278 A. M. Banta 



when subjected to an 80 CM. or a greater intensity, and occur- 

 red almost without exception in all such experiments. The re- 

 sponse was noticeably more prompt and more vigorous in animals 

 at the end nearer the lamp than in those farther from the source of 

 light. The activity, as stated before, took the form of random 

 movements. There was no evidence of a selection by the individ- 

 ual in the direction of these movements with resulting orienta- 

 tion such as occurs in the random movements discussed by Holmes 



The ultimate positions in which the animals settled were the 

 result of a certain degree of attunement to a definite intensity 

 of light, which rendered them less subject to the photokinetic 

 influence, so that when their random movements carried them 

 into the negative end they were less and less affected by the inten- 

 sity of light prevailing there and finally became so attuned that 

 the intensity at the negative end of the tank did not affect them 

 in a photokinetic way. Hence the settling at the negative end 

 was the result of photokinesis. 



Caecidotea, although for some time very active, sooner or later 

 became acclimated and came to rest under any intensity of 

 illumination. In the experiment recorded in Table VHI the 

 animals after appearing to respond in a negative photo tactic way, 

 became in the course of 2 hours and 50 minutes so acclimated 

 to the existing conditions of illumination that w^hen the light 

 was transferred to the opposite end of the tank, they responded 

 much less definitely than at the beginning of the experi- 

 ment. Before the experiment began they had been subjected to 

 diffuse daylight; when settled in the negative end of the tank 

 they were acclimated to two-fifths the intensity to which they 

 were subjected after the light was transferred to the opposite end 

 of the tank. These and other similar experiments afford good 

 evidence of acclimatization to light and seem to support the 

 statement that the responses of Caecidotea to horizontal illumina- 

 tion are due to photokinesis and not to phototaxis. 



From the foregoing experiments with Caecidotea when sub- 

 jected to horizontal illumination the following conclusions are 

 drawn : 



