MOVKMBNTS, ETC., OF FRESF-WATER PLAXARIANS. 691 



observed at the anode end is, as shown by the form taken by 

 the part reacting-, a contraction of the longitudinal muscle* 

 fibres, Avhile the fibres of the circular and transverse system 

 are relaxed. In other words, the current only affects those 

 fibres which bear a definite orientation Avith relation to direc- 

 tion of its flow, viz. those which are parallel with it. Now it 

 has been shown in an earlier part of this paper that in the 

 ordinary negative reaction the turning away from the stimulus 

 is produced by a contraction of the circular, transverse, and 

 dorso-ventral fibres (principally the circular) on the side 

 stimulated. Evidently when the animal is at right angles to 

 the direction of the How of the current the only muscle-fibres 

 in the body whose longitudinal axes are in line with the 

 current are the fibres of the circular and transverse systems. 

 Unless it is assumed that the current acts differently in one 

 case from in another there is no apparent reason why, when 

 the animal is in the transverse position, the fibres which are 

 in line with the direction of the current on the anode side of 

 the body should not contract. If the fibres fulfilling these 

 conditions as to location and orientation (the circular system) 

 do contract, they will cause the anterior end to be turned 

 towards the kathode and the anode side to become convex, — 

 in other words, produce the actually observed result. The 

 fibres of the longitudinal system should not be affected, and 

 there is no evidence that they are. This explanation assumes 

 that the current produces its efl^ect by directly causing the 

 contraction of properly oriented muscle-fibres, possibly, or 

 even probably, without relation to the stimulation of any 

 sense-organs of the animal. Or, to put it in another way, the 

 responses according to this view might not necessarily be 

 reactions of the organism at all, in the sense of being* some- 

 thing that the animal does after receiving and transforming- 

 a stimulus, but are direct effects of the stimulus acting on the 

 motor organs. It has doubtless occurred to the reader that 

 another explanation is possible for these reactions, namely, 

 that they are in no way essentially different from what would 

 be produced if the animal were given strong mechanical 



