198 EDWIN B. POWERS. 



tage is indicated by the experiments heretofore described. 

 Virilis, however, in its stream habitat would seldom encounter 

 carbon dioxide concentrations of anything more than a very 

 low degree and thus we find that this species does not react 

 definitely to 'the gas. Virilis, then probably, while sensitive 

 to carbon dioxide, orients itself in its environment by reactions 

 to some other factor or factors. Immunis and dio genes are 

 pond forms and are less sensitive to the acids but both avoid 

 high concentrations. Diogenes although shown to be more sensi- 

 tive than immunis orients less definitely as is shown by the per 

 cent, of turnings from the high and low concentration acid ends. 

 The specificities are again in coordination with the habitats of 

 the species, immunis being a pond mud loving form, remaining 

 in burrows only at times while diogenes remains almost wholly 



in burrows. 



SUMMARY. 



1. Crayfishes sense the increase in carbon dioxide and acetic 

 and hydrochloric acids in a gradient. 



2. Both propinqnus and virilis are intoxicated by carbon 

 dioxide; virilis is also intoxicated by acetic acid but to a lesser 

 degree. 



3. The four species are susceptible to high concentrations of 

 carbon dioxide and when subjected to high concentrations die in 

 the following order, virilis, propinquus, diogenes and immunis. 



4. Propinquus reacts negatively to the higher concentrations 

 of carbon dioxide in a gradient, but when the total amount of acid 

 present is large, the negative reaction may be interfered with by 

 the direct detrimental effect of the acid. 



5. Virilis reacts less definitely to the higher concentrations of 

 carbon dioxide in a gradient than does propinquus. This is true 

 whether the total concentration of the acid is large or small. 



6. Both diogenes and immunis react more or less irregular to 

 carbon dioxide due possibly to the lesser sensitiveness of these 

 two species to this acid. 



7. Both propinquus and virilis react negatively to the higher 

 concentrations of acetic acid in gradients of this acid ; propinquus 

 reacts definitely in the presence of both high and low total con- 

 centrations; virilis reacts definitely to low total concentrations, 



