552 RAYMOND PEARL. 



variably causes the portion in front of the cut to advance by 

 tlie crawling movement, and, as has been mentioned in the 

 preceding section, at the same time frequently causes the 

 posterior piece to crawl backward. There is no reason to 

 suppose that the operative procedure acts in this respect in 

 any otlier way than merely as a strong mechanical stimulus 

 applied at the point of the cut. Other stimuli which induce 

 the backward crawling have been taken up in the preceding 

 section. 



c. Movement on the Surface Film. — Motion on the 

 surface film is practically confined to the gliding movement. 

 This gliding is slower in rate than that on the bottom, largely 

 on account of the greater flexibility of the surface on which 

 the animal is moving. While the mechanism of the move- 

 ment is the same in the two cases, the surface film is elastic, 

 and does not give so firm a basis as does a solid body. The 

 effect of this elasticity of the film is very well seen when the 

 animal attempts to change its course and turn to one side. 

 The film offers little resistance to the posterior end, so that 

 this cannot easily serve as a fixed point for the anterior part 

 to turn about. Furthermore, in case the anterior end is left 

 in contact with the film when the turn is attempted, as is 

 usually the case, there is almost as much resistance against this 

 turning of the anterior part as there is resistance to hold the 

 posterior end fixed as a pivot support. The consequence is 

 that the worm is unable to change its direction of movement 

 quickly when on the film, and it has to go through a succes- 

 sion of muscular twists and jerks towards one side before the 

 result is attained. I have not been able to induce well 

 co-ordinated crawling movements in a worm while on the 

 surface film. The preliminary contraction of the posterior 

 part of the bt>dy occurs when that region is stimulated, but 

 the subsequent stretching out of the head and drawing up of 

 the body does not usually follow. I have tried stinuilating 

 both the exposed ventral surface of the animal and the dorsal 

 surface from below, but neither method is effective. 'I'he 

 reason for this is probably to be found again in the elasticity 



