330 



W. C. ALLEE 



Table 28 gives the summary from eight trials made with isopods 

 from water having an oxygen content of over 5 cc. per liter. The 

 grand totals show a decided positive response to the tap water 

 end, altht)ugh the control animals are almost equally distributed 

 between the two ends. This mass result was confirmed by 

 transferring the introducers and thus gradually changing the 

 amount of oxygen present at either end, and also by stirring the 

 isopods back to the center. In all cases the final reaction was in 

 favor of the tap water end. Generally this reaction was the 

 result of a series of movements about the box during which the 

 isopods went from one end to the other but in a few cases there 

 was a definite turning back when the boiled water was encountered. 



Table 29 shows the response of isopods kept in an oxygen supply 

 of less than 3 cc. per liter. Even under these conditions they did 

 not collect in the boiled water end of the gradient in greater num- 

 bers than in the tap water end, yet the response to the tap water 

 was cut from 77 to 53.3 per cent of the total number of readings 

 taken. This probably means that having been kept in low oxy- 



TABLE 28 

 Stream isopods from high oxygen in an oxygen gradient 



EXPEBIMENT 



Control 



O2 gradient cc. per liter. . 



response 



gradient 



response 



response 



gradient 



response 



Totals 



Percentage of total trials 



LOW CENTER 



2.56 

 15 



3.07 



6 

 20 



1.82 

 28 

 87 

 32 



9 



197 



16 



5.58 

 5 



5.47 

 2 

 21 



10 



22 



18 

 4 



82 



7.68 



40 



7.63 



122 



67 



8.14 



178 



388 



64 



73 



932 



77 



cuhbent: 



cc. per rmn. 



1200 

 600 



200 



LOW center high 



20 



53 



102 



160 



62 



37 



434 



39 



39 

 23 

 35 



168 



278 



25 



17 

 68 



no control 



75 



169 



36 



45 



410 



36 



