410 W. C. ALLEE 



with a fixed standard. This topic will immediately be discussed 

 more fully in dealing with the reactions of individual isopods. 



Thus far I have been dealing with averaged results. For 

 generalizations these are necessary and in order to save space the 

 individual records have not been given. How far does the work 

 with individuals support the averaged results? Before taking up 

 this point it is necessary to point out that each individual has in 

 all probability a rate of metabolic processes slightly different from 

 that of any other isopod and further, that it is not a fixed standard 

 rate of metabolism that finds its expression in a high rate of 

 positiveness but rather a relative rate. That is, when the metab- 

 olism of an isopod is rapid for that individual it tends to go posi- 

 tive to the water current, when less rapid, negative, when still less 

 rapid, indefinite, and when least rapid no reaction at all is given. 

 But a rate of metabolism that is rapid for one isopod may be 

 slow for another, and intermediate for a third. Now all three 

 would show the same survival-time in potassium cyanide and yet 

 their rheotactic reaction would be highly different. Thus for exam- 

 ple, isopods 96 and 101 from Stock III had a survival-time of three 

 hours and forty-five minutes in 0.001 molecular cyanide solution. 

 The former gave 50 per cent positive, the latter 100 per cent positive 

 reactions. Isopod No. 97 was killed in the same solution with 

 the other two with a survival-time of three hours and fifteen 

 minutes, yet its rheotactic reaction was only 60 per cent positive. 

 Since it is obviously impossible to test the survival-time of an 

 isopod in its varying rates of metabolism, one is forced to do the 

 next best thing, that is, take average results. 



There is still another complication in this regard. An individ- 

 ual isopod may give identical rheotactic reactions when its rate 

 of metabolism varies greatly. Thus an isopod kept at 20°C. 

 comes to have a normal mean metabolic rate, also a normal mean 

 rheotactic response. When the metabolic rates goes above this 

 normal mean the isopod tends to become more positive, when 

 below, less positive to water currents. Put the same isopod at 

 5°C. The metabolic rate is depressed and the positiveness also 

 decreases. But in time the isopod becomes acclimated; a new 

 metabolic mean is established and the rheotactic reaction goes up 



