RHEOTAXIS IN ISOPODA 279 



In the figure the starting point is indicated by a large dot ; the end 

 point, by a cross. The arrow on the outside of the circle indi- 

 cates the direction of the current. 



In all cases, excepting no. 23, the animal responded positively 

 to the current, that is, it gave a 96 per cent positive response. 

 Evidently its normal reaction was a positive one and the one fail- 

 ure to go against the current would then mean a chance turning 

 that was not corrected. That is, in this case there is an experi- 

 mental error of 4 per cent, which checks well with that shown in 

 table 1. These results also show that the isopods will give their 

 normal response for at least twenty-five successive trials, so that 

 it is entirely safe to take the first ten responses as indicating the 

 normal behavior of the animals tested. 



During these experiments the oxygen content of the water 

 has been determined by the Winkler method. During the first 

 part of the work, the method was followed as outlined in the report 

 of the committee on standard methods of water analysis to the 

 laboratory section of the American Health Association ('05, pp. 

 74-77) . The free carbon dioxide was determined by direct titra- 

 tion with N/22 sodium carbonate using phenolphthalein as an 

 indicator. This method is described in the same report (pp. 

 72-73). After the appearance of Birge and Juday's work ('11, 

 pp. 13-24), their methods were followed wherever the technique 

 appeared better. Birge and Juday compared this method of 

 determining the oxygen content of the water with that of boiling 

 and found (1. c, pp. 11-12) that the amount of variation in results 

 from the two methods was not more than was the case in dupli- 

 cate determinations by the same method. V. E. Shelf ord and the 

 writer verified these results and the methods used have proved 

 eminently satisfactory for rapid biological work. 



