REACTIONS OF CRAYFISHES. 185 



response as well as an avoidance of the acids. The tendencies to 

 crawl out of the water and upon the side of the tank are more 

 clearly acid avoiding reactions. In the controls there were no 

 attempts to crawl on the sides of the tank and few animals showed 

 an inclination to crawl out of the water. 



i. Carbon Dioxide Gradient. 



(a) C. propinquus. The crayfishes, upon invading the high 

 carbon dioxide end, showed a tendency to turn before reaching 

 the screen, or they would back a short distance and then turn 

 and pass to the lower carbon dioxide end. Of all the individuals 

 tried 55 per cent, turned back on the first encounter of the 

 gradient and 14.8 per cent, of these turnings were accompanied 

 by backings (Table I.). There was a greater number of turnings 

 from the high concentration end than from the low, there being 

 a total of 124 from the high end to 51 from the low. The fact 

 that turning from the high end is an avoiding reaction is empha- 

 sized by 1 6 per cent, of the turnings being accompanied by back- 

 ing. There were fewer crossings of the center in the experiments 

 than in the controls, in the latter there being a tendency to travel 

 the entire length of the tank. In some cases the reduction of 

 crossings of the center represents the extent of anesthetization 

 of the animals. 



In the experiments with low concentrations (low 3.5 and high 

 20 c.c. of carbon dioxide per liter) and low temperature the 

 turnings were rhythmical (Chart I., Expt. 2). Long invasions 

 of the high concentration end were followed by shorter invasions 

 and periods of rest at the low concentration end. These were 

 then followed by very short invasions and very long periods of 

 almost complete rest in the low concentration end. Later there 

 would be a second similar period of invasion and rest. Three 

 such periods are shown in the graph. Thus there is a period of 

 increased sensitiveness or a period of increased reaction to the 

 same incoming sensation, after invasion of the high concentration 

 end. In either case there is a rapid modification of behavior. 



With higher concentrations (24-47 c.c. per L., low end and 

 58-94 c.c. per L. high end), if the crayfishes were not too greatly 

 overcome, there was greater rapidity of reaction and a more 



