RHEOTAXIS IN ISOPODA - 321 



starting suddenly when no apparent stimulus was acting. Even 

 when the rate of positive response was already high, as shown in 

 the second part of the table, increasing the amount of oxygen 

 present caused a large increase in the positive reaction. The 

 third part of the table 22 shows the effect of allowing the oxygen 

 in the water to escape gradually, and under these conditions the 

 animals returned to a normal pond response while the amount of 

 oxygen present was still much higher than that in their normal 

 habitat. However it was very low in comparison with the 

 amount of oxygen to which they had been previously exposed. 



(2) Juvenile mores. Table 23 gives the effect of keeping juven- 

 ile pond Aselli in high oxygen for long periods of time. Other 

 tests run for shorter times, give similar results but with less increase 

 in positiveness. The right hand side of the table gives averages 

 from the response during normal development and is placed here 

 for comparison. Two things are brought out by the table. First, 

 the pond isopods kept in a large amount of oxygen, develop not as 

 normal pond mores but as stream Aselli, and second, that the time 

 taken to acquire a given size is less when a larger amount of oxy- 

 gen is present. The final results are especially interesting. At 

 the age of 123 days the pond Aselli in high oxygen were 6 mm. long 

 and gave a 72 per cent response. It took those in low oxygen 

 193 days to attain the same size and then they gave only a 38 

 per cent positive response. There is no evidence of a return to 

 the response normal for pond isopods. 



(3) Res'ponse in straight current. Again the tests with the 

 continuous straight current support the results with the dis- 

 continuous circular one. The results of these trials are sum- 

 marized in table 24. Isopods from all three methods of furnish- 

 ing increased oxygen were used with similar results, and one is 

 forced to the conclusion that increasing the amount of oxygen 

 present in the water makes juvenile pond Aselli give a positive 

 rheotactic response comparable with that given normally by 

 stream isopods. 



