538 A. FRANKLIN SHULL 



Response to oxygen in the Nebraska line 



The failure of the Nebraska Hne in experiment 5 (table 7) to 

 show any marked effect of oxygen is the only inharmonious result 

 obtained in this study. It seemed possible that this line was not 

 as sensitive to oxygen as the New Jersey line used in the other 

 experiments. If this were the case, any other response to oxygen 

 might prove less marked in the Nebraska line than in the New 

 Jersey line. Such a response to oxygen in this species was found 

 in earlier experiments. In a former paper (Shull, '15a) it was 

 shown that races of rotifers might differ very markedly in the 

 place of egg-laying. One race laid its eggs very largely on the 

 bottom or sides of the dishes; another race chiefly attached to 

 the surface film of the water. In connection with later oxygen 

 experiments (Shull, '18), it was shown that placing the dishes in 

 an atmosphere containing an excess of oxygen caused the eggs 

 to be laid more largely on the bottom than when the dishes were 

 kept in air. If the Nebraska line lacked responsiveness to oxy- 

 gen, this lack might be evidenced by failure of oxygen to alter the 

 place of egg-laying. This possibility gave rise to the following 

 experiment. 



Experiment 7. On each of the days named in table 9 several 

 rotifers of the Nebraska line were placed in each of two dishes of 

 water and fed manure scum. One dish was set under a bell jar 

 in an atmosphere of which 60 per cent was oxygen, the other 

 placed under a bell jar in ordinary air. After a period of from 

 sixteen to eighteen hours, the eggs laid on the bottom and at the 

 surface film in each of the dishes were counted. As a check upon 

 the results, a similar test was simultaneously made upon the 

 New Jersey line. Table 9 gives the results. 



While the Nebraska line is here shown to be responsive to oxy- 

 gen, the controls show that it normally laid more of its eggs at the 

 bottom than did the New Jersey line. In view of this normal 

 difference between the two lines, it is difficult to judge of the rela- 

 tive effect of oxygen upon them, and the discrepant results in 

 experiments 5 and 6 probably remain unexplained. 



