692 KAYWOND PEARL. 



stimuli on tliosc parts of the body wliich arc nearest the anode 

 in the several positions. In other words, the constant current, 

 from the standpoint of the planarian, produces the same 

 effect on the anode side or end of the body that a strong- 

 mechanical stimulus applied in the same place would. 



Which of these two views is the correct one the planarian 

 does not show cleai'ly. Yet there is some inferential evidence 

 which makes it seem probable that the first view as to the 

 cause of the reaction is the correct one, viz. that the current 

 produces direct contractions of muscle-fibres oriented in line 

 with its direction. The evidence for this view is as follows : — • 

 (1) In the case of specimens which have been for some time 

 under the action of the current, and are, as has been 

 mentioned, almost completely paralysed, the essential features 

 of contraction on the anode side or end can still be pro- 

 duced by a fairly weak current. At the same time it takes a 

 very strong mechanical stimulus to get any reaction from 

 these pieces, indicating that their sense-organs are almost 

 completely paralysed, and their general sensitivity gone. If 

 the current acts merely as a stimulus qualitatively like 

 others which produce the same reactions, it is not apparent 

 why it should be effective in weak intensities when another 

 stimulus fails in strong intensity. If it acts directly on the 

 muscles we should expect that it would be capable of pro- 

 ducing an effect after the general sensory functions had been 

 lost. (2) The contractions produced by the current are 

 sharply localised, i. e. they involve only a certain definite 

 part of the body whether the current is strong or weak 

 (within certain limits) ; whereas mechanical stimuli applied 

 to the same places with an intensity sufficient to cause the 

 same definitive reaction will also cause a marked general 

 response of the whole organism. This is just what would ])c 

 expected if the current affects only the muscles oriented in 

 line with it and lying at the anode pole of the worm, (o) 

 By analogy with other forms — for example, the Protozoa — it 

 Avould be expected that the curreiit would |)roduce some 

 other effect than that of an ordinary stimulus applied at the 



