Reactions of Isopods to Light 26 1 



quickly and a very general and consistent migration toward the 

 negative end continued for about seven minutes, when the 

 photokinetic effect became apparent in the turning back of many 

 individuals upon reaching the negative end of the tank. At 

 3:29 p.m., more seemed to be moving toward the positive than 

 toward the negative end. These movements were clearly due 

 to photokinesis, for the animals had by this time been in the tank 

 long enough to have become thoroughly adjusted to it. At 3:40, 

 22 minutes after the position of the light was changed, consider- 

 able wandering about in the tank was noticeable, but 51 minutes 

 after changing the light, at 4:09, the animals seemed pretty well 

 confined to the negative end, though wandering about there to 

 some extent. After 4:27 the movements were slight. During 

 the \\ hours following the reversal of the direction of the light, 

 the mean average position changed from 1.91 to 5.45, representing 

 an average movement of 3.54 in a negative direction. 



The above experiment is typical for the reactions of Asellus. 

 The phototactic influence is often slower in asserting itself than 

 in the experiment here recorded, but the other influences appear 

 in this experiment in a characteristic way. A number ofexperi- 

 ments with the same intensity yielded similar results, bearingout 

 the conclusion that Asellus is decidedly negative to such an in- 

 tensity after previously being in diff"use daylight. 



The results of another experiment with Asellus following ex- 

 posure to diffuse daylight are given in Table III, the intensity 

 of the light in this case being only 3 CM. (19 c.p. incandescent 

 at 2.75 m. from middle of tank.) 



As these animals had been in the tank in diffuse daylight within 

 the glass ring for 55 minutes before the experiment began, they 

 seemed fairly well settled and were near the center of the tank. 

 While they were not very active at the time the ring was removed, 

 yet, with freedom to move in any direction, they responded very 

 promptly to the directive light. The response was so prompt and 

 the movements so general that at first accurate counts could not 

 be made. A part of this movement was due to photokinesis, 

 rather than phototaxis, for the number and position of those in 

 the positive end varied considerably. Whereas two or three min- 



