RHEOTAXIS, RESISTANCE TO POTASSIUM CYANIDE 405 



percentage, show in each case that the lower rate of positive rheo- 

 taxis accompanies a relatively long survival- time in the cyanide. 

 That is, the isopods giving a low positive rheotactic reaction have 

 on the average a slower rate of metabolism than do animals 

 giving a higher percentage of positive responses. We are dealing 

 here with average results only; individual results will be considered 

 later (p. 410). 



The position in the metabolic gradient of isopods that give a 

 high percentage of negative reactions has been discussed in an 

 earlier paper (Allee '13). From the evidence then at hand it was 

 decided that under these conditions isopods occupied a place 

 midway between that which accompanies a high degree of posi- 

 tiveness and a high degree of indefiniteness. That this conclusion 

 is true is indicated by the results given in table 2, where averaged 

 responses of 52 individuals from three sets of experiments have a 

 survival-time intermediate between that given with a high per- 

 centage and that with a low percentage of positive responses. 



In all the individuals tested in these experiments the negative 

 rather than the indefinite response increased as the positive 

 response lessened. For this reason there were too few instances 

 where the isopods gave a majority of indefinite reactions to make 

 their inclusion in the table of any value. However, the position 

 upon the metabolic gradient of animals giving a high percentage 

 of indefinite responses is firmly established by the earlier experi- 

 ments (Allee '12, '13). These clearly show that the indefinite 

 reaction accompanies a rate of metabolism lower than either of 

 the other responses. 



The survival-time of animals showing a high percentage of 

 inaction in the current varies greatly. At times the survival- 

 time is much longer than when movement occurs but on the 

 average it is shorter than that of highly positive isopods. This is 

 not surprising if one remembers that there is no rheotactic method 

 for measuring the metabolic activity of the isopods that give no 

 reaction to the current. They may be just lower than the lowest 

 rate that would result in an indefinite reaction, or they may not 

 be strong enough to turn over on their feet after the current has 

 turned them- on their backs. It is the first type that gives the 



