Reactions of I so pods to Light 275 



tinued to be rather conspicuous for about 2\ hours. The records 

 set down in Table VIII were not made at sufficiently frequent 

 intervals to show well the random movements due to this influence. 

 As these movements became less general and less vigorous, the 

 animals were found to be more and more in the negative parts 

 of the tank. 



By 10:45, 2 hours and 50 minutes after the experiment started, 

 most of the animals were fairly settled and 18 out of 20 were in 

 the negative half of the tank. This represented a change in mean 

 average position of 1.88 in a negative direction. Possibly further 

 change in this direction would have occurred if the experiment had 

 been continued without modification, but at this time the light 

 was changed to the other end of the tank. Having already been 

 exposed to a high intensity for about 3 hours, the response to this 

 change was not very prompt, and the animals did not shift as 

 near to the new negative end as might have been expected. How- 

 ever, the change in mean average position was very decided in 

 the 6| hours during which the experiment was continued, being 

 2.10 in a negative direction. 



From these experiments, which are typical of the series, it is 

 evident that Caecidotea responds negatively to intensities from 

 80 to 2855 CM. In general the maximum negative response was 

 reached in approximately 2 to 3 hours, but there was consider- 

 able variation in the length of this period depending somewhat 

 upon the intensity of illumination and also upon the length of 

 time the animals had been in the tank before the experiment 

 began. After once responding to horizontal illumination, and 

 becoming fairly well settled in the negative end of the tank, the 

 animals did not move toward the positive end again to any marked 

 extent, i.e., they did not become less negative. 



Experiments were made with intensities from less than i CM., 

 (i c.p. incandescent at 1.3 m. from middle of tank) to 80 CM. 

 It was shown conclusively that Caecidotea is never positive in its 

 response to horizontal illumination; but whether or not it is more 

 responsive following retention in darkness or whether it then 



THE JOURNAL OF EXPERIMENTAL ZOOLOGY, VOL. 8, NO. 3 



