MOVEMENTS, ETC., OF FRESH-WATER FLANARTANS. 679 



mens will remain in tlie normal position on the bottom of a 

 disli when there is a layer of plant debris a half-centimetre 

 in thickness above them, and necessarily in contact with the 

 dorsal surface. The negative thigmotaxis of the dorsal 

 surface plays its part in the righting reaction in determining 

 in which direction the turning shall take place. 



It has so far been shown that the view of the mechanism 

 of the righting reaction presented is in accord with all the 

 mechanical principles necessary to produce the observed 

 results. The attention may now be turned to an examination 

 of the evidence that this mechanism is the one which actually 

 brings about the reaction. This evidence is obtained from 

 experiments with worms on which operations have been per- 

 formed. Obviously, if the mechanism described is the one 

 by which the reaction is produced, any operation which 

 destroys or throws out of working order any essential part 

 of the mechanism will cause the typical reaction to disappear, 

 or be greatly modified. 



We may first consider the reactions of the pieces resulting 

 from cutting the animal in two ti-ansversely in the middle of 

 the body. It is found that each of the pieces resulting from 

 such a cut will perform the righting reaction in the typical 

 manner. The spiral is formed, but there is usually only one 

 half-turn of the body, i. e. just enough to bring the anterior 

 end ventral side down. This then attaches itself to the 

 bottom and starts gliding, unwinding the spiral just as under 

 normal circumstances. There is observable the same rela- 

 tion between the side of the body, which is in contact with 

 the bottom and the direction of the turn as in the normal in- 

 dividual. The only striking difference in the behaviour of 

 the anterior and posterior pieces is that the reaction time of 

 the former is much shorter than that of the latter. Tlie 

 anterior piece rights itself practically as quickly as does the 

 normal animal, while the posterior piece took in one series of 

 experiments 1 minute and 38* 1 seconds (average of ten trials) 

 for complete righting. This slower rigliting reaction is 

 another expression of the generally lowered tonus of such 



