550 RAYMOND PEARL. 



is usually rather short. A single strong stimulus at the 

 posterior end of the body — such, for example, as is given by 

 running a needle through the body — will not usually cause 

 more than three of the strong contractions of the crawling 

 nioveraentj and then the animal will relapse into the usual 

 glide. The limits in this matter I found to be from a single 

 contraction and expansion as a minimum up to six or seven 

 as a maximum. This is, o£ course, in response to a single 

 stimulus only. By repeating the stimuli the animal may be 

 made to continue the crawling indefiuitel3^ 



The effective rate of this form of progression is faster than 

 that of the gliding movement. The crawling rate of one of 

 the worms used for the measurement of the rate of gliding 

 (the specimen 11 mm. long) was measured in the same way 

 as was the gliding rate. The worm was stimulated Avith a 

 needle at its posterior end just enough to keep it crawling, 

 i. e. prevent it from settling into the regular glide. The 

 average rate of the crawling was found to be I'GO mm. per 

 second. Merely qualitative observation shows that the worm 

 gets along somewhat faster in the crawling than in the 

 gliding. 



1. Direction. — The crawling may take place so as to 

 carry the animal either forward or backward. The back- 

 ward crawling is induced by very strong stimulation of the 

 anterior region of the body. It does not alwa3's occur tiven 

 after such stimulation, there being apparently some in- 

 dividual differences among the specimens in this respect. One 

 factor which will call forth persistent backward crawling is 

 partial desiccation. If the dorsal surface of the organism is 

 allowed to dry, it will attempt to crawl backward violently. 

 The mechanism of the backward crawling is jnst the reverse 

 of that which obtains when the animal moves forward. The 

 posterior end is extended and fastened to the bottom ; then 

 a wave of contraction, starting in this case from the posterior 

 end, draws the remainder of the animal backwards, and then 

 the posterior end is again extended. The backward crawl- 

 ing is usually induced when the worm is excessively stimu- 



