RHEOTAXIS IN ISOPODA 281 



tion being 18 per cent from the mean. These variations may be 

 due to length of time since moulting or age. The group giving 

 the highest positive response had been carefully selected to repre- 

 sent animals in the best possible condition. In all the other 

 cases the animals were simply picked at random from the general 

 stock. The variation in oxygen content of the water within the 

 limits given in table 2 does not appear to have any marked signi- 

 ficance although in general the lowest positive responses are 

 found with the lowest oxygen content. Apparently there is 

 sufficient oxygen here for the usual activities to be carried on at 

 the usual rate and not enough to stimulate them greatly. The 

 length of time the animals have been in the laboratory does not 

 affect their response providing they are kept in conditions resem- 

 bling as nearly as possible, those in their usual habitat. 



The response of these stream isopods to the current is vigorous 

 and usually definite, that is, they are either definitely positive 

 or negative. When the current is reversed they tend to reverse 

 within the first ten seconds after the new current is set up, and 

 often they are all reversed before the stirring is stopped. They 

 also move vigorously, sometimes completing two circumferences 

 of the pan in the minute reaction time allowed. This would mean 

 a rate of about 80 cm. per minute. They sometimes pivot on 

 their posterior end, turning their head in a complete circle before 

 starting a definite reaction and this testing reaction is usually 

 followed by a positive response. 



In all these cases the amount of free carbon dioxide in the water 

 was very low. That is, it never exceeded 3 cc. per liter of water 

 and usually ran much lower, the average being about 2 cc. per 

 liter. Thus the variations of the free carbon dioxide are too 

 small to be of any significance, and the fact that it was present in 

 such small amounts will have to be considered in determining the 

 cause of the high percentage of positive reactions. 



6. Breeding season 



The breeding season of these isopods begins before the ice is 

 out of the water in the spring. It reaches its culmination by the 

 last of April and gradually diminishes. Occasionally breeding 



THE JOURNAL OF E XPERI.MENTAl. ZOOI.OC.Y, VOL. 13 NO. 2 



