Jennings, Behavior of Paramecium. 471 



ing resisted. As we know, exactly such changes act as stim- 

 uH, and the animal reacts, as we have seen, in the usual way. 

 It swings its anterior end about in a circle, so that the body 

 axis occupies successively many positions, and continues or re- 

 peats this reaction as long as it is subjected to the changes men- 

 tioned But when it comes into a position such that its rela- 

 tion to the current remains constant, it no longer reacts, for to 

 constant conditions, unless destructive, Paramecium soon be- 

 comes acclimatized. Such a position is found only when the 

 axis of the spiral path coincides with the direction of the cur- 



Fig. II. Diagram to illustrate the cause of the reaction to currents of 

 water. The straight arrows indicate the direction of the current. The 

 swinging of the unoriented Paramecium in its spiral course from the position b 

 to a is resisted by the current, while the movement from a to ^ is assisted. (The 

 same diagram illustrates the conditions in the reaction of gravity, if the straight 

 arrows represent the direction of gravity). 



rent. In this position the animal of course still swims in a 

 spiral, the anterior end describing circles about the axis of the 

 spiral. But in every phase of the path the axis of the body 

 lorms the same angle with the axis of the spiral, and hence 

 with the direction of the water current, so that its relation to 

 the current remains constant, and there is no farther cause for 

 reaction. Orientation has been attained through the "method 

 of trial and error." 



But why do the majority of the animals become oriented 

 with anterior ends against the current ? Our description thus 

 far accounts for the position of the body axis, but not for the 

 more usual direction of the anterior end. We know that as a 

 rule when Paramecium is subjected to changes of opposite char- 



