334 



W. C. ALLEE 



The unbroken line (A) in figure 6 gives the effect of a decreased 

 oxygen supply upon adult stream Aselli. The response consists 

 of four periods. First there is the lessening of the positive re- 

 sponse which is followed by partial acclimatization. This is 

 followed bj' the breeding season after which the isopods are either 

 more vigorous or else are more capablp of becoming acclimated 

 than before and so give a stronger positive response. The broken 

 lines (B) show the effect produced by keeping the pond Aselli in 

 tap water, the steeper curve being plotted for younger animals. 

 In neither case is there any sign of acclimatization. 



Fig. 6 Adult isopods in reversed oxygen supply. A, Adult stream; B, Adult 

 pond isopods. 



The broken line {B) in figure 7 gives the results produced by a 

 low oxygen supplj^, upon the rheotactic response during the breed- 

 ing season. In general there is first an increase of the positive 

 response, followed by a decided reversal. The unbroken line {A) 

 shows the effect of an increased carbon dioxide supply The 

 effect of the decreased oxygen is probably due to its action first, 

 upon the germinal glands, causing a cessation of their activity, 

 which gives more energy for other activities. This is followed 

 by the direct depressing effect of the low oxygen upon the tissues 

 themselves. The results so far obtained with carbon dioxide 

 may be explained by its general narcotic action. 



