328 



W. C. ALLEE 



stream isopods were all kept in still water, the higher amount of 

 oxygen being supplied by means of the aerating device. In the 

 pond isopods, however, those from the higher oxygen were kept 

 in running water so that they were in a medium free from their 

 waste products, which other workers have found to cause a diminu- 

 tion of size (Colton, 1. c). The results indicate that the amount 

 of oxygen present is one of the factors and probably a major one 

 in causing the size difference between pond and stream Aselli. 



TABLE 27 



Effect of oxygen on size 

 Stream isopods 



4. REACTIONS TO GRADIENTS OF GASES 



Earlier experiments had shown that isopods would collect in 

 their optimum light or temperature conditions if subjected to a 

 series of graduated changes in either of these conditions. Since 

 their rheotactic activity is more dependent on the oxygen supply 

 than upon either light or temperature, experiments were tried 

 to determine whether or not the Aselli would respond to gradients 

 of oxygen and carbon dioxide. 



These experiments were run in galvanized iron boxes especially 

 designed for the purpose. Each box was 50 x 30 x 7.5 cm. in 

 dimensions. The bottom was covered with a layer of beeswax to 

 give the isopods a better foothold for crawling. All the metal 



