RHEOTAXIS IN ISOPODA 261 



RESPONSE TO CONCUSSION COMPARED WITH THE SIGN AND 

 EFFICIENCY OF THE RHEOTACTIC REACTION 



METHODS 



In testing for the sign of the rheotactic reaction in a circular 

 current the methods described in the preceding paper ('12, p. 

 276) were largely used. Forceps wrapped with soft cloth were 

 substituted for the rounded glass rod in producing the current 

 since they are less apt to cause injuries. The isopods were 

 stirred loose from the bottom in order to diminish the inter- 

 ference of positive thigmotaxis with the reaction to the water 

 current. Not more than three animals were tested at one time 

 and records were kept of their individual reactions. 



Method for determining efficiency in a circular current 



During the earlier work the sign of the response was not dis- 

 tinguished from the vigor of the reaction in determining the 

 positiveness of the animals under experimentation; but it is evi- 

 dent that an isopod might head upstream at each trial and yet 

 be unable to make headway against the current. Its physio- 

 logical condition then would be obviously different from that of 

 an isopod that could make rapid progress against the current 

 although in percentage of positive responses their reactions might 

 be identical. 



In order to test the relation between the sign and the efficiency 

 of movement, the following arbitrary standard of efficiency in 

 the current was adopted. This represents numerically the dis- 

 tance 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 (approxi- 



mately 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 



