272 W. C. ALLEE 



The responses accompanying low negativeness and low indefi- 

 niteness may be seen in the first parts of table 4. Of necessity 

 where either response is above 60 per cent of the total number 

 of reactions the other two must be low. From table 6 it is evi- 

 dent that a high percentage of negative reactions may accom- 

 pany a high rate of efficiency. But this condition was found 

 only fifty times in the course of these experiments while the 

 opposite response of high positiveness and high efficiency 

 occurred 357 times as shown in table 4. For these 357 times 

 the positive responses averaged 91 per cent, and the negative 

 ones 4 per cent of the total response. From this it follows that 

 under natural conditions the negative response does not vary 

 with the efficiency or vigor of movement as does the positive 

 response. The relationship may be stated as follows: 



The rate of positiveness is usually high when the efficiency is 

 high and is always low when the efficiency is low. Normally 

 the negative response is low in the stream mores. It is always 

 low when the efficiency is very low and is usually low when the 

 efficiency is high. However it is never high unless the efficiency 

 is fairly high and in this alone it agrees with the positive re- 

 action. 



On the other hand the indefinite response is rarely given with 

 high efficiency and is usually given when the rate of efficiency is 

 low. It is not high in stream isopods under normal conditions 

 but always occurs when the metabolic rate of the isopod has 

 been experimentally decreased. 



From the data presented so far the response to shock has 

 apparently no direct relation to any of the other factors. When 

 the positiveness and efficiency of the rheotactic reactions that 

 accompany concussion responses with a vigor of 1.2 or more are 

 compared with those accompanying 0.8 or less, the following 

 average results were obtained: 160 trials with the shock reaction 

 1.2 or more averaged 1.52. The average positive response was 

 71 per cent and the average efficiency, 2.6. Ninety-four trials 

 with the shock response 0.8 or less averaged 0.46. The accom- 

 panying rheotactic reactions averaged 59 per cent with an average 

 efficiency of 2.1. Eight of the twenty isopods gave as strong or 



