578 



RAYMOND PEARL. 



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

 tion is the characteristic reaction given to all strong stimuli, 

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

 same tjpe of reaction which most organisms with fairly well- 

 differentiated reactions give in response to strong stimulation. 

 It takes the animal away from what might be a dangerous 

 object. 



In Planar i a 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 onl}^ the head to be turned 

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



FiG, 14. —Diagram sliowing the form of the nen;ative reaction to mccliaii- 

 ical st.iinuli. A shows the position just, bel'oie the slimuhis is 

 applied, and P> 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 tn^st often observed form of the 

 negative reaction the portion of the body which turns away 

 is that anterior to a point 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 poiut of turning 

 is farther back on the body. 



The number of degrees through which the head is turned 

 in the negative reaction depends on the intensity of the 



