302 A. M. Banta 



be shifted occasionally in order to keep the sun's rays approxi- 

 mately atrightangles to the long axisof the tank. This mechanical 

 disturbance, while exceedingly undesirable, was non-directive in 

 its effects. It did, however, tend to increase the activity of the 

 animals and thus indirectly increased the average percentage in 

 the illuminated region, rather than that in the dark region. Fur- 

 thermore, since the shifting did notoccuroftener than every 15 min- 

 utes, and since within fifteen minutes after the experiment started 

 practically the maximum response had occurred, the results 

 were already virtually attained before any shifting was necessary. 

 In these experiments the temperature of the water rose rapidly, 

 but since the water used was 3 cm. in depth and the sun's rays en- 

 tered it somewhat obliquely, they passed through a thickness of 

 more than 3 cm. of water. Hence it is safe to say that heat was 

 not the effective stimulus in these experiments. 



J. J sell us 



The following table (XIII) shows the results of one of these 

 experiments, in which 43 Asellus communis were used. After 

 obervations at intervals of sixty seconds for about half an hour, 

 at 1 1 :oi the illuminated end of the tank was qu'ckly darkened and 

 the one previously dark was suddenly illuminated. At 11:30 a 

 return to the initial illumination was made. 



This table shows that Asellus very promptly avoids direct sun- 

 light under the conditions of these experiments. The animals in 

 the illuminated region showed great activity. This appeared very 

 promptly with most individuals, nearly all beginning to move 

 within one minute after the sunlight was allowed to reach them. 

 This soon resulted in bringing all the individuals into the dark 

 area. In no case did it require more than about five minutes for 

 nearly all the animals to find their way into the dark region; 

 once there, very few came back at all. In most cases those which 

 did come back remained in the sunlight for only a minute or even 

 less. After each disturbance caused by shifting the tank to 

 compensate for the earth's motion, several usually came out into 

 the illuminated region; but they remained in the light only a 



