RHEOTAXIS, RESISTANCE TO POTASSIUM CYANIDE 399 



these experiments individual tests were made of the reactions of 

 257 isopods of the first two groups, and of 195 of the third lot, 

 making a total of 452 isopods. Moreover, these experiments 

 consistently support results of the preceding four years experi- 

 mental study on this subject. 



METHODS 



The isopods were tested for their rheotactic reaction in a cir- 

 cular pan the bottom of which was covered with wax. The 

 responses of an individual isopod for ten successive one-minute 

 reaction periods were taken as a fair indication of the rheotactic 

 tendencies of the annual. The approximate distance covered 

 during each reaction was recorded and will be found in a stand- 

 ardized form in the tables under the head of efficiency in the 

 current.^ 



Immediately following the rheotaxis test the animals were 

 placed in potassium cyanide solution in Erlenmeyer flasks of 150 to 

 2000 cc. capacity. The size of the flask depended upon the 

 amount of oxygen to which the isopod was accustomed and upon 

 the strength of cyanide used. In all instances, had the cyanide 

 been omitted the isopods would have shown no ill effects of con- 

 finement for several days. A 0.1 molecular solution of potassium 

 cyanide was made up in distilled water. From this, solutions of 

 the desired strength were made in water similar to that in which 

 the isopods had been kept. During the survival periods in the 

 flasks the temperature was the same as in the stock aquarium. 



Exact determination of the death point gave considerable 

 difficulty. It was found that close examination with a lens for 



' The following arbitrary standard of efficiency in the current was adopted in 

 the earlier experiments and is used in those reported here. This represents numer- 

 ically the distance covered by an isopod in a minute's reaction period: 



no reaction 



1 slight movement 



2 any response between 1 and 3 



3 progress one-third around the pan positive (approximately 27 cm.) or two-thirds, negative 



(54 cm.) 



4 progress two-thirds around the pan positive or one and one-third negative 



5 progress once around the pan positive or twice around, negative 



6 any distance over 5 



For further details see Allee '13. 



