

 of intensity causes a tendency toward negative geotaxis. This 

 tendency seems to be stronger the greater the change in in- 

 tensity. It seems also that these tendencies are persistent for 

 a considerable length of time. 



