262 



W. C. ALLEE 



In indefinite reactions the efficiency was estimated as closely 

 as possible from the varying course taken by the isopod. 



The basis upon which the relation between positive and nega- 

 tive reactions was worked out is shown in table 2. In this 

 work one isopod was used at a time. One person held the watch 

 and recorded observations while another with the aid of a read- 

 ing glass counted the number of movements of the fourth 

 thoracic leg during a minute's reaction period and afterward 

 measured the distance covered in that time. Distance covered 

 per leg movement with the isopod going with the current was 

 found to be approximately double that given when the isopod 

 was reacting positively. 



In testing for efficiency of response in a water current the 

 rate of the current becomes of much greater importance than it 

 is in finding the sign of the rheotactic reaction. Therefore even 



TABLE 2 



