276 W. C. ALLEE 



tions gave identical responses 11 times in 33 trials. These 

 are typical of the responses of the highly positive isopods. 



Some of the isopods show greater variation, typically repre- 

 sented by no. VII. This isopod had as strong tendency to give 

 negative as positive reactions (table 7). The positive response 

 varied greatly from day to day often without a corresponding 

 variation in the efficiency of the reaction, although this latter 

 item varied more than in the more uniformly positive isopods 

 cited above. 



Attention should be called to the facts illustrated in table 7. 

 This table shows that those stream isopods which naturally give 

 a low percentage of positive responses give a higher percentage 

 of negative than of indefinite reactions. That is, if a stream 

 isopod does not go positive to the current under normal condi- 

 tions it has a distinct tendency to go more negative than indefi- 

 nite. Exactly the reverse is true of pond isopods. ^ 



Another state of affairs is illustrated by the reactions of isopod 

 no. V, table 7. This individual was injured in handling on the 

 sixth day of observation. Trials were continued until its death 

 28 days later. In this case, however, although the positive 

 response shows the decided cut that would be expected, the 

 indefinite, not the negative response increased. Under these con- 

 ditions the efficiency was also markedly decreased, bearing out 

 the experimental results of a high degree of indefiniteness accom- 

 panying low efficiency in stream isopods, however the low effi- 

 ciency be induced. 



The cases of the isopods (nos. VII, XII, XV and XIX) that 

 gave an unusually high negative response for the stream mores 

 have been discussed at some length (p. 272) and will be summed 

 up later (p. 280). In general the table shows that there is a 

 relation between the positiveness of the rheotactic reaction and 

 the rate of efficiency and there is slight evidence of a relationship 

 between these two rheotactic factors and the concussion response. 



^ Of 827 trials of normal pond isopods ('12, table 6, p. 290) 29 per cent of the 

 reactions were positive, 30 per cent were negative and 41 per cent were indefinite. 

 This shows the high percentage of indefiniteness in the typical rheotactic reac- 

 tion of pond isopods. 



