298 



W. C. ALLEE 



TABLE 10 

 The effect of low oxygen upon the rheotactic response of adult stream Aselli 



the breaking point in the reaction to current for these animals at 

 this time. However if they had been accustomed to a lower 

 amount of oxygen, they might have reacted positively when kept 

 in this amount of oxygen. The effect of increasing the amount 

 of oxygen is shown by the fact that after one day in 7.51 cc. oxygen 

 per liter, these isopods gave an 82 per cent positive reaction. 



Something of the details of the effect of a lowered amount of 

 oxygen is shown in table 11. The first set of results is taken 

 from the early work. The amounts of oxygen did not rise, dur- 

 ing the time shown in the table, above 4.5 cc. per liter nor fall 

 below 2 cc. The effect of the lack of oxygen in this case became 

 evident after the animals had been in quiet water for twenty- 

 four days. An entry in the laboratory notes for that day reads as 

 follows: "The reaction to current is slower, less definite, and is 

 maintained for a shorter period than with the freshly collected 

 isopods." Unfortunately the exact oxygen content at that time 

 is unknown. The most important thing shown by this section of 

 the table is that the Aselli tend to become acclimated to the low 

 oxygen and at length react with a normal amount of positiveness 



