458 Journal of Comparative Neurology and Psychology. 



same time it begins to swim backward. The moment the more 

 slender form is restored, the animal begins to swim forward. 

 Frequently only the anterior half or anterior tip is contracted 

 (Fig. 7, c); then the cilia are reversed in this region alone. 

 Again, one often sees the aboral side contract strongly, so that 

 the animal curves toward this side. At the same time the cilia 

 are reversed on this side, while they continue to strike as usual 

 on the oral side ; the animal then of course turns toward the 

 aboral side (Fig. 7, cf). Is this coincidence of the reversal of 

 ciliary movement with contraction to be considered a necessary 

 relation, so that whenever contraction occurs, the cilia must be 

 reversed? Statkewitsch (1903) shows that the same relation 

 exists in the reaction to induction shocks, so that the general- 

 ization seems very probable. 



4. Tlie Avoiding Reaction as a Factor in Behavior. — 

 Let us now leave the detailed physiology of the avoiding reac- 

 tion, and consider it as a factor in behavior; that is, its effect 

 on the relation of Paramecium to the environment. We may, 

 for the sake of a vivid realization, put the conditions in the 

 form of a problem, with a slightly subjective tinge. The Para- 

 mecium has been swimming forward without stimulation ; on 

 reaching a certain region it is stimulated. What is to be done 

 in order to avoid or escape the stimulation ? 



The first featijre of the reaction — the swimming backward 

 or stopping — of course either removes the animal from the re- 

 gion where it is stimulated, or prevents it from entering farther. 

 This reaction is, logically if we may so express it, an abso- 

 lutely correct one. Since the animal was not stimulated till a 

 certain point is reached, then zvas stimulated, in order to avoid 

 the stimulation it is sound practice to retrace the course ; in 

 other words, to restore the condition which did not stimulate. 

 With the swimming backward the direction of the water cur- 

 rents is likewise reversed, so that no more of the water from 

 the stimulating region is brought to the mouth. 



The next problem is, in what direction shall the Parame- 

 cium now swim forward so as to avoid further stimulation ? To 

 determine this, it would be well if a trial could be made of the 



