THE MOVEMENTS AND REACTIONS OF AMCEBA. 227 



CLASSES OF STIMULI TO WHICH AMCEBA REACTS. 



The simple naked mass of protoplasm reacts to all classes of stimuli 

 to which higher animals react (if we consider the auditory stimulus 

 merely a special case of the mechanical stimulus). Mechanical stimuli, 

 chemical stimuli, temperature differences, light, and electricity — all 

 control the direction of movement, as they do in higher animals. 



TYPES OF REACTION. 



Amoeba has two chief types of reaction, by one or the other of which 

 it responds to most stimuli. These we may call the positive and the 

 negative reactions. As a third type we must distinguish the food 

 reaction, which cannot be brought completely under either of the two 

 chief types of reaction above mentioned. 



(i) The positive reaction consists in pushing out the body substance 

 toward the source of stimulus and rolling in that direction. 



(2) The negative reaction consists in withdrawal of body substance 

 and rolling in some other direction — not necessarily in the opposite 

 direction. 



(3) The food reaction is not sharply definable. Its most character- 

 istic features consist in the hollowing out of the anterior end and in 

 the pushing out of pseudopodia at each side of and over the food body. 

 It involves also the positive reaction above characterized. 



RELATION OF THE DIFFERENT REACTIONS TO DIFFERENT STIMULI ; 

 ADAPTATION IN THE BEHAVIOR OF AMCEBA. 



(a) The positive reaction is known to be produced by weak mechanical 

 stimuli ; it is probably produced also by weak chemical stimuli (in the 

 reactions to food, pp. 193-202). The positive reaction to weak mechan- 

 ical stimuli serves the purpose of bringing the floating animal to a sur- 

 face on which it can creep. The positive reaction to food substances 

 (mechanical and chemical stimuli), of course, serves to obtain food. 

 The positive reaction is thus, as a rule, performed under such circum- 

 stances as to be beneficial to the organism ; /. c, it is directly adaptive. 



(l>) The negative reaction is produced liy powerful stimuli of all 

 sorts. Such powerful stimuli are, as a rule, injurious, and the nega- 

 tive reaction tends to remove the Amoeba from their action ; it is, there- 

 fore, directly adaptive. This is true of the negative reaction to light 

 as well as to other stimuli, for light is known to interfere with the activi- 

 ties of Amoeba. The reaction to the electric current is of exactly the 

 character that would be produced by a strong stimulus on the anode 

 side, but owing to the peculiarity of the current the reaction does not 

 assist the Amoeba to escape. The reaction to the electric current can 

 not then be considered adaptive ; this stimulus forms, of course, no 

 part of the normal environment of an Amoeba. 



