EHEOTAXIS IN ISOPODA 279 



7 A.M. to midnight without any evidence of such a variation. 

 Thus the tests upon which this paper is based were made largely 

 from 8 A.M. to 12 noon. Those recorded in the preceding publi- 

 cation ('12) were made largely in the afternoon and evening up 

 to midnight, although some tests were made in the early morn- 

 ing. This leaves the interval from midnight till 7 a.m. untested. 

 In order to find if there was evidence of a daily rhythm three 

 uniformly positive isopods (nos. I, II and VI) were tested every 

 hour for twenty-four hours. The results are shown in chart 7, 

 p. 283. The reaction of isopod no. I is not given since it does 

 not differ essentially from that of no. II. There is no evidence 

 of a daily rhythm. The decrease in the positiveness of the reac- 

 tion of no. VI towards the end of the period is due to fatigue 

 as is shown by the fact that it was three days before the isopod 

 recovered its normal positiveness. Since there was no evidence 

 here of a daily periodicity of reaction and since the earlier data 

 all point against the existence of such a daily rhythm, no further 

 tests were made along this line. 



EFFECT OF LOW OXYGEN^ AND POTASSIUM CYANIDE ON THE 

 REACTIONS OF INDIVIDUAL ISOPODS 



It has been shown that both potassium cyanide and low oxy- 

 gen decrease the positiveness of stream isopods ('12, pp. 298 and 

 314). Experiments were run to determine the effect of these 

 conditions upon the reactions of individuals, especially as regards 

 the efficiency of the rheotactic response and the vigor of the 

 concussion reaction. Typical effects of potassium cyanide may 

 be seen in charts 1 and 2 (p. 275) of low oxygen in charts 3, 4 

 and 5, pp. 277-281. 



Both the positiveness and the vigor of the reaction to current 

 were diminished, though from one to five days might elapse 

 before the reaction was greatly affected. The length of this 

 period differed with different isopods. On the other hand, the 

 response to concussion at first increased (charts 1, 2, 3 and 5) 



* Potassium cyanide, joo^ooo solution. Low oxygen ranged between 0.50 and 

 1 cc. per liter. 



