EDWIN B. POWERS. 



"M "K.k 



-to 



CHART I. Showing reaction of the crayfishes of gradients to carbon dioxide 

 and acetic acid. The horizontal distance between the scales represents the length 

 of the tank. The vertical scale represents time in minutes. Different tracings 

 represent individual animals. Three animals were tested simultaneously. 



2. Acetic Acid Gradient. 



(a) C. propinquus. The avoidance of the acetic acid in 

 gradient was sharp and definite (see Chart I., Expt. 14). Orien- 



