HHEOTAXIS IN ISOPODA 291 



from this viewpoint, but regarding the efficiency of movement 

 of pond isopods there is some data. By comparing the graphs 

 of thirty-five individual trials*' of stream with fifty of pond iso- 

 pods it appears that the relative vigor of the pond is about half 

 that of the stream mores, while on the average the positive 

 rheotactic response is about three-eighths as strong. Here again 

 there is apparent agreement between the degree of positiveness 

 and the rate of efficiency. 



The response to concussion failed to furnish an exact method 

 for testing the metabolic state. This was first suggested in the 

 responses listed in table 3. Further evidence is found in charts 

 1 to 6, which show that the variations of the vigor of the reaction 

 to shock oppose those of positiveness and efficiency almost as 

 often as they agree; also by the fact (tables 4, 5 and 6) that 

 practically the same vigor of concussion is given with both high 

 and low positiveness, high and low indefiniteness and high and 

 low efficiency. It seems probable that the vigor of the concussion 

 reaction varies inversely as the thigmotactic response rather than 

 directly as the rheotactic reaction. 



In the early periods of depression the vigor of the concussion 

 response often increases. This action corresponds with that usu- 

 ally given during early stages of the action of depressing agents. 

 The cause of the increased reaction is now generally regarded 

 as being due to the decrease of the normal inhibitory power of 

 the nervous system (Sherrington '06, p. 106). Later with in- 

 creased action of the depressing agent this decrease in the action 

 of the inhibiting nerves gives way to depression of the entire 

 nervous system. This period corresponds to the time when 

 weakened shock responses were given n the later stages of 

 depression. 



In the pre\dous experiments ('12, p. 285) it was shown that 

 stream isopods cannot permanently maintain themselves in a 

 stream where they cannot withstand the current during the 

 weakened responses of the breeding season. From the present 

 experiments it appears that the molting period offers for brief 



^ These graphs are those from which figures 1 and 2 (pp. 278, 291) of the pre- 

 ceding paper were taken. 



