TOXICITY OF ACIDS TO CILIATE INFUSORIA 469 



varies with several factors — the species used, the acid, and the 

 concentration. The acids are unequally affected by different 

 temperatures. The coefficients for Paramoecium and Euplotes 

 are scarcely alike at any point. Most of the coefficient of 10° 

 for Paramoecium lie between two and three and suggest a chemi- 

 cal reaction. The coefficients for Euplotes, however, are rarely 

 so high and agree better with the idea of a physical factor. 

 Indeed, with butyric and acetic, though not with any of the 

 other acids tried, a decrease in temperature below 20° accelerates 

 the toxic action upon Euplotes, so that their toxicity at 10° is 

 nearly as great as at 30° and much greater than at 20°. 



A comparison of the acids at various equinormal concentra- 

 tions shows that a rough parallelism exists between toxicity and 

 degree of dissociation, as would be expected if the H ion is the 

 most important factor in toxicity. When, however, the acids 

 are compared in solutions of equal Ph, it is evident that other 

 factors must enter, for the toxicity of the various acids is mark- 

 edly different. The order of toxicity, unlike that of equinormal 

 acids, is .closely similar to the toxic order of salts of these acids. 

 This suggests that the anions of at least some must be toxic. 

 Additional evidence of the toxic action of certain anions (mainly 

 of the fatty and the cyclic acids) is afforded by the action of acid 

 mixtures, in which the ionization of the organic acid is progres- 

 sively diminished by addition of a strong acid, HCl. 



All the acids used bring about sw^elling followed by precipita- 

 tion of part of the cell contents and by solution of the cilia. 

 There is clear evidence of change in surface tension, for the cilia 

 always become sticky before they stop beating. It seems prob- 

 able that the toxic effect of the various acids depends upon their 

 solubility in the tissue as well as upon capillary activity and the 

 changes in colloidal state wrought by the H ion. 



THE JOURNAL OF EXPERIMENTAL ZOOLOGY, VOL. 29, NO. 3 



