Reactions of Isopods to Light 25 1 



to be experimented with were placed within a glass ring (17 cm. 

 in diameter, located in the center of the inner tank) and allowed 

 to settle there. This insured the settling of the animals in the 

 beginning of the experiment in a neutral position, and facilitated 

 the interpretation of their movements when subjected to light 

 stimulation. 



When all the conditions were favorable for the experiment, the 

 glass ring was carefully lifted. It often happened that numbers of 

 Asellus gathered in bunches at one edge or in one corner of the 

 tank. The individuals were then very slow to leave the bunch, 

 even under intense light stimulation, so that in such cases definite 

 reactions were much delayed and sometimes did not appear at all. 

 Bunches were less often formed when the animals were confined 

 within the glass ring than when left free. Moreover, in removing 

 the ring any aggregation formed at the angle between it and the 

 floor of the tank was somewhat disturbed mechanically and the 

 individuals composing the bunch were more quickly scattered 

 than when the ring was not used. This mechanical stimulation 

 lasted only a second and was wholly non-directive; consequently 

 it in no way interferred with the influence of the light. The main 

 advantage of the ring, however, was due to the retention of the 

 animals m the middle of the tank, so that when they were sub- 

 jected to light stimulation their movements were readily inter- 

 preted. 



In the light experiments, as in all other experiments with Asellus 

 and Caecidotea, the same conditions were observed for both species, 

 the two forms being studied one after the other in quick succession. 



The relative inactivity and lack of responsiveness to light made 

 it desirable to use a considerable number of individuals in each 

 experiment. Although the numbers employed varied from 12 to 

 40, the most desirable number was found to be from 20 to 25. 

 Because of the tendency of Asellus to collect in groups, and because 

 of the thigmotactic responses of the species upon contact with 

 one another, a great number of individuals were less responsive 

 to light, and therefore unfavorable for experimentation. In the 

 case of Caecidotea, too, a larger number than 25 proved to be un- 

 desirable, as these animals are likewise very responsive to contact 



