196 EDWIN B. POWERS. 



points are suggested by the comparison of the ratings of each of 

 the other three species in the hydrochloric acid, acetic acid and 

 carbon dioxide experiments. It is interesting to note that when 

 turnings only are considered that in all four species in the carbon 

 dioxide, acetic acid and hydrochloric acid experiments that there 

 is an increase of per cent, of turnings from the high acid concen- 

 tration end over the low concentration acid end in the order 

 named. Thus the hydrochloric acid again has the greatest inten- 

 sity in avoidance of the high acid concentration end. These 

 points are shown by the experiments with propinquus although 

 the acetic acid experiments were of longer duration and were 

 performed at a lower temperature than the hydrochloric acid 

 experiments. The conclusion that intensity of negative reac- 

 tion is directly proportional to the concentration of H ions can 

 only be suggested, as sufficient data to warrant a definite con- 

 clusion are wanting. 



The above suggestion might receive objection on the ground 

 that there may be specific differences in the effect of the three 

 acids used. Such an objection ia supported by the fact that 

 propinquus, in the low temperature experiment, is intoxicated 

 by carbon dioxide and not by acetic acid. This apparent 

 difference may, however, be explained on the ground that the 

 carbon dioxide, since it diffuses more rapidly than the acetic 

 acid, really produces a higher hydrogen ion concentration in the 

 blood of the crayfishes than does the acetic acid, in spite of the 

 fact that the latter acid is more highly ionized. The carbon 

 dioxide would also tend to increase the carbon dioxide in the 

 animal's blood by preventing the escape of the supply of this gas 

 that is constantly being given off by the tissues of the animal. 

 The acetic acid and hydrochloric acids would not offer any such 

 hindrance to the diffusion of the internal gas into the water, 

 and would not, therefore, be as detrimental as the carbon dioxide. 



The periodicity and final cessation of invasion of the high 

 concentration end of the experimental tank is a modification 

 of behavior that may be brought about by increase in sensitive- 

 ness on the part of the crayfishes, or by a more rapid reaction to 

 the same sensation. In the one case the cause is physiological, 

 in the other the explanation must be psychological. If the 



