178 EDWIN B. POWERS. 



gradients. The experiments were conducted between January 

 13 and April 27, and between June 16 and July 31, 1913. The 

 first set will be designated as low-temperature experiments and 

 the second set as high-temperature experiments. 



II. MATERIAL AND METHODS. 

 i. Apparatus and Method of Experimentation. 



In the study of reactions of the crayfishes in gradients of carbon 

 dioxide and acetic and hydrochloric acids, the method and 

 apparatus were devised by Shelford and Allee ('13) for the 

 study of reactions of fishes to gases or solids in solution. The 

 apparatus, a full description of which is given in Shelford and 

 Allee ('13) "The Reactions of Fishes to Gradients of Dissolved 

 Atmospheric Gases" (pp. 225-229), consists of two tanks each 

 120 cm. long by 20.5 cm. wide by 14 cm. deep with outlets at 

 the center of both sides, near the top. These outlets are guarded 

 by a screen-bottomed tube which extends across the tank. The 

 tanks are placed side by side in an aquarium, beneath a hood 

 under identical and symmetrically surrounding conditions. Tap 

 water was introduced at both ends through perforated tees 

 behind screens. In the experiments with carbon dioxide, the 

 flows were 600 c.c. per minute. At one end carbon dioxide was 

 introduced into the inlet so that a gradient was produced length- 

 wise of the tank between the tap water and the water high in 

 carbon dioxide. This was shown by titrations made of samples 

 taken from different portions of the tank. In the acetic and 

 hydrochloric acid experiments the conditions were the same, 

 except for the flow at the tap water end and corresponding end 

 of the control, which was 1,200 c.c. per minute, thus producing a 

 sharper gradient. This flow was also used in all high temperature 

 carbon dioxide experiments. The acetic and hydrochloric acids 

 were introduced by means of a separate tee. 



After everything was made ready for the keeping of records, 

 the crayfishes were dropped into the center of the tank and were 

 observed through a slit in the hood, and the back and forth 

 movements recorded in the form of a graph on paper especially 

 prepared for this purpose. Records were also kept of specific 

 reactions observed in any individual. These records were made 



