1913] Gee: Behavior of Leeches 209 



2. Random IMovements 



Too mueli stress cannot be placed upon the part played by 

 random movements in the behavior of the leech. Its life is one 

 of repeated trial, and the adaptiveness of its selective action from 

 among the many varying factors of its environment usually serves 

 to steer the animal effectively out of regions unfavorable to its 

 continued existence, and into proper environmental conditions. 



These random movements are effected mainly by the action 

 of the dorsi-ventral and lateral longitudinal muscles of the body, 

 the circular muscles performing a more or less accessory function. 

 The movements, depending largely upon the strength of the 

 evoking stimulus and the ph.ysiological condition of the organism, 

 may vary from a slight, slow vertical or lateral turn to a com- 

 plete and rapid turn to the right or left, resulting in an entire 

 reversal of direction of movement in the animal. Upon the 

 animal's coming into an unfavorable environment, such as an 

 area of superoptimum light-intensity, the number of random 

 movements is very much increased, with the final result that it 

 becomes oriented and moves away in a direction opposite to the 

 source of light. This in its normal habitat would serve in most 

 cases to remove the leech into a region more nearly its optimum 

 of light-intensity. Chemical stimuli, favorable or unfavorable, 

 serve to intensify the random responses: the one being followed 

 up, the other usually successfully avoided through many trials. 

 Higher temperatures than normal and increased sensitivity 

 resulting from mechanical stimuli such as contact or jars produce 

 the same general effect. 



Nor is it necessary, particularly in the case of the nephelid, 

 for some stimulus to be applied to observe this tendency to 

 random movements. Almost every looping response of the ani- 

 mal in its progress forward is preceded by a series of random 

 movements of the anterior end. So thorough is the exploration 

 of the advance ground, at times, that the body of the animal 

 describes several very wide arcs of a circle before orientation 

 is finally effected. This tendency to "prove all things, hold fast 

 that which is good" is perhaps the most striking single charac- 

 teristic of the behavior of leeches, and undoubtedly, due to the 

 flexible nature of the response, serves a most important adaptive 

 purpose in the life of the animal. 



