578 



RAYMOND PEARL. 



a. Reactions to Strong Stimuli. — Tlie negative reac- 

 tion is tlie characteristic reaction given to all strong stimuli, 

 wliether mechanical or of some other sort. It is, further, the 

 same type of reaction which most organisms with fairl}' well- 

 differentiated reactions give in response to strong .'stimulation. 

 It takes the animal away from what might be a dangerous 

 object. 



In Planaria the portion of the body which takes part 

 in the turning away varies with the strength of the stimulus 

 to a certain degree. Stimuli just strong enough to call forth 

 the negative reaction will cause only the head to be turned 

 away. The first turn away of the definite reaction never in- 



FlG. Id. — Diaf^ram sliowincj ilie form of tlio no|Tative reaction to mcclian- 

 iral stimuli. A sliows tlio position just, before the stimulus is 

 applied, and 1' the position after the reaction. 



eludes any of the body back of the pharynx, so far as I have 

 observed, except in the case of very strong and repeated 

 stimuli. In the typical and most often observed form of the 

 negative reaction the portion of the bodj' which turns away 

 is that anterior to a poiut about halfway between the level 

 of the eyes and the point of origin of the pharynx. This is 

 shown in Fig. 14. With stronger stimuli the point of turning 

 is farther back on the body. 



The uiiinb(>r of degrees through which the head is turned 

 in tli(^ negative reaction dop(Mids on ihc intensity of the 



