180 REPRESENTATIVE SINGLE-CELLED ANIMALS 



proceeds forward by a resumption of the spiral course. A com- 

 plete cone-shaped figure is not always described, but there is 

 always a portion of such a figure. The angle of the cone may 

 vary to such an extent that in extreme cases the animal swings 

 about almost in one plane, as would be the case if a cone became 

 flattened out to form a plane surface. Under mild stimulation, 

 however, the response is not so extreme. The paramoecium backs 

 away from the obstacle, stops, describes part of a cone by con- 

 tinuing to swerve and rotate, and starts forward once more on a 

 spiral course but with the axis of the spiral at a new angle. If 

 this Une of progress brings it again in contact with the obstacle, 

 it repeats the process of backing off, describing the cone-shaped 

 figure, and proceeding in a new direction. Thus, by a series of 

 what may be called " trials," some of which may be " errors," 

 the paramoecium may eventually succeed in finding its way around 

 an obstacle. This automatic and reflex-like response has been 

 calbd the " avoiding reaction." The same kind of reaction occurs 

 in response to other situations, as when the animal is confined 

 within the trap-like meshes of the cotton fibers sometimes used in 

 laboratory study, or in response to other forms of stimulation 

 likely to be encountered in nature. With electrical and other 

 stimuli not encountered in nature, the responses may be different. 



When many paramoecia are crowded together, the individuals 

 seem to be merely darting backward and forward in aimless fashion. 

 When, however, the response of isolated individuals is studied 

 vvith reference to particular stimuli, the behavior is found to be 

 as above described. The avoiding reaction is given in response 

 to mechanical, thermal, and chemical stimuli, water currents, 

 gravity, and centrifugal force. This form of activity so much 

 resembles the " trial and error " behavior of higher organisms that 

 it seems justifiable to extend this term even to the protozoa. The 

 use of such a phrase does not imply that the paramoecium acts 

 with any degree of inteUigence, that it " tries " and " errs," and 

 recognizes its mistakes; but only that it responds mechanically 

 with a stereotyped form of behavior, which, if repeated a sufficient 

 number of times, will usually bring the individual under conditions 

 that are favorable for its normal activities. 



As the animal swims through the water or swerves in giving the 

 avoiding reaction, the beat of the cilia in this region draws a cone of 

 fluid against the buccal groove, which thus " samples " the water 



