MOVBAIENTS, ETC., OF I'RKSH-WATEli PLANAKIANS. 657 



cause well-marked positive reactious iu all cases. This is 

 the ouly chemical which I liave found that causes only one of 

 the reactions. 



Distilled Water. — To distilled water applied by the 

 capillary method the organisms give a well-marked positive 

 reaction in all cases. That the reactions to very dilute solu- 

 tions of chemicals were not due to the distilled water in cases 

 where this was used as the solvent^ rather than to the chemical 

 itself, was proven in the following way : — Parallel experiments 

 were performed, using tap water as a solvent, and in every 

 case the same reaction was given to the tap-water solution as 

 to that in distilled water. At the same time the specimens 

 would not react to clear tap water applied in the same way by 

 the same tube. 



2. General Summary. — Putting all the results on the 

 effects of localised chemical stimuli together, we are forced to 

 the somewhat remarkable conclusion that practically all sub- 

 stances are both " attractive " and '' repellent " to planarians. 

 Evidently, then, the chemical composition of a substance is 

 not of the first importance in determining how the individuals 

 shall react to it; but, on the contrary, its concentration is the 

 important matter. To Aveak solutions of any chemical the 

 animals give positive responses, while to strong solutions they 

 give negative. 



Between the behaviour towards chemical stimuli and 

 towards mechanical stimuli there is a very close parallelism, 

 or, perhaps better, identity, which is evidently something of 

 fundamental importance. In order to bring this out more 

 clearly it may be well to arrange in tabular form the results 

 of the study of the reactions to these two stimuli. 



