Harper, Reactions of Penchceta. 573 



under different degrees of stimulation. For reasons to be given 

 it is convenient to classify the negative reactions under five types. 

 Negative Eeactlon Types. — I. To a very weak stimulus the v^orm 

 usually responds by checking forward movement, with sometimes a 

 slight pause, after which it ordinarily turns away from the stimu- 

 lated side and creeps forward. 



II. To a slightly stronger stimulus, the preceding reaction may be 

 accompanied by some shrinking back of the head during turning. 

 This is transitional to the second ^type in which the anterior end 

 recoils rather suddenly, after which a pause may sometimes occur. 

 The worm then usually turns away from the stimulated side and 

 creeps forward. 



III. With increasing stimulus, stronger shrinking movements 

 occur, after which in the third type of response the worm creeps 

 backward for a while, then forward and turns ordinarily away from 

 the stimulated side as in the other reactions. In a typical case, 

 during the interval of backward creeping there is a period of unco- 

 ordinated movements in which the anterior end attempts to extend. 

 At each interval in the extension of the posterior end the anterior 

 end makes a negative turn and these increase in strengih at each 

 trial until it succeeds in regaining the lead. In some other cases 

 the activity of both ends subsides and a pause in the reaction occurs. 



IV. After a still stronger stimulus and a stronger shrinking move- 

 ment of squirming or leaping, in the fourth type of reaction the 

 worm begins immediately to roll sidewise for a time, after which it 

 creeps backward usually for a while, then forward as before. 



V. In the strongest type of response, after leaping about for a 

 time the worm begins at once to creep rapidly forward, manifesting 

 a tendency sooner or later to turn away from the stimulated side. 

 Another common form of this reaction is for the worm to make a 

 single leap, turning end for end and beginning at once to crawl 

 rapidly forward. This movement differs only in extent and sud- 

 denness from the second type. It is the ordinary reaction of Lum- 

 bricus to a maximum stimulus. Perichseta more commonly gives its 

 characteristic leaping movements for a time instead of a single 

 sudden turn. This strong turning end for end may occur at the 



