RHEOTAXIS IN ISOPODA 259 



are so few who are prepared to make field observations and to 

 fill in logically the inevitable gaps in such data, it seems the 

 more important that ecological views be subjected to the most 

 rigorous experimental tests before they are accepted. 



Fortunately there is a large mass of data that has been col- 

 lected in the fields of general physiology and general psychology, 

 called 'animal behavior,' upon which the ecologist may draw. 

 But unfortunately for the ecologist, most of this experimental 

 work has been done either from the standpoint of the physi- 

 ologist, where a minimum of emphasis is placed upon the natural 

 environment and a maximum upon the method of the reaction 

 (Loeb '05; Jennings '06 and Mast '11); or from the standpoint 

 of the comparative psychologist (Holmes '11). It is true that 

 even in much of this work the idea of response to environment 

 is present, sometimes in a marked degree, as with Dawson ('11) 

 and more rarely it is the dominant thought (Shelford and AUee 

 '13). 



STOCK 



The isopods were all Asellus communis of a stock collected 

 in County Line Creek (Shelford '11a, maps) near Glencoe, 

 Illinois. They belong to the same general stock which furnished 

 material for almost all the earlier experiments on stream isopods. 

 After being kept in an aquarium supplied with Chicago city tap 

 water, at the University of Chicago for a year the isopods were 

 transferred to the University of Illinois and placed in aerated^ 

 university tap water. The chemical analyses of the two waters 

 are shown in table 1. 



The change in water did not greatly affect the rheotactic 

 reaction of the isopods. One lot received October 24, 1912, 

 which had been two days in transit in small containers, gave 

 an average of 62 per cent positive for 150 trials when first re- 

 ceived. Four days later the positive response had increased to 



^ The aeration was accomplished either by allowing the water to flow slowly 

 through a series of pans (Marsh '10) or by allowing air from the university com- 

 pressed air system to bubble through small openings in a rubber tube laid around 

 the bottom of the aquarium. 



