EDWIN B. POWERS. 



tion with a greater intensity of avoiding reactions as is shown 

 by the rating and per cent, of time spent in the halves of the pan. 

 See Tables II. and III., and Chart II., graph 30. While in 

 experiment 35 where the concentration was very high there was 

 a falling off of the intensity of the avoiding reaction due to one 

 specimen becoming more or less anaesthetized. See Table II. 

 and Chart II., Expt. 35. 



(c) C. diogenes. In the acetic acid experiments as well as the 

 carbon dioxide experiments the intensity of the avoiding reac- 

 tions are rather low with the exception of experiment 31 which 

 is rather high, the rating being 78. There was a stronger ten- 

 dency to stop and then move forward and with longer periods of 

 rest than was noted in the carbon dioxide experiments. 



(d) C. immunis. The avoiding reactions of immunis to acetic 

 acid was definite in the time spent in the halves of the pan, by 

 turnings and by turnings accompanied by backings. See Table 

 II., Chart II., Expt. 37 and 34. The intensity of the reactions 

 in the acetic acid as well as in the carbon dioxide was rather low 

 as is shown by Tables I. and II. 



3. Hydrochloric Acid Gradient. 



After having completed the experiments with carbon dioxide 

 and acetic acid it was thought advisable to test the crayfishes 

 with some inorganic acid and thus determine the difference or 

 similarity of the reactions of the crayfishes to the different 

 acids and to better compare the reactions of the four species of 

 crayfishes. Hydrochloric acid was selected for this purpose. 

 Since the ion constant of hydrochloric acid in very weak concen- 

 trations is approximately one (Stiegletz, 'n) very low, concentra- 

 tions were used in all the experiments. By an inspection of the 

 tables it is seen that the same intensity of avoiding reaction was 

 obtained in the low concentrations of hydrochloric acid but of high 

 ion concentration as was obtained by the higher concentrations 

 of acetic acid and carbon dioxide of lower ion concentration. 

 Not only was the intensity of the avoiding reactions high in 

 proportion to the concentration, but the reaction of all four 

 species was more definite as is indicated by the turnings from 

 the ends and turnings accompanied by backings. See Table II. 

 and Chart II. Expt. 47, Chart II. was extended over a period 



