460 Journal of Coinparatwe Neurology and Psychology. 



With a very powerful stimulus, such as a strong chemical, 

 this reaction makes the impression of being violent and dis- 

 ordered, as indeed may the reactions of a human being under 

 similar conditions. But with a moderate stimulus the reaction 

 may be very delicate. This may be illustrated by the behavior 

 of Paramecia within an area of water containing carbon diox- 

 ide. Part of the reaction under these conditions was described 



tSu 



Fig. 9. Diagram of the swinging of the anterior end about a large circle, in 

 reacting to a strong stimulus. The revolution on the long axis has entirely ceased. 



in one of my earlier paper (Jennings, 1899, p. 331), though 

 without a full appreciation of its real significance. The Para- 

 mecium, swimming slowly within the area of carbon dioxide, 

 comes near to the edge of the area, where it receives water 

 containing none of the gas in solution. This change acts as a 

 very mild stimulus ; the organism merely stops and swings its 

 anterior end gently toward the aboral side, "trying" a new di- 

 rection. If the water now received is still without the carbon 

 dioxide, the Paramecium swings its anterior end still farther, 

 at the same time continui-ng to revolve on the long axis, which 

 changes the direction of swinging. As soon as the water it re- 

 ceives contains carbon dioxide, it swims ahead, changing its 



