RHEOTAXIS IN ISOPODA 



335 



The solid lines in figure 8 give the effect of low {S2) and high 

 (Si) oxygen content upon the rheotactic reaction of juvenile stream 

 Aselli. The rheotactic reactions are too indefinite during the 

 first month to be considered. The second month shows the same 

 state of development in each environment but from that time on 

 the responses are entirely different. It is especially noteworthy 

 that the isopods kept in' a low supply of oxygen throughout their 

 lives, did not become strongly positive even after the breeding 

 season, which came in their fifth month. That is, they did not 

 show the same capacity for regulating, given by adults under the 

 same conditions. The broken lines give the reactions of isopod 



-1 — I — I — I — I — I — I — I — I — I — I — I — I — 1 — I — I — I — I — r 



— I — i — I — I I I I I I I I I I L__i I I I 1 



1 2 34 5 6 7 8 S 10 11 12 13 14 15 16 17 18 19 20 



Fig. 7 The eflfect of increased carbon dioxide and decreased oxygen upon the 

 rheotactic reaction during the breeding season. A, carbon dioxide; B, oxygen. 



of pond parentage (Pi) in high, (P2) in low oxygen supply, and 

 it is evident that their rheotactic reactions do not depend on their 

 ancestry but on their environment. 



Figure 9 compares the results of different depressing agents at 

 the concentrations used. Apparently a rapid decrease in tem- 

 perature {A) is most effective and most transitory. Carbon 

 dioxide (C) from 200 to 300 cc. per Hter seems to be second in 

 both these qualities. The isopods become slowly acclimated to 

 chloretone (B) at the strength used, but show no such tendency 

 with the potassium cyanide {D) in the concentration used. Star- 

 vation {E) also acts as a continued depressant. 



