304 W. C. ALLEE 



positive response. Later if the external depressing factors are 

 continued in their action, the rate of metabohsm is cut below that 

 which supplied energy for the body alone and this results in a 

 decrease in positiveness below that usual in the breeding season. 

 Besides being supported by the rheo tactic response this hypothe-* 

 sis is supported by the fact that the isopods tend to lose their 

 general breeding reactions if kept in a decreased oxygen supply. 

 The hypothesis is capable of being experimentally investigated, 

 and this is one of the points upon which the writer expects to 

 do further work. 



(5) Juvenile stream Aselli. It has been shown in dealing 

 with the rheotactic response of juvenile stream isopods in the 

 ■field (table 4) that in at least one case the Aselli are known to 

 have given an increase to a strong degree of positiveness in the 

 presence of a small amount of oxygen. All the tests that have 

 been carried on in the laboratory have failed to give a correspond- 

 ing result. These experiments are of two kinds. First, the less 

 carefully controlled type of experiments that characterized the 

 early part of the work. In these experiments the actual amount 

 of oxygen present in the water was not under control although it 

 was always below air saturation at that temperature. Results 

 from this type of experiment are shown in the first part of table 

 13. The other experiments were more carefully controlled and 

 these results are shown in the second part of the same table. 



In the first part of table 13 the results given for the response 

 of the isopods under high oxygen conditions have been summa- 

 rized from table 4. That is, the average response shown in that 

 table has been taken as giving the reaction of the isopods of that 

 size under normal conditions. The oxygen content given is also 

 the averaged amount shown for the same trials in the same table. 

 It will be seen that until the isopods are about 5 mm. long, they 

 tend to give the same response regardless of the amount of oxy- 

 gen supplied, but that after that size is reached, the animals with 

 the higher oxygen supply increase much more rapidly than do the 

 ones with a sub-normal supply. The figures in this case for the 

 animals in the low oxygen content represent the entire life his- 

 tory of the isopods up to the time of their first breeding season. 



