276 A. M. Banta 



responds to lower intensities than when previously in light, was not 

 directly determined. There is, however, convincing evidence 

 that after exposure to strong light for some time Caecidotea is 

 less responsive to light. This was shown in the experiment de- 

 tailed in Table VIII, in which at the beginning of the experiment 

 a definite negative response, occurred in 2 hours and 50 minutes, 

 three-fourths of the animals under observation having in that time 

 collected and become settled in the extreme negative end of the 

 tank. But when the light was then changed to the other end of 

 the tank, there resulted a less definite response even after a much 

 longer time (6| hours) of exposure. The mean average position 

 was changed a great deal, to be sure, but part of that change 

 would have occurred as the result of the normal non-directive 

 movements of the animals. No very pronounced tendency to 

 collect in the extreme negative end appeared in the course of the 

 entire 6| hours. Hence it seems evident that these animals after 

 exposure to 2855 CM. for 2 hours and 50 minutes were less respon- 

 sive to the influence of directive light. 



It seems very probable that with Caecidotea there are not two 

 factors (i.e., phototactic and photokinetic) in the response to 

 horizontal illumination, as there clearly are with Asellus, for 

 with Caecidotea the whole reaction seemed clearly attributable to 

 photokinesis. Conviction that such was the case led to a par- 

 tial test of the matter, as the result of which it may be said that 

 no direct phototactic reactions were observed with Caecidotea. The 

 responses were clearly and purely photokinetic, the animals starting 

 up in any direction in which they happened to be turned. Defi- 

 nite orientation or direct movements in a negative direction did 

 not occur at all. It was different with Asellus, in which definite 

 orientation and direct responses on the part of individuals very 

 generally occurred. 



In order to determine if the difference in luminosity between the 

 two ends of the tank was sufficient to account for the collecting 

 of Caecidotea in the end farthest from the light under the influ- 

 ence of photokinesis alone, tests were made to determine the 

 actual luminosity in various parts of the tank. The following 

 method was employed. A six-glower Nernst lamp of 772 c.p. 



