RHEOTACTIC REACTION OF STREAM ISOPODS. 63 



Not enough data has been collected to warrant any conclusions 

 being drawn regarding the possible relationship between the 

 clinging ability of isopods and their metabolic rate as measured 

 either by their resistance to potassium cyanide or their carbon 

 dioxide production. However the data given in Tables II. and 

 III. show that there is a variation in the clinging power of posi- 

 tively oriented isopods accompanying more or less closely the 

 variation in the positiveness of the rheotactic reaction. Since 

 the degree of positiveness of this reaction depends on the rate of 

 metabolism (loc. '/.) it seems fair to conclude that there is some 

 correlation between the physiological states of isopods and their 

 ability to maintain themselves in a water current. This means 

 that a decrease in the metabolic activities of isopods not only 

 causes a decrease in the positiveness of the rheotactic reactions 

 and hence indirectly decreases the clinging power of isopods in a 

 current (p. 60) but that it also directly decreases the clinging 

 power. 



DISCUSSION. 



In all probability the sum total of the reactions of an organism 

 largely determines its distribution (Shelford and Allee, '13; 

 Wells, '14). With different animals in different environments 

 now one reaction or group of reactions may be the more important 

 and now another. It is obvious that the maintenance of position 

 is of primary importance in the distribution of stream animals 

 and it is likewise obvious that this must be done either by some 

 means of clinging or by the rheotactic reaction or by a combina- 

 tion of the two. Bryozoa show the maximum development of 

 the clinging reaction while certain fishes as Notropis show a 

 maximum development of the rheotactic response since they 

 rarely rest on the bottom (Shelford and Allee, '13). A study of 

 maintenance methods of stream animals would probably establish 

 a completely graded series between these two extremes. In 

 such a graded series all crawling animals, to which group the 

 isopods belong, would of necessity depend a great deal upon their 

 ability of clinging to the substratum to maintain themselves. 



To what extent positive orientation in the rheotactic response 

 increases the clinging efficiency of crawling stream animals other 

 than isopods is a matter for experimentation. Judging solely from 



