296 A. M. Banta 



boundary plane; only 172 such pauses occurred at the arbi- 

 trary plane. These results, while not very decided, still indicate 

 that Caecidotea is sometimes affected by the abrupt change in 

 the intensity of illumination at the plane of separation between 

 the light and dark regions. 



The general movements of Cascidotea, when experimented on 

 in numbers, under the conditions already described (one half of 

 the tank being dark, the other half being under vertical illumina- 

 tion of 6983 CM.) were fairly definite and no lengthy series of 

 experiments was necessary to demonstrate them. In addition to 

 the general photokinetic effect, which incited the animals to move- 

 ment and kept those in the illuminated region more active than 

 those in the dark one, there existed a very pronounced tendency 

 for the animals to congregate in the dark region. 



Table XI gives the results of a characteristic experiment of the 

 series. In this experiment 18 Caecidotea were placed in the tank 

 already described and under the same conditions of vertical illum- 

 ination (6983 CM. intensity) as were employed with Asellus 

 (p. 288). These animals had been in the tank in diffuse daylight 

 for i\ hours before the records began, but just before the light was 

 turned on they were all driven into the region that was about to be 

 illuminated. Naturally, if all the conditions were the same in 

 both halves of the tank, one would expect the animals to distri- 

 bute themselves equally in the two. But when, the other condi- 

 tions remaining the same, the light conditions are different in the 

 two halves, any marked difference in distribution is clearly attrib- 

 utable to reaction to light. 



In this experiment, starting with practically all the animals in 

 the illuminated region, an equality of distribution had become 

 established in the course of 16 minutes. But this equality did 

 not persist; after a few fluctuations on either side of equahty dur- 

 ing the next period of about thirty minutes, the number in the 

 dark region became permanently greater than that in the illumi- 

 nated part, and although there were some fluctuations, the tend- 

 ency was toward a constant increase of numbers in the dark re- 

 gion, which finally reached a maximum in about two and a half 

 hours. Although this result was accomplished rather slowly, it 



