MOVEMENTS, ETC., OF FKESH-WATKE PLANARIANS. 693 



same point. In the case of the Infusoria the current causes 

 an entirely different reaction from that produced by any 

 other known stimulus. 



For these reasons, then, I am inclined to think that in the 

 case of the flat-worm the current affects certain definitely 

 oriented muscle-fibres directly, and by this means produces in 

 the main the characteristic reactions. That the current does 

 not also stinuilate the sense-organs, and so act like other 

 stimuli applied to the same places, I am not prepared to say, 

 but it seems probable that the phenomena observed are not 

 primarily caused by such action. 



It has been brought out by inference that the cilia play 

 no part in the electrotactic reaction of planarians. This is 

 the true state of the case. The current in any intensity 

 sufficient to cause the definite reactions stops immediately, so 

 fur as I have been able to observe, all ciliary movement. The 

 evidence for this is twofold. First, all gliding movement 

 stops in effective currents ; and second, by direct observation 

 of specimens crawling ventral side up on the surface film no 

 ciliary currents can be observed while the electric current 

 acts. This result is of interest in connection with the 

 reactions of the rhabdoccele Steno stoma leu cops, O. 

 Schm. This form, which normally moves freely through the 

 water by the activity of its cilia, reacts to the electric current 

 in essentially the same Avay as do the Infusoria (cf . Pearl, : 00) . 

 That is to say, the cilia on the kathode half of the body take 

 a reverse position when the current is made, and their effective 

 stroke is towards the anterior end. The different relations of 

 the cilia in different positions of the body are shown in 

 Fig. 48. This relation of the ciliary beat, coupled with the 

 form of the body, causes, as a mechanical necessity (cf. 

 Ludloff, ^95), the animal to orient with the anterior end 

 towards the kathode. This method of reaction of S t e n o s t o in a 

 I worked out by precisely the same methods as I used in a 

 previous study of the electrotaxis of tlic lul'iisdria (: 00). 

 This reversal of the position of the cilia as a result of tlie 

 action of the current has hitherto been observed only in 



