The Protozoa 141 



stimulus is applied at its anterior end, the cilia reverse themselves and 

 the animal swims backward for a short distance away from the object 

 or substance causing stimulation. The forepart of the animal then 

 swings about, using the posterior part as a pivot, and the animal again 

 moves forward. If it again comes into an undesirable medium, the 

 same process is repeated. As the animal backs up from an unpleasant 

 stimulus, using its posterior end as the pivot upon which to turn, various 

 samples of the surrounding medium are brought into the oral groove, 

 so that, as soon as these samples of liquid no longer contain the unpleas- 

 ant stimulus, the animal moves forward. 



The important point to remember here is that Paramoecium has 

 only two reactions, the going forward and the going backward. 1 Much 

 erroneous interpretation may be avoided if this be remembered when 

 the study of the animal mind, or animal psychology, is taken up later; 

 for, no matter how many hundreds of times an animal of this kind may 

 make an experiment, it always continues this trial-and- error method of 

 going forward, bumping into something that is antagonistic to itself, 

 backing up, and again coming forward until it accidentally gets into a 

 medium that is satisfactory. In fact, there are some substances, such 

 as acetic acid, to which Paramoecia react in a peculiar manner. If a 

 drop of this acid be placed before the animal, it will enter the liquid ; 

 but once within the acid-drop it will react to the surrounding water 

 in a negative manner ; that is, it will come to the edge of the acid-drop 

 and then back away again and again. Then, the trial-and-error method 

 may be observed when heat is applied to the surrounding media. The 

 animal tries almost every direction until it finds some method of escaping 

 from the unfavorable stimulation. The optimum temperature is nor- 

 mally between 24° and 28° C. 



There are positive reactions of Paramoecium also, such for example 

 as its habit of lying against solid objects. Paramoecia are negatively 

 geotropic, in that they usually come toward the upper portion of the 

 water in which they are placed. The animals usually swim upstream, 

 and it is supposed that the reason for this is that the current might 

 interfere with the beating of the cilia if another direction were taken. 



It is generally supposed that it is the physiological condition of 

 Paramoecium which determines the character of any response to a given 

 stimulus. This means merely that the actions are more or less spon- 

 taneous and due to the internal condition of the animal (autogenous). 

 This internal condition changes, however, with the different amounts 

 and qualities of food and digestion. One physiological state really 

 resolves itself into another. This "becomes easier and more rapid after 

 it has taken place a number of times," giving ground for the belief that 

 stimuli and reaction have a distinct effect upon succeeding responses. 



x These two reactions are, of course, in addition to the animal's regular revolving method of 

 locomotion. 



