MOVEMENTS, ETC., OF FRESH- WATER PLANA PvIANS. 667 



tliey are iintnersed in it. They will simply squirm about aud 

 make no progressive movements, and hence not get into 

 the drop of substance to which they are positive. But it is 

 quite possible that by making a long enough series of experi- 

 ments on this point, one might get a solution just strong 

 enough to cause a negative reaction, and in which the 

 organisms would still move well. We would then get a 

 collection in the positive drop. The important thing, how- 

 ever, is that to the water in which they live the animals do 

 not, under any circumstances, give a negative reaction, and 

 hence under normal conditions no collections can be found by 

 a "motor reflex" method. 



It may be well, before leaving this subject, to point out tlie 

 fundamental physiological difference betAveen the Infusoria 

 and the planarians, on which the difference in the behaviour 

 towards chemicals is based. It is that in the case of tlie 

 Infusoria there is but one form of reaction (the "motor reflex" 

 turn towards a structurally defined side) regardless of whether 

 the stimulus is strong or weak, while in the case of the 

 planarian there is a qualitatively different reaction to strong 

 stimuli from that which is given to weak. When the in- 

 fusorian passes into the drop of acid it is apparently not 

 stimulated at all (for what reason we do not know). When 

 it attempts to pass from acid to water it is given a stimulus 

 which must be in the nature of things a rather weak one, yet 

 it responds with the only reaction it has, and is, as a 

 consequence, kept in the acid. With the planarian any slight 

 change in environmental conditions gives a weak stimulus, 

 aud the sjDCcimen turns towards the source of stimulation. 

 This serves, together with random movements, to get it into 

 the drop of solution ; but when it strikes again the water, 

 which again must furnish a weak stimulus, it gives the same 

 positive reaction and passes out into the water. The ability 

 to differentiate in the reactions between the strong and weak 

 stimuli gives the organism a far greater range in its 

 activities. 



Another problem whicli is of interest in connection with 



