1 68 ^Journal of Comparative Neurology and Psychology. 



Owing to the deflection to the left there were but very few colonies 

 within 2 cm. of the right end of the field immediately after they 

 had crossed the aquarium; but a few minutes later it was clearly 

 seen that a large majority were swimming toward the right along 

 the glass wall. In this movement, some followed the wall closely 

 but most of them made a zigzag course coming in contact with the 

 wall at short intervals. This zigzag course seems to have been the 

 result of the interaction between contact and light stimuli. In 

 about 15 minutes most of the colonies collected within 5 cm. of 

 the brightest end of the aquarium. At first they were closely 

 packed together near the wall, but after a short time they began 

 to spread out in the form of a right angled triangle, the perpen- 

 dicular of which coincided with the end of the aquarium. Some 

 entered the dark region near the end of the aquarium and thus no 

 longer stimulated by light wandered back from the side of the 

 aquarium facing the light, others left this side without entering 

 the dark region. These evidently became acclimatized or nega- 

 tive after exposure for some little time. Thus they continued to 

 move back and forth, gradually spreading out toward the darker 

 end of the aquarium, until finally they began to become less nu- 

 merous along the bright border of the field of light, the region of 

 highest intensity. Then the whole aggregation appeared to work 

 itself very slowly into the regions of lower light intensity, gradually 

 spreading back from the side of the aquarium facing the source of 

 light; thus at the close of the experiment, five hours after it was 

 beo[un, most of the colonies were within ^ cm. of the darker end 

 of the aquarium. Here they were scattered over a triangular 

 area which extended from the side of the aquarium nearest the 

 source of light almost to the opposite side. The light intensity 

 within the limits of this area varied from zero at the left tc 47 ± 

 candle meters at the right. The organisms were, however, most 

 numerous in the portions most strongly illuminated. The limits 

 of the area which contained most of the colonies were, in every 

 instance, very indefinite. There was always quite a number 

 scattered about in other parts of the aquarium. 



This experiment was entirely, or in part, repeated seven times 

 and the reactions and results in each repetition were in general like 

 those described above. The optimum intensity, as was to be 

 expected, varied greatly, as did also the time it required the colo- 

 nies to reach the optimum. Thus on August 9 it required only 



