RHEOTAXIS IN ISOPODA 267 



and depressed animals and since all were tested under uniform 

 conditions, this source of error is probably not vital as far as 

 the larger variations in physiological condition are concerned. 



Two average vigors of response are given in table 3. The 

 first (A) is based on the principle that an animal may respond 

 to a given stimulus by remaining quiescent, when its reaction 

 must be expressed numerically as zero; that this reaction is as 

 significant as when the most violent movement occurs and should 

 be included in a scheme of averages. The second (B) follows 

 the reasoning that only movements should be counted as reac- 

 tions. This system was followed by Banta in his work with 

 concussion. The fallacy of the reasoning may be seen by com- 

 paring the reactions listed in columns 4, 7 and 8. According to 

 the second system of averaging all these would be considered 

 as equivalent while a glance at the responses in detail shows 

 that this is not the case. Consequently the first method has 

 been taken in this work to summarize the reactions into one 

 significant figure. 



INTERRELATIONS BETWEEN RESPONSE TO UNIFORM CONCUSSIONS 

 AND THE SIGN AND EFFICIENCY OF RHEOTAXIS 



As the percentage of positive rheotactic responses varied from 

 day to day, did the efficiency of movement and the reaction to 

 shock vary in like or opposite manner? If all are the direct 

 expression of the physiological state of the animal the variations 

 should be similar and a decrease in positiveness should be accom- 

 panied by a decrease in efficiency and in the vigor of the con- 

 cussion reaction. 



The relation or non-relation between these factors is shown 

 in charts 1-6 (pp. 275-282). These show that in the main the 

 larger variations in efficiency and positiveness are in agreement 

 although this differs somewhat with individuals, notably with 

 isopod no. XIV, chart 6. The larger concussion variations show 

 little agreement with the other factors although there are some 

 cases of notable disagreement which will be discussed later (p. 

 273). 



