BEHAVIOR OF UNICELLULAR ORGANISMS. 109 



of the animal. Now, as already stated, this posterior end is 

 not at all sensitive, so that no reaction is caused. The chemical 

 continues to diffuse until it finally reaches the very sensitive 

 anterior end, when at once the typical reaction occurs, and the 

 animal swims backward into the strong solution. The reaction 

 to a chemical is perhaps then always due to stimulation at the 

 anterior end. 



Psychologically considered, we seem to have here a remark- 

 able transitional condition toward a perception of the localiza- 

 tion of the stimulus by the organism a reaction with reference 

 to the localization of the stimulus so far as motion along the 

 axis of the body is concerned, a blind reflex, without regard to 

 the localization of the stimulus, so far as motion to one side is 

 concerned. 



We may now summarize briefly the essential facts in regard 

 to the reactions of the unicellular organisms studied. The 

 reactions of these organisms may be classified into three reac- 

 tion forms : 



(1) One is the thigmotactic reaction. Starting with the 

 moving infusorian, we find that it reacts to contact with solid 

 bodies of a certain physical texture by suspending part of the 

 usual ciliary motion, so that locomotion ceases and the organism 

 remains pressed against the solid. Whether anything more 

 than this cessation of part of the usual ciliary motion is con- 

 cerned in the thigmotactic reaction is very difficult to say. 



(2) If we start with the resting individual, the simplest reac- 

 tion to a stimulus is the resumption of the usual forward motion. 

 This is the reaction that is produced when the solid substance 

 against which the creature is resting is removed ; it is also pro- 

 duced in some Infusoria when the posterior part of the body is 

 stimulated mechanically. 



(3) The third, and, for our purpose, most important reaction, 

 to which most of the so-called tactic or tropic phenomena are 

 due, may occur in either active or resting animals. It is a 

 reflex consisting of the following activities : the animal swims 

 backward, turns toward one structurally defined side, then 

 swims forward. This reaction is produced by chemical stimuli 

 acting upon any part of the body or upon the entire body at 



