286 



W. C. ALLEE 



they were about a month old. As table 4 shows, the general 

 positive response of the young isopods is much lower than that 

 of the adults but that the positiveness increases with age. 



Again there-is more variation in the positive response than seems 

 consistent, for, as the table shows, the isopods give at times a 

 low response regardless of size and of the external conditions here 

 controlled. 



It will be noted that the increase in positive reaction was not 

 due primarily to the oxygen content of the water because this was 

 high when the positiveness was low. The carbon dioxide was also 

 low as it has been in all cases discussed. Evidently the growth in 

 size was the most important factor in the increased positive 

 response. 



TABLE 4 

 Rheotaxis in juvenile stream Aselli 



