302 W. C. ALLEE 



length. All were as near the same size and general condition 

 as could be selected from their external appearance. The stock 

 had first shown signs of the approach of the breeding season in 

 the latter part of December and these trials were begun January 

 31, 1912, so that at that time they were in the midst of the breed- 

 ing season. 



The isopods were taken from a temperature of 4° C. and with 

 the exception of the first trial in the control they were kept 

 within two degrees of this temperature throughout the experi- 

 ment. The heightened response obtained from the control in 

 their first trial may be due to the fact that the temperature was 

 4° above that to which they were accustomed, or it may mean that 

 the isopods used were not so completely under the influence of 

 the breeding response as they became later. It will be noted that 

 during the time the experimental animals were giving a changed 

 reaction, the control gave almost no variation in response. 



In the experiment the variation in the first two days is not 

 strong enough to make one sure of the cause without more data, 

 but at the ei^^d of the third day there is a strong increase in the 

 positive reaction which is entirely different from anything that 

 we have yet seen in the response to a sudden lowering of the oxy- 

 gen content of the water. This increase in positiveness was 

 maintained three days in which time it had increased until the 

 response was almost that of the normal adult isopod under high 

 oxygen conditions. Then the response fell off and at the end of 

 the experiment it was decreased almost to the zero mark. This 

 decrease started at the time of the highest oxygen supply so that 

 it was not due to the variation of the amount of oxygen present. 



The following explanation of these phenomena is suggested. 

 When the breeding activity set in, energy normally spent in 

 general body metabolism is taken by the reproductive organs. 

 As these increase in activity, the amount of energy left for bodily 

 activities decreases and hence the positiveness of the animals to 

 currents of water is diminished. But the presence of an external 

 medium tending to decrease the rate of metabolism decreases that 

 of the reproductive system and so gives an increased amount of 

 energy for bodily activities. This results in an increase of the 



