108 BEHAVIOR OF THE LOWER ORGANISMS 



are those shown in what we have called the avoiding reaction. This, 

 as we have seen, consists essentially in reversing, stopping, or slowing 

 up the forward motion, then swerving more than usual toward the aboral 

 side, while at the same time the rate of revolution on the long axis is 

 decreased. By this combination of movements Paramecium responds 

 to most effective stimuli that act upon it. By it are produced both 

 negative and positive reactions. 



B. Causes of the Reactions, and Effects produced by them 



Examination has shown us that the cause for this reaction is some 

 change in the conditions; usually some change in the relation of the 

 animal to the environmental conditions. Such changes are brought 

 about chiefly by the movements of the animal. In certain cases 

 they are due to the direction of movement, carrying the animal into 

 environmental conditions which stimulate it; in other cases they are 

 due to the axial position taken by the animal, this resulting in internal 

 or external disturbances which act as stimuli. 



These stimuli produce, as we have seen, not a single, simple, definitely 

 directed movement, comparable to the typical reflex act. On the con- 

 trary, stimulation is followed by varied movements, made up of several 

 simultaneous or successive factors, each of which may vary, as we have 

 seen in detail, more or less independently of the others. These move- 

 ments produce varied effects, as follows: (i) They place the animal 

 successively and in a systematic way in many different axial positions 

 (see Fig. 38); (2) they cause it to move successively and systemati- 

 cally in many different directions ; (3) they subject it successively 

 to many different environmental conditions, of temperature, light, 

 chemicals, mechanical stimuli, etc. Now, it is evident that in this way 

 the animal is practically certain to reach finally a position, direction of 

 movement, or environmental condition, that removes the cause of stimu- 

 lation, since the latter was due to something wrong in one of these 

 respects. The reaction then ceases, since its cause has ceased; the 

 animal therefore retains the axial position, direction of motion, or en- 

 vironmental condition thus reached. The method of reaction is then 

 of such a character as to bring about whatever is required for putting 

 an end to the stimulation, whether this requirement is one of orienta- 

 tion, of general direction of locomotion, or of the retention of certain 

 environmental conditions. 



Thus the behavior and reactions of Paramecium consist on the whole 

 in performing movements which subject the organism to varied condi- 

 tions (using this word in the widest sense), with rejection of certain of these 



