RHEOTAXIS IN ISOPODA 



291 



rest of the year although it is somewhat less positive. That is, 

 the depression found in the stream Aselli during the breeding 

 season is not so marked in the pond isopods. 



The external condition with which the low positive response 

 seems to be correlated is the low oxygen content of the water. 



17 18 19 20 



Fig. 3 Twenty successive rheotactic reactions of one pond Asellus 



Thus the highest oxygen content of the pond is a little below the 

 lowest found in the stream during a period of normal response, 

 while the normal amount of oxygen found in the pond water is 

 much below that of the stream. The amount of free carbon diox- 

 ide present in the pond water is higher than in the stream, but 



