64 The Animal Mind 



strongly than the rest, and that the animal compensates 

 for the turning thus occasioned by turning on its long axis. 

 Its reactions to stimulation Jennings has shown to be only 

 two in number. First, there is a very definite avoiding or 

 negative reaction. This is given in response to decided 

 mechanical stimulation at the anterior end, as when the 

 animal swims rapidly against an obstacle, and also in 

 response to chemical stimulation, to strong ultra-violet 

 rays (299), and to temperatures above or below a certain 

 middle region called in this case, as in analogous cases with 

 other animals, the optimum. For Paramecium it lies be- 

 tween 24 and 28 C. The negative reaction consists, 

 according to Jennings, of the following process : the animal 

 darts backward, reversing the beat of its cilia, turns to- 

 ward the aboral side (that opposite to the oral groove) 

 by increasing the beat of the oral cilia and lessening the 

 compensating rotation, and continues on a forward course 

 that is now at an angle with its former line of motion. 

 If this new course carries it clear of the stimulus, it con- 

 tinues on its way ; if not, repeated contact with the stim- 

 ulus causes a second reaction, the Paramecium always 

 turning in the same direction, so that ultimately it avoids 

 the source of stimulation (361, 378) (Fig. 4). Differing 

 strengths of stimulus produce the reaction with different 

 degrees of violence. When a very strong stimulus is en- 

 countered, the animals " respond first by swimming a long 

 way backward, thus removing themselves as far as possible 

 from the source of stimulation. Then they turn directly 

 toward the aboral side, the rotation on the long axis 

 completely ceasing. In this way the animal may turn 

 directly away from the drop [the stimulus] and retrace its 

 course" (378, p. 50). On the other hand, when the stim- 

 ulus is very weak the reaction may be reduced to the 



