REACTIONS OF BRANCHIPUS TO EXTERNAL STIMULI 235 



between the temperatures of 14° and 17°. Thej^ were kept in 

 the pan thirty minutes, and during that time they stayed out 

 of regions where the temperature was higher than 17° C. 



Various other tests of the behavior of Branchipus to different 

 temperatures were made with results confirming those just given. 

 From 14° to 17° C. may, therefore, be considered the optimum 

 temperature for the majority of the animals tested, that is, the 

 temperature chosen by them when free to move into either 

 warmer or cooler water. When confined in water as warm as 

 28°C. or above Branchipus soon perishes. 



BEHAVIOR WITH RESPECT TO GRAVITY 

 Reaction to gravity in light 



When exposed to diffuse light these crustaceans are usually to 

 be seen lying at rest on the bottom of the aquarium, or quietly 

 swimming near the bottom. When the animals were put into 

 distilled water in tall glass cylinders for a more critical test of 

 what seemed to be their positive geotropism, they went down at 

 once, and stayed near the bottom as long as the light was diffuse 

 enough to be non-directive. 



An experiment was devised to test the behavior with respect 

 to gravity in directive light. A glass cylinder, twelve inches high 

 and six inches in diameter, was used. It was filled with distilled 

 water thoroughly aerated, and was then placed vertically in the 

 dark box between two 12 c. p. incandescent lamps, one above and 

 one below, each being about 6 inches from the cylinder. When the 

 light above was turned on, the animals were attracted upward 

 toward it, and a record was kept of the time required to make the 

 trip. When the light was turned on below the animals traveled 

 downward, and a record was kept as before. Records were also 

 taken of trips made under similar conditions with the cylinder 

 lying horizontally on the floor of the box, and the lights 6 inches 

 from the ends. Numerous tests were made with different indi- 

 viduals on different days, the results in all cases being similar. 

 Two typical laboratory records of this kind are given below, one 

 for the cylinder in a vertical position, and one, for the cylinder 

 in a horizontal position. 



