Reactions of I so pods to Light 293 



Nernst at 50 cm. from surface of water) . In the first case 12 ani- 

 mals were used and the experiment was continued for 3 hours. In 

 the second 24 animals were used for 2^ hours. Little or no photo- 

 kinetic efFectappeared in either case, and theanimalsdidnotexhibit 

 a tendency to remain in the dark region rather than in the light 

 one. When two days later, the same animals were exposed to 

 6983 CM. intensity, very definite reactions were obtained. It 

 is therefore safe to conclude that Caecidotea is little responsive 

 to vertical illumination of an intensity of 172 CM. or less. 



As with Asellus, vertical illumination of 6983 CM. intensity 

 produced a decided photokinetic effect upon Caecidotea. 



Random movements due to recent transference to the tank like- 

 wise appeared in the experiments with Caecidotea. But such 

 movements are of only passing interest here, for they merely 

 resulted in a delay in the ultimate settling of the animals. During 

 the period of these random movements the activity was greater 

 in the illuminated than in the dark area. This difference in activ- 

 ity in the two areas became more evident as time passed, the in- 

 fluence due to recent transference to the tank presumably becom- 

 ing less strong. 



The photokinetic effect was less marked and less prompt in its 

 appearance with Caecidotea than with Asellus, but there was no 

 mistaking its existence. However, in the case of Caecidotea, when 

 it had once become settled in the tank, the response to this influ- 

 ence was quick compared with the usual tardy response to light. 

 With many individuals the response came in the course of a min- 

 ute or two, and the majority of the individuals were stimulated to 

 begin moving within ten minutes. 



Fromgeneral preliminary observations it seemed that Caecidotea 

 sometimes reacted in a direct and definite manner to the change 

 in illumination at the plane of division between the illuminated 

 and the dark regions. In a general way these reactions were like 

 those of Asellus under like conditions, but they occurred much 

 less often, so that it was hard to decide whether there really were 

 definite reactions, and whether the observed actions at the bound- 

 ing plane were not due to other causes than the change in illumina- 

 tion. To test these supposed reactions of Caecidotea an arbitrary 



