450 Journal of Comparative Neurology and Psychology. 



2. Modification of the Movevicnts 2indcr Stinmlatiofi ; 

 Reaction lypcs. — In the behavior of Paramecium under the ac- 

 tion of stimuli we ma}' recognize a certain number of distinct 

 reaction types. (i) The chief one of these is that which I have 

 in former papers called the "motor reflex" or "motor reaction," 

 and which I shall call here, for reasons given later, the "avoid- 

 ing reaction." The others are (2) the movement forward from 

 the resting condition ; (3) the coming to rest of a moving indi- 

 vidual ; (4) certain features of the reaction to the electric cur- 

 rent ; (5) local contractions- of the body, and possibly (6) the 

 discharge of trichocysts. The list of reaction types thus rises 

 to a considerable number, but the last three named play almost 

 no part in the regulation of the relations of Paramecium to its 

 environment under natural conditions. We shall deal in extcnso 

 here only with the most important reaction type — the "avoiding 

 reaction." 



5". The Avoiding Reaction. — Through this reaction t}'pe 

 occur most of the marked reactions of Paramecium that have 

 often been spoken of as "tropisms" or "taxes;" in other words, 

 the reactions to stronger stimuli of all sorts. The avoiding re- 

 action consists, when well marked, of the following : the ani- 

 mal swims backward, turns toward the aboral side, then resumes 

 the forward motion. I have called this in former papers the 

 "motor reaction" or the "motor reflex." But the former is a 

 general term, properly used for any movement that takes place 

 as a response to a stimulus, and hence not fitted for character- 

 izing a special reaction type. To the second, objection has been 

 raised on thd ground that the word reflex should be used only 

 when a nerve cell is concerned ; there are perhaps other and 

 better grounds for leaving open the question whether the move- 

 ment in this reaction is in the nature of a reflex or not. For 

 these reasons I have sought for a simple expression which shall 

 bring out the essential character of the reaction without preju- 

 dice to its nature in other respects. The most general effect of this 

 reaction is to remove the reacting organism from the source of 

 stimulation and direct it elsewhere ; it may, therefore, be ap- 

 propriately called the "avoiding reaction." By this reaction. 



