Reactions of Isopods 471 



response persisted in most cases as long as the experiment lasted, 

 and in one instance it lasted three days. 



In some experiments the animals appeared altogether indif- 

 ferent to the current and either moved about freely or tended to 

 remain in the lower half of the trough. Two such results (experi- 

 ments Nos. 2 and 6) may have been due to poor conditions of 

 the animals, which had only 2 or 3 days before arrived from the 

 caves of Indiana. Vigorous animals nearly always gave a defi- 

 nite and persistent response. A record of some of the experi- 

 ments of this series is given in Table IX. 



As compared with Asellus, Caecidotea does not respond to the 

 influence of a current so quickly; the maximum response with the 

 former appears on an average in about 1.4 hours after the experi- 

 ment starts, with the latter in about 2.1 hours. But the response 

 of Caecidotea continues longer than that of Asellus and usually 

 lasts indefinitely. 



In the experiments combining the effects of light and currents 

 of water, where the upper end of the trough was darkened and the 

 lower end subjected to strong illumination, both Asellus and Caeci- 

 dotea collected pretty generally in the upper darkened end. With 

 Asellus this response reached its maximum in about three-quarters 

 of an hour; with Caecidotea in about an hour. After about an 

 hour Asellus seemed to remain less generally in the dark end than 

 before, while with Caecidotea the reaction usually persisted without 

 much diminution as long as the experiment was continued. 



When the lower end of the trough was darkened and the upper 

 strongly illuminated both Asellus and Caecidotea showed tenden- 

 cies to enter the upper end in spite of the light. With Asellus 

 this tendency was very pronounced for a time, but the animals 

 became so active on entering the light area that they were often 

 carried off their feet and swept back into the lower end again. If 

 Asellus was not swept back into the lower end it crawled back 

 after a time and the rheotactic stimulus finally failed to bring 

 it again into the upper end. Caecidotea for a time entered and 

 moved about in the upper light end to some extent. This per- 

 sisted for various lenghts of time so that on an average several 



