June. 1921] Resistance of Eiiglenidce to Citric Acid 269 



glasses, 5 c. c. of liquid in each watch glass. To each series were 

 added, in similar glasses, both neutralized and unneutralized 

 pond water without citric acid, these to serve as controls; and 

 each series was then inoculated with a dififerent species, about 

 five or six individuals being introduced into each watch glass. 

 The quantity of culture medium introduced with these was so 

 slight as to be negligible. As the watch glasses were kept 

 stacked (close to a north window and away from the direct 

 rays of the sun) except when actually being handled, it is 

 evident that evaporation was also a negligible factor. 



vSince the object was purely to get a measure of the com- 

 parative resistance to the acid, no attempt was made to deter- 

 mine the length of time that the organisms could live in a 

 given concentration of the acid. Instead, all the watch glass 

 cultures were very carefully examined just 24 hours after 

 inoculation, and the condition of every individual was noted. 

 In every case all the organisms in the controls (both neutralized 

 and unneutralized pond water) were found to be alive and 

 normal in every way, showing that the results observed in the 

 other solutions were actually due to the addition of the citric 

 acid, and not to anything in the original pond water, nor to 

 the HCl introduced in neutralizing it. 



The results from the citric acid solutions are concisely 

 expressed in the accompanying table. In each case where all 

 the individuals in the watch glass were still alive after 24 hours, 

 the proper space has been left blank. Where part of the indi- 

 viduals were dead, an asterisk (*) is used; and where all were 

 found dead, a large X. In the case of each species, therefore, 

 the column with the last blank space (counting from left to 

 right) indicates the highest percentage of the acid successfully 

 withstood by one hundred per cent of the individuals tested; 

 while the column with the first X indicates the lowest per- 

 centage causing the death of one hundred per cent. 



The criterion used in determining whether an individual 

 was dead or alive was the unmistakable change in color and 

 appearance which ensues shortly after death has occurred. 

 Movement can not be used as a criterion in this group, since 

 individuals may temporarily remain perfectly rnotionless for a 

 considerable length of time. 



