RHEOTAXIS IN ISOPODA 



293 



shown (table 8, p. 280) that under experimental conditions posi- 

 tive isopods may come to give negative reactions when the effi- 

 ciency or vigor of response is intermediate between that which 

 usually accompanies high positiveness and high indefiniteness.* 



Then if we use the degree of positiveness as an index of the 

 physiological state of the stream isopods we shall have the fol- 

 lowing results: 



That is, the work with individuals has shown that in the 

 physiological states that accompany the highest efficiency there 

 is something as yet unanalyzed that sometimes causes negative 

 responses where positive reactions are usually given. Also that 

 the varying physiological states that produce the minor varia- 

 tions are as yet inexplicable in their entirety (cf. Pearl '02, with 

 Planaria and Jennings '02, with Stentor) although each class 

 may be in part explained by the influence of the molting period. 

 One other factor that may affect the minor variation is the fact 

 that the isopods tend to continue in the direction in which they 

 are started regardless of the response they will ultimately make 

 ('12, p, 276), so some of the variations may be due to this error 

 in the method. It is probable that factors enter into the control 

 of the positive rheotactic response other than the rate of metab- 

 olism of the organism. Thus during the molting period there is 

 an interval when the mechanical difficulties are such that move- 

 ment is limited regardless of the metabolic condition of the 

 isopod. Yet on the average the data at hand strongly indicate 

 that the degree of positiveness and the efficiency of Asellus com- 



