Jennings, Behavior of Paramecium. 



459 



different conditions immediately in advance. This is exactly 

 what the Paramecium does. It begins to turn toward the aboral 

 side, at the same time continuing to revolve slowly on the long 

 axis. In this way the anterior end swings about in a circle and is 

 pointed successively in many different directions (Fig. 8). From 

 each direction a little water is brought to the anterior end and 

 mouth by the oral cilia. Thus the Paramecium is given oppor- 

 tunity to "try" the water in many different directions. When 

 the water coming from a certain one of these directions does 

 not show the conditions which acted as a stimulus, the ani- 



^?;y^ 



Fig. 8. Diagrams of the way in which Paramecium swings its anterior end 

 about in a circle, in reacting to stimuli, a, reaction to weak stimulus ; b, reac- 

 tion to a stronger stimulus. From each difl'erent direction a current of water is 

 brought to the anterior end. (The forward or backward component of the mo- 

 tion is omitted from the diagram). 



mal may move forward in that direction, since now there is no 

 further cause for reaction. If the original stimulus was weak, 

 the anterior end is swung about in a small circle, "trying" the 

 water 'from a number of directions varying only a little from 

 the original one (Fig. 8, a). If the stimulus was very strong, 

 after swimming backward a long distance the animal swings its 

 anterior end about a larger circle, a circle of which the longi- 

 tudinal axis forms one of the radii ; thus directions are "tried" 

 which diverge as much as possible from the original one (Fig. 9). 

 If in any of these "trials" the stimulus is again strongly re- 

 ceived, the animal may repeat the whole reaction from the be- 

 ginning — retracing its course anew, and beginning a new set of 

 "trials." 



