RHEOTAXIS OF ISOPODS 213 



of carbon dioxide per ten minute respiration period than did 

 No. 12 (length 7 mm.). After making all due allowance for the 

 effect of the slight size difference No. 102 had the higher meta- 

 bolic activity yet the average percentage of positive rheotactic 

 reactions was lower than with No. 12. Also the pair of isopods 

 Nos. 86 and 87 (table 1, page 206) gave identical rheotactic 

 responses but the carbon dioxide production and resistance to 

 potassium cyanide both indicate a difference in the metabolic 

 activity of the two animals. The same is true of isopods 90 

 and 91. 



So far as the facts at present are known it appears that the 

 relation between metabolic activity and the rheotactic reaction 

 of isopods is as follows: When the metabolism of an isopod 

 is rapid for that individual it tends to go positive to a water 

 current, when less rapid, negative, when still less rapid, indef- 

 inite, 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. Also an individual 

 may give identical rheotactic responses at times when its meta- 

 bolic activities measured by an absolute scale vary widely. Thus 

 isopods kept under favorable conditions including a high oxygen 

 tension come to have a normal mean metabolic rate and also a 

 normal mean rheotactic response. When the metabolic rate 

 goes above its mean the rheotactic reaction tends to become 

 more positive; when below, less positive. Put the same isopod 

 under similar conditions except that the oxygen tension is low 

 and the metabolic rate is depressed and the positiveness of the 

 rheotactic reaction also decreases. But in time the isopod be- 

 comes acclimated to the new conditions and the rheotactic reac- 

 tion may go up to about its old average (table 4, isopod 12, 

 reaction for 8/14) and plays up and down as the metabolic rate 

 changes about its new mean. So the rheotactic reaction is an 

 expression, not of the absolute metabolic rate of the animal 

 but of the relative metabolic rate to which the isopod is accli- 

 mated for the time being. 



SUMMARY 



1. The resistance of isopods to relatively strong solutions of 

 potassium cyanide has an inverse relation to their carbon dioxide 

 production; the higher the rate of carbon dioxide production, 



