Spatially Determined Reactions 173 



determined responses that is not, indeed, ideally satis- 

 factory, but may serve our purpose, divides them into five 

 groups : 



1 . Reactions adapted to the position of a single stimulus 

 acting at a definite point on the body. , 



2. Reactions to a continuous stimulus, which involve the 

 assumption of a certain position of the whole body with 

 reference to the stimulus : orienting reactions. 



3. Reactions to a stimulus that moves, i.e., that affects 

 several neighboring points on the body successively. 



4. Reactions adapted to the relative position of several 

 stimuli acting simultaneously. 



5. Reactions adapted to the distance of an object from 

 the body. 



These forms of behavior will be successively discussed. 



46. Class I : Reactions to a Single Localized Stimulus 



Responses to stimulation that are adapted to the point 

 of application of the stimulus are to be found among very 

 simple animals. They may be subdivided into three 

 groups: first, cases where the part of the animal that 

 reacts is the part directly affected by the stimulus ; second, 

 cases where the whole animal reacts by a movement in the 

 appropriate direction; and third, cases where a part of 

 the body not directly affected by the stimulus moves toward 

 the point stimulated. 



i. Amoeba furnishes an example of the first class. Its 

 negative reaction occurs by the checking of protoplasmic 

 flow at the point where a strong mechanical stimulus affects 

 the body ; its positive reaction by a flowing forward of the 

 protoplasm at the point where a weak stimulus acts, and its 

 food-taking reaction by an enveloping flow on both sides of 



