280 



W. C. ALLEE 



but later, as the animals became more affected, this response 

 also decreased to zero some time before death occurred. 



As the positive rheotactic movement decreases either the nega- 

 tive or indefinite reactions must increase. Under normal con- 

 ditions it has been shown that in stream isopods the negative 

 rather than the indefinite reaction tends to be given in cases of 

 low positive response. But little evidence was forthcoming con- 

 cerning the relative metabolic condition of the animal giving 

 highly positive or highly negative responses. 



Under experimental conditions when the positive response is 

 cut by depressing agents some evidence is available and is given 

 in table 8. The high positive response may give way directly 

 to a state of high indefiniteness as in the first, sixth, seventh 

 and ninth instances cited in the table. But when the negative 

 response is increased it stands intermediate between the two and 

 when it is given under these conditions the efficiency has always 

 decreased. Later with further decrease in efficiency the reac- 

 tion becomes largely indefinite. The tenth item shows that this 



TABLE 8 



Showing the relative effect on the sign of the rheotactic reaction when the metabolic 

 rate is known to have been reduced enough to cause an intermediate rate of 

 efficiency 



1 By 'intermediate' is meant a rate between that given when the isopod is 

 highly positive and when it is almost completely indefinite. 

 " In this instance molting resulted in death. 



