672 RAYMOND PKARL. 



befoi'e reacliing the bottom, provided the distauce tlirough 

 which the drop was made was greater than 7 — 10 cm. This 

 result seems to indicate that there is something more con- 

 cerned in the righting reaction than the negative thigmo- 

 taxis of the dorsal surface for the following reasons : — (1) the 

 dorsal surface is not in contact with any solid of this experi- 

 ment ; (2) it is in contact with water only, just as is normally 

 the case when the animal is right side up. It may be 

 objected that the experiment is not conclusive, because, as a 

 result of the falling, there is an increased water-pressure on 

 the dorsal surface, and this may act as a thigmotactic 

 stimulus. This objection is met by two different facts. 

 First, the animal gives the righting response in some cases 

 while falling ventral side down, under which circumstances 

 there can be no increased pressure on the dorsal surface. 

 Second, if a stream of water from a pipette is directly 

 squarely against the dorsal surface of a worm normally 

 gliding about on the bottom the righting reaction is not 

 induced, regardless of the force of the stream. Evidently 

 this stream of water against the dorsal surface produces a 

 pressure on the dorsal surface similar to that when the 

 animal is falling, and if the righting reaction in the falling 

 is due to increase of pressure on the dorsal surface, we might 

 suppose that some indication of it would be produced in this 

 case. As a matter of fact it is not. We must conclude, 

 then, that the righting reaction is due, at least in very large 

 part, to some other cause than the negative thigmotaxis of 

 the dorsal surface. This is indicated also by the fact that 

 when solid bodies are laid on the back of a specimen in its 

 normal position, the reaction which is caused is not the 

 righting action, as would be expected if the latter were 

 due solely to the negative thigmotaxis of the dorsal surface. 

 'J'he righting reaction is clearly not due to gravitation, since 

 the flat-worms move on the surface film with the dorsal 

 surface downward. This leaves, as the only factor to which 

 the reaction can be duo, the positive thigmotaxis of the 

 ventral surface. I am convinced that it is to this factor that 



