398 W. C. ALLEE 



measuring the resistance of the animal. The effect is due to the 

 action of the cyanide which (Geppert '99) decreases the abihty of 

 the tissues to take up oxygen. Child ('13 a) suggests that the 

 effect of the potassium cyanide solution depends upon the mrniber 

 of chemical bonds in the organism which are opened in a given 

 length of time. This means that the higher the rate of reaction, 

 the greater is the opportunity of the cyanide to produce its effect. 

 In a much weaker solution (0.00001 mol.) acclimatization occurs 

 and the animals with the higher rate of reaction live longer than 

 those with a lower rate. This is called the 'indirect method.' 



Although Professor Child found this method held for planarians 

 it did not necessarily follow that the chitinous-covered, gill- 

 breathing isopods would be affected in the same manner. My 

 problem then was, first, to find whether or not the cyanide method 

 would hold for isopods and then whether or not there was a rela- 

 tion between the rheotactic reaction and the resistance to potas- 

 sium cyanide. 



THE STOCK 



Three groups of Asellus communis were tried in these experi- 

 ments : 



I. The offspring of a stock that was originally collected in 

 August, 1911, from County Line Creek, north of Glencoe, Illinois 

 (Shelf ord '11a, maps) and which had been in well-aerated labora- 

 tory aquaria since (Allee '13). These isopods were half -grown 

 when used. 



II. A large number of breeding isopods collected in the above 

 stream April 13, 1913, and kept under the same conditions as the 

 first group. Both adults and young were tested. 



III. A series of collections of pond isopods, also A. communis but 

 mainly juvenile, from small fresh-water ponds near Woods Hole, 

 Massachusetts. These came from silt- and debris-bottomed ponds, 

 low in oxygen. They were kept under similar conditions in the 

 laboratory. 



These groups will be refered to in the body of the paper as 

 Stocks I, II and III respectively. The mortality rate of all three 

 groups was low while under observation. During the course of 



