MOVEMENTS, ETC., OF EKESH-WATEK PLANAHIANS. 



629 



time, since the stimulus is a rather slroug uue, the reaction 

 is a very precise one, and tlie subsequent movement carries 

 the animal directly to the food (Fig. 28, 7). This behaviour 

 is typical for this sort of stimulation, but may vary in its 

 component phases, depeuding on the relative strength of the 

 stimulus — the distance from the food Avheu first stimulated. 



N B 



Fig. 28. — Diagram sliowiiig llie reactions of Plaiiaria l.o food (A) 

 from wliicli juices have been diU'using into t.lie waitr fur some time. 

 B, B, B, represent tlic elFective margin of llie dilFusiDU area of llie 

 food A. 1, 2, 3, 4:, 5, G, and 7 are successive positions taken by 

 the organism. 



Thus either two or as many as four positive reactions niay be 

 necessary to bring the animal to the food. This shows 

 clearly that with reference to chemical stimidi, the precision 

 of localisation of the jxjsitive reaction decreases as the in- 

 tensity of the stimulus diniiuishes. Indeed, 1 have observed 

 what is evideutly au uulucalised positive reaction, although 



