THE BEHAVIOR OF INFUSORIA; PARAMECIUM 51 



sample of such water merely stops, or progresses more slowly, and 

 begins to swing its anterior end about in a circle, as in Fig. 37, thus 

 "trying" a number of different directions. As long as the oral cilia 

 continue to bring it the weak salt solution or the warmed water, the 

 animal holds back, and continues to swing its anterior end about in a 

 circle. When the anterior end is finally pointed in a direction from 

 which no more of the stimulating agent comes, the Paramecium 

 swims forward. The reaction in this case is a very precise and delicate 

 one; in a cursory view the animal seems to turn directly away from 

 the region of the stimulus, the revolution on the long axis and swing- 

 ing of the anterior end in a circle being easily overlooked. 



Between this delicate reaction and the violent one first described 

 there exists every intermediate gradation, depending on the intensity of 

 the stimulation. 



Paramecia react to most of the different classes of stimuli which act 

 upon them, in the way just described. Mechanical stimuli, such as 

 solid obstacles, or disturbances in the water; chemicals of all sorts; 

 heat and cold ; light that is sufficiently powerful to be injurious ; electric 

 shocks, and certain disturbances induced by gravity and by centrifugal 

 force, all cause the animal to respond by the avoiding reaction, so that 

 it escapes if possible from the region or condition that acts as a stimulus. 

 Certain peculiarities and special features in the action of the different 

 classes of stimuli will be taken up separately in the following chapters. 



Stimulating agents produce the same reaction when they act on the 

 entire surface of the body as they do when they reach only the anterior 

 end or oral groove. This is shown by dropping the animals directly 

 into a \ per cent solution of sodium chloride, or into corresponding 

 solutions of other chemicals ; or into hot or cold water. They at once 

 give the avoiding reaction ; they swim backward, turn toward the aboral 

 side, then swim forward, and this reaction may be repeated many times. 

 If the stimulating agent is not so powerful as to be directly destructive, 

 the reaction ceases after a time, and the Paramecia swim about within 

 the solution as they did before in water. 



This experiment shows clearly that the cause of the avoiding reaction 

 does not lie in the difference in the intensity of the chemical on the two 

 sides or two ends of the animal, as is sometimes held. For as we have 

 just seen, the animal reacts in the same way when the entire surface of 

 the body is subjected equally to the action of the chemical or the changed 

 temperature. It is clear that the cause of the reaction is the change 

 from one solution or temperature to another. This is evident further 

 from the fact that the animal reacts as a rule when the change occurs, 

 but ceases to react after the change is completed. To constant con- 



