MOVEMENTS, ETC., OF FHESH-WATKR TLANAHIANS. 623 



There ai-e a nninbcr of other points in the general behaviour 

 which are directly rehited to the reactions to nieclianical 

 stimuli, wliich \Yill be taken up later in connection with the 

 other reactions, 



/. Summary. — Before passino- on to a discussion of the 

 next subject, it may be well to summarise briefly the chief 

 findings with reference to the effect of mechanical stimuli on 

 planarians. 



It has been shown that the planarian responds in a well- 

 nigli perfect manner to the localisation and intensity' of 

 meclianical stimuli. Tt turns awa}- from strong stimuli (in 

 the long run harmful) applied to the side of the body ; turns 

 towards weak stimuli (in the long run beneficial, almost never 

 harmful) ; it crawls rapidly away from strong stimuli applied 

 to the posterior end; backs and turns away from similar 

 strong stinmli applied at the anterior end. 



It has been shown, further, that these reactions have all 

 the characteristics of reflex actions, complex, it is true, but 

 still reflexes. 



The mechanisms of the reactions to unilateral stimulation 

 are unilateral, and lie in the side stimulated. 



Discussion of the implications of these results on 

 mechanical stimulation, with reference to the psychology of 

 the organism and the general theories regarding the reactions 

 of oro-anisms to stimuli, is deferred till the results from other 

 sorts of stimuli are in hand. 



II. Reactions to Food and Chemical Stimuli. 



Evidently one of the most important factors in the sum 

 total of the activities of any aquatic organism is its reactions 

 to chemical substances. Its ability to receive chemical 

 stimuli and react to them must be of prime importance in its 

 struggle for existence, for in its natural habitat such an 

 aquatic organism must be almost constantly encountering 

 different chemical substances. Some of these may be harm- 

 ful and some beneficial, and it would seem that if a species is 



