Reactions of Isopods to Light 287 



A few of the records of observations on the actions of individual 

 Asellus at the plane of division between the light and the dark 

 areas are here transcribed to illustrate the details upon which are 

 based the general statements which precede. 



April 28, 1906. ii:io. One Asellus climbed up side of tank at the plane. Came 

 from dark and returned to dark. 



J/.'/j. An individual came from dark headed into light and immediately turned 

 about. 



Ii:2§. A large cJ* moved back and forth across the plane several times and for 

 eight minutes did not get far from the plane. It finally went into dark. 



2:i§. One individual came to the plane from light, paused and then crawled up 

 side just on the plane remaining on plane for approximately a minute, then went to 

 dark. 



2:1 §. At start it was noted that after the Asellus crossed the plane into dark they 

 often paused much longer than usual. At one time five were noted just within the 

 dark, all having heads toward dark. 



2:^2. Ono individual headed into light but soon turned back, then moved along 

 plane for sometime, moving partly into light, and after about \\ minutes turned 

 away into dark. 



2:^2. Another went into light and immediately hurried off and scarcely stopped 

 until it had made a circuit of tank and gotten into dark, where it stopped for some 

 time and then moved away very deliberately. 



2.37. One started into the light end but stopped as soon as it reached the plane, 

 and turned back in two or three seconds. 



The above were notes made hastily the first time Asellus was 

 observed under these conditions, and are given here, first, 

 because they show characteristic reactions of the animals; and, 

 secondly, because they were made before I had formed any 

 very definite conception of the movements and actions of the 

 animal under these conditions of experimentation, and when I 

 was entirely unprejudiced toward any explanation of the animal's 

 actions at the bounding plane. 



The eyes of Asellus, as might be expected, played a conspicuous 

 part in the reactions at the bounding plane. Whether the animal 

 was headed from the illuminated region toward the dark region, 

 or vice versa, the effects appeared as soon as the eyes were across 



