1913] Gee: Behavior of Leeches 257 



a turn of the body to the left and away from the stimulus. A 

 much stronger stimulation with the animal in the same position 

 will produce a still farther turn to the left; while a stimulus of 

 still greater intensity may cause the animal completely to reverse 

 its direction of movement, and combine with the turn the swim- 

 ming response. A very slight stimulus applied to the anterior 

 end while the animal is moving forward will often produce no 

 effect further than a local contraction of the part stimulated. 

 It has already been mentioned that to slight stimulation of the 

 posterior end. the looping response is usually given ; a much 

 stronger stimulus will immediately evoke the swimming response. 



(&) Localization of stimulus. — A slight stimulus along the 

 extreme anterior edge of the body often produces a distinct 

 positive response in which the animal inclines toward the stimu- 

 lating rod, and applies to it the anterior sucker, withdrawing 

 it quickly. The same stimulus applied a few millimeters pos- 

 terior to this region will usually cause a turn of the body in the 

 opposite direction. Other conditions continuing uniform, the 

 same stimulus applied to the right and left sides of the anterior 

 end of the leech will produce turns in opposite directions — in 

 each case a turn away from the stimulus. Stimulation of parts 

 anterior to the middle region of the body will usually produce 

 a typical anterior end response, while a strong stimulus applied 

 to the parts posterior to this will evoke the swimming response. 

 The reactions produced upon stimulating the posterior end are, 

 as previously stated, of a considerably less plastic nature, and 

 of a type quite different from those usually produced by stimu- 

 lating the anterior end. 



These diverse types of response resulting from localization 

 of the contact stimulus are what one would expect from an 

 animal which is at all complex in its structure. The stimulation 

 of a certain region affects reflex arcs so correlated in their action 

 upon end organs that unless the normal equilibrium of the 

 mechanism is upset usually an adaptive reaction is secured. 



(c) Position of the body. — The position of the body of the 

 leech at the time of stimulation is often a cause of differences 

 of response. If the anterior end of the leech is inclined rather 

 sharply towards the left, stimulation of either the right or left 



