288 A. M. Banta 



the plane. If the animal, upon turning back after it had crossed 

 over into the illuminated region, got into such a position that one 

 eye was in the dark region, its immediate return to the dark space 

 was almost certain. Meeting the plane at a sharp angle brought 

 one eye into the light, while the other was still in the dark; hence 

 the turnino; back into the dark that almost invariablvoccurred in 

 such cases. When an Asellus followed along the bounding plane 

 for a short distance before turning into either area, the head was 

 the part of the body which "found" the plane and directed the 

 animal in following it. Whatever the direction of the animal's 

 movements, they were never modified until the eye reached the 

 boujiditig plane. This was taken to indicate that the animal 

 when approaching the illuminated region was not capable of 

 distinguishing that region until the eyes were very near the plane 

 or were actually illuminated by the strong light; likewise that 

 when moving in the light toward the dark region it was equally 

 incapable of detecting that region until the eyes were quite near 

 the plane or actully carried beyond the reach of the light. The ac- 

 tions of the animal in following along the plane and in finding its 

 way back into the dark region are clearly due to the effects of 

 unsymmetrical stimulation of the two eyes. 



The general movements of the animals as a whole and the ulti- 

 mate positions taken by Asellus under such conditions will next 

 be considered. 



In Table IX are s^iven the details of an experiment with vertical 

 illumination by means of the apparatus previously (p. 280) 

 described. In this case 24 individuals (Asellus) were transferred 

 to the tank and left for one hour in diffuse daylight. The room 

 was then darkened and the artificial light (6983 CM.) turned on. 

 The observations given above (p. 287) were made during this 

 experiment. In the first column, at the left, are given the epochs 

 at which observations were made; in the two other columns are 

 given the numbers of individuals found in the illuminated 

 region and in the dark region respectively at these epochs. 



At the beginning of this experiment the number of individuals 

 in the illuminated end was very nearly equal to that in the dark 

 end. But the photokinetic effect appeared very promptly, and 



