684 EAYMOND PEAEL. 



formed for tlic purpose of testing the righting reactions after 

 operations, but since none of them bring out anything 

 different in principle from the results already given, they will 

 not be reported here. But it may be said in general, that all 

 the experiments gave the same results with reference to the 

 mechanism of the reaction, namely, that so long as the 

 mechanism described above was intact the typical spiral re- 

 action was given ; when this mechanism was destroyed or 

 injured the reaction was not given, but the animal, if it 

 righted itself at all, did it by a different method. 



When the animal falls freely in the water the righting re- 

 action is induced because the ventral surface is no longer in 

 contact with a solid. There is no reason for thinking that 

 the mechanism of the reaction in this case is any different 

 from what it is when the animal is placed in an inverted 

 position on the bottom. The direction in which the spiral is 

 thrown in the case of the falling animal is probably deter- 

 mined by slight differences of pressure on the two sides of the 

 body. 



c. Summary. — The flat-worm is positively thigmotactic on 

 its ventral surface, and negatively thigmotactic on its dorsal 

 surface. As a result of this it gives a characteristic righting 

 reaction whenever the normal relations of either surface are 

 changed. This righting reaction consists in throwing the 

 body into a spiral in such a way as to bring the ventral sur- 

 face of the anterior end down into contact with a solid (in all 

 cases except when the animal is dropped into free water). 

 The anterior end starts gliding and unwinds the spiral, 

 thus righting the whole body. The thigmotactic reaction 

 may be modified by chemical and other stimuli. All the 

 evidence shows that the spiral righting reaction is due to a 

 lengthening of the side whose dorsal surface is in contact 

 with a solid, while the other side of the body keeps the long 

 axis straight. The direction of the turn in the spiral is 

 determined by the side of the body which is in contact with 

 the solid. This reaction is thus seen to be closely related to 

 the negative reaction to mechanical and chemical stimuli, so 



