678 RAYMOND PEAT?L. 



ing of one side of the body. The determinant of this is 

 evidently a difference of tension on the uppei^ and lower sides, 

 the spiral turning towards the side of greatest tension,^ This 

 greatest tension is evidently, then, in the normal reaction on 

 the dorsal surface, as we should expect on a priori grounds, 

 since that is the part directly stimulated. 



To sum up, the spiral righting reaction of the planarian, 

 as I have worked it out, is due to an elongation of that side 

 of the body whose dorsal surface is in contact with the solid, 

 while the opposite side of the body actively maintains its 

 original length. As the elongation occurs the various parts 

 of the body rotate freely about its long axis, and hence the 

 whole worm takes on the spiral form. The spiral turns 

 towards the dorsal surface in every case (i. e. so as to bring 

 the ventral surface of the head down), as a result of the 

 greater tension of the dorsal musculature on the elongating- 

 side. 



The reaction is thus seen to be of almost the same cha- 

 racter as the ordinary negative reaction to strong mechanical 

 stimuli, in that the primary reaction is an extension of the 

 side stimulated. The difference between the two is that in 

 one case there is a bending of the longitudinal axis of the 

 body, while in the other there is a rotation about this axis. 

 On the view just given of the mechanism of the righting 

 reaction the specific parts played by the positive and negative 

 thigmotaxis of the ventral and dorsal surfaces are evident. 

 The positive thigmotaxis of the ventral surface is the primary 

 cause of the whole reaction, and is evidently the stronger 

 factor of the two, as shown by the experiments of laying 

 solid bodies on the dorsal surface of the animal when in a 

 normal position. It will be recalled that such treatment does 

 not call forth the specific righting reaction. Further 

 evidence of this same thing is found in the fact that speci- 



' The statements as to tlie mechanical principles of a spiral have been veri- 

 fied with different sorts of models, including plastic clay, rubber bands, etc. 

 Lack of space will not permit the enumeration of these experiments in detail, but 

 anyone can verily for iiimself the various statements with very little trouble. 



