MOVEMENTS, EW., OF FRESH-WATER PLANARIANS. 613 



towards the stimulus when applied to the cut edge was 

 obtained in several other series with this same specimen, and 

 many times with other specimens similarly mutilated. It 

 will be seen that this is the result which Avould be expected 

 if the turning away is due to lengthening of the side stimu- 

 lated. Stimulation of either side of the cut portions, inner 

 or outer, causes turning in the same direction, and that 



h 



Fig. 21. — Diagram to show the reactions to meclianical stimuli and 

 tlieir meclianisms in tlio case of a specimen in which the head 

 has been split longitudinally. Por further explanation see text. 



direction is the one in which turning would be caused pro- 

 vided each piece did actively lengthen on its outer side. 

 There seems to be no reason whatever, if the turning away 

 were due to contraction of the side opposite thnt stimulated, 

 why the specimen should not turn away from stimuli applied 

 to the cut inner edges. This it does not do. There seems 

 to be no escape, then, from the conclusion that the turning 



