MOVEMENTS, ETC., OF FRESH-WATER PLANARTANS. GOO 



out slig-litly ventrally, thus bringing the ventral cilia in close 

 contact with the bottom, as is necessary for their effective 

 working. At the same time it lengthened along its outer 

 side, thus forcing the anterior end around towards the side 

 opposite from the stimulus. That the '' side opposite " had 

 nothing to do with the turning could be observed in many 

 cases directly, for this side (B) would remain in an almost 

 entirely relaxed condition after the stimulus was given, and 

 not get any effective hold on the bottom so that it could 

 affect the movement. It was further possible by a little 

 manipulation to get the piece B laid over on A so as to be 

 practically entirely supported by it, as shown in Fig. 17, h. If 

 with such conditions the worm was stimulated rather strongly 

 on the A side of the head, it gave a strong negative reaction, 

 the poiiit about which the turn was made being as far back 

 as X. Evidently with part B up on the dorsal surface of A, 

 and consequently having no hold on the bottom, it could 

 have no effect in the reaction. The reaction must have been 

 due to the side A alone. The same thing could be shown by 

 very gently lifting on a needle the side B so that it svas not 

 in contact with the bottom, and then stimulating A, when 

 again the negative reaction occurred. This experiment I 

 have repeated with variations many times, but always with 

 the same result, showing that the side stimulated is the 

 effective one in producing the turning. 



It may be mentioned here that the effect of strongly 

 stimulating the posterior end of either of the two pieces of a 

 specimen slit in this way was to cause a local contraction of 

 the piece stimulated, and a crawling movement of the short 

 portion of the body in front of the slit. This crawling was 

 not very effective, since so small a portion took part in it, 

 but it is of interest to note that what crawling appeared 

 involved only the uncut part of the body. 



It being established that the side stimulated produces the 

 turning, the question may be raised, how, supposing in these 

 longitudinally split individuals that this side does produce 

 the reaction, is it known that it does this by lengthening 



