MOVEIFENTS, ETC., OF FRESH-WATER FLANAIJIANS. 589 



case in tlie head region. The nearer to the anterior end of 

 the middle region the stimulus is applied, the more easily 

 will the negative reaction he produced, while back in the 

 pharyngeal region it follows even strong stimulation in fewer 

 cases. In all cases where this reaction is not produced the 

 direction of movement is either unchanged by mechanical 

 stimulation, or the anterior end may be brought around 

 vei'y slightly towards the side stimulated as a mechanical 

 i*esult of the local contraction on that side. Hy repeating 

 the strong stimuli on one side of the middle region, summa- 

 tion effects similar to those described above as taking place 

 when the head is similarly treated are not produced. The 

 animal crawls faster and faster away from the stimulus. Its 

 direction of movement is changed, but usually not more than 

 thirty to forty degrees. We see here evidence of precise 

 response to localisation of stimulus. Stimulation of the head 

 causes the animal to turn to one side, and, in case the 

 stimulus is very strong, to contract longitudinally strongly 

 before doing so. As we go back from tiie head in the 

 middle region of the body, the tendency to crawl rapidly 

 ahead away from the stimulus increases. At the same time 

 the turning away from the stimulus becomes less and less 

 marked the farther back it is applied. In no case do we get 

 any strong retraction of the anterior end, which, in case of 

 stimulation of the middle region of the bod}^, would tend to 

 bring the animal, or at any rate a part of it, into further 

 contact with the stimulus. 



Strong stimulation applied to the dorsal surface of the 

 middle region of the body causes the animal to change from 

 the glide to the crawl. This change of the form of motion 

 may be regarded as a specific reaction in response to strong 

 stimulation of the middle or posterior regions of the body. 

 Stimulation of the dorsal surface of the middle region does 

 not change the direction of the movement unless the stimulus 

 is applied near the lateral margin, in which case it may 

 cause the negative reaction, as mentioned above. 



/3. Reactions to Weak Stimuli. — Weak media nical 



VOL. 4G, PAUT 4. NEW SEUIES. Q Q 



