Reactions of I so pods to Light 299 



the tank in the dark region so as to reenter the illuminated region, 

 then the animals remained in the dark region. 

 The foregoing experiments indicate: 



1 That under the conditions described, Caecidotea was little 

 responsive, either in increased activity or in collecting within the 

 dark area, when exposed to intensities of 97 CM. and 72 CM.; 

 but 



2 That with an intensity of 6983 CM., a very decided photo- 

 kinetic effect was produced; 



3 That this photokinetic influence caused most of the ani- 

 mals to become active in from 30 seconds to about ten minutes 

 after they were exposed to the light; 



4 That thus the activity was greater in the illuminated region; 

 less pronounced in the dark region; 



5 That sometimes Caecidotea, like Asellus, apparently reacted 

 at once to the sudden transition in illumination between the dark 

 and the illuminated regions; 



6 That this reaction sometimes appeared as a sudden turning 

 back into the dark region after the stopping at the plane between 

 the dark and the illuminated regions, when headed towards the 

 illuminated part. 



7 The influence of the change in illumination was also shown 

 by the fact that Cxcidotea when headed toward the dark region, 

 did not often turn back into the illuminated region af^er stopping 

 at the bounding plane; whereas, when headed in the opposite di- 

 rection such turnings mto the dark region were not rare. 



8 In addition to the general photokinetic efi^ects upon Caecidotea 

 the vertical illumination of 6983 CM. intensity caused the 

 animals ultimately to collect and remain within the dark region. 



9 The majority of the animals began to appear in the dark 

 portion of the tank within 17 minutes after the beginning of the 

 experiment. 



10 Pract-cally the maximum number was collected in the 

 dark region within about i| hours, and when once settled there 

 thev remained in that region. 



11 Certain reactions of Caecidotea at the plane of division be- 

 tween the dark and the illuminated regions are apparently due to 



