Sensory Discrimination: The Chemical Sense 65 



following form: the Paramecium " merely stops, or pro- 

 gresses more slowly, and begins to swing its anterior end 

 about in a circle." As long as it does not thus get out of 

 range of the stimulus, the movement is continued. "When 

 the anterior end is finally pointed in a direction from which 

 no more of the stimulating agent comes, the Paramecium 

 swims forward" (378, p. 51). Evidently, however, these 

 are but differing degrees of a reaction whose essential 

 features are the same. 

 While Paramecium definitely avoids by means of this 



/. JL 



FIG. 4. Negative reaction of Paramecium. A is the source of stimulation. 

 1-6 are the successive positions of the animal. After Jennings (378). 



negative reaction certain chemicals introduced into the 

 water, it shows a tendency to collect in the neighborhood 

 of others. Such is the case with weak acids, with a bubble 

 of oxygen if air has been long excluded from the slide, and 

 with carbon dioxide, which in water of course produces 

 acid (378). Jennings pointed out that the inclination of 

 Paramecium to gather in groups is very likely due to the 

 attraction for them of the carbon dioxide which they ex- 

 crete. But he has also shown that this "attraction" to 

 certain chemicals does not mean the presence of a special 



