62 



W. C. ALLEE. 



positive and gave no response to the current in 10 per cent, of 

 the trials. Thus the rheotactic reaction averaged 21 per cent, 

 less positive when the isopods had the lower clinging power. 

 Since this is four times the probable error it must be accepted 

 as strongly indicating that isopods with high clinging ability 

 when positively oriented, tend to give a higher percentage of 

 positive rheotactic reactions than those with low clinging power. 

 The converse data of this relationship is shown in Table III,. 

 which lists the average current strength necessary to sweep 

 isopods off their feet that had given: (a) highly positive rheo- 

 tactic reactions; (b) low positive rheotactic reactions; (c} a high 

 percentage of failures to respond to the current. In all cases 

 the only variation known was in the metabolic state of the 



isopods. 



TABLE III. 



Showing the relation between clinging ability of positively oriented isopods 

 accompanying a rheotactic reaction of 60 per cent, or more positive; 40 per cent, 

 or less positive: and 40 per cent, or more no reaction. For details of method see 

 pp. 57 and 62. 



The data presented in Table III., show that isopods having a 

 13 per cent, positive rheotactic reaction gave 23 per cent, less 

 clinging ability than those that were 90 per cent, positive in their 

 rheotactic reactions. With isopcds that gave a high degree of 

 inaction in the rheotactic tests there was a further reduction in the 

 clinging power. The change in the strength of current success- 

 fully resisted is not at all proportional to the variation in the 

 rheotactic reaction with which it is compared but the difference 

 in clinging power accompanying high and low positiveness of 

 the rheotactic reaction is 4.5 times the probable error and is 

 therefore significant. 



4 For explanation of average efficiency see footnote 3, p. 61. 



