468 Journal of Comparative Neurology and Psychology. 



causing changes in the intensity of the stimulus, and hence in 

 determining the reactions. While in Paramecium there is no 

 reaction to light, certain other reactions are produced in the 

 manner just set forth. These reactions we shall analyse in the 

 next section of this paper. 



III. Re.actions to Certain Stimuli, with Special Refer- 

 ence TO THE Part Played by the "Action System." 



A. Reactions Produced through the ''Avoiding Reaction.'' 



I. Reactions to Water Currents ; Rheotaxis. — Under rheo- 

 taxis is usually understood the orientation of the organism in 

 line with a water current, and movement with or against the 

 current. I have come across a reference to such a reaction to 

 water currents in Paramecium only in two papers dealing pri- 

 marily with reactions to the electric current — namely the papers 

 of Dale (1901) and Statkevvitsch (1903, rt). Dale says : "It 

 is sufficient to watch the behavior of Paramecium in water con- 

 tained in a tall jar in which convection currents have been pro- 

 duced, in order to be convinced of its tendency to swim with a 

 stream of water" (Dale, /. c, p. 354). He attempts to use 

 this tendency to swim with the current in explaining the move- 

 ment to the cathode in the reaction to electricity, but has no 

 farther observations on rheotaxis itself. Statkewitsch (1903, 

 a, pp. 102-104) likewise observed that Paramecia swim with 

 currents caused by the absorption of water by porous sub- 

 stances, but showed that this has nothing to do with the move- 

 ment to the cathode, since the latter occurs in the same way, 

 whatever the direction of the water currents. 



I have carefully examined the reaction of Paramecium to 

 water currents under various conditions. The reaction varies 

 with different individuals, and it is difficult to arrange the con- 

 ditions in such a way as to make the reaction a very precise 

 one. But in all my experiments a large majority of the ani- 

 mals showed the opposite relation to the direction of the current 

 from that mentioned by Dale and Statkewitsch. They turned 

 the anterior end up stream and moved against the current. 

 There were usually a number of individuals, however, that 



