408 



W. C. ALLEE 



varied =t 4°. The isopods were examined each morning and night. 

 As soon as one died it was removed with a long pipette. In 

 general the method is, I believe, a trifle more reliable than the 

 direct method since the exact determination of the death point is 

 of less significance. However, since the method is so much slower 

 it was used only to check the results obtained with the direct 

 method. The findings are summarized in table 5. They support, 

 as far as is possible with a limited number of cases, the findings 

 with the direct method. 



TABLE 5 



Showing the relation between survival-time in 0.00001 molecular potassium cyanide 

 solution and the rheotactic reaction in Stock II. Temperature range 14 to 22° C. ; 

 average 18° 



DISCUSSION 



It was shown in an earher paper (Allee '12) that isopods of 

 the pond mores are less positive in their rheotactic reaction than 

 those of the stream mores. This is again illustrated by these 

 studies and light is thrown on the metabolic condition of the 

 isopods giving this difference in response. Thus the rheotactic 

 reactions of 72 isopods of the stream mores (Stocks I and II), 

 taken partially during the lowered responses of the breeding 

 season, averaged 61 per cent positive, 28 per cent negative, and 11 

 per cent indefinite, with an average efficiency in the current of 

 2.5. These gave a mean survival-time in 0.0002 molecular potas- 

 sium cyanide of five hours and fifty-nine minutes. 



Of the pond mores 51 were killed in 0.0002 molecular cyanide 

 solution with an average survival-time of twenty-three hours and 



