MOVEMENTS, ETC., OF EUESH-WATEll PLANAKIANS. 549 



has beeu stimulated iu certain ways, aud is ol mucli less 

 frequent occurrence than the gliding. 



The crawling is always induced when the posterior end of 

 the body is strongly stimulated. The characteristics of the 

 movement are as follows : — If the posterior end of a worm 

 which is gliding smoothly along is touched with a needle the 

 posterior half of the body immediately contracts longi- 

 tudinally; an instant later the anterior end stretches out far 

 in front and fastens to the substrate. Then there is a longi- 

 tudinal contraction which begins just back of the head and 

 runs posteriorly. This, of course, at once draws forward the 

 posterior part of the body, and as this comes forward and 

 gets a hold on the surface on which the animal is crawling-, 

 the anterior end is again extended far in front and attached 

 to the substrate. This process is repeated until the animal 

 settles down into the regular glide again. It consists essen- 

 tially in a stretching out of the head followed by a pulling 

 of the body forward by an active muscular contraction. 

 When the animal is very strongly stimulated the por- 

 tion of the body posterior to the pharynx usually takes no 

 part in the crawling after the first general contraction. In 

 fact, the posterior half of the body may even be held 

 slightly raised off the bottom, while the region between the 

 head and the origin of the pharynx is actively expanding and 

 contracting and sending the body ahead. These strong ex- 

 pansions and contractions of the anterior end which make up 

 the crawling movement may follow each other in rapid 

 succession as described above, or there may be a considerable 

 interval between one contraction and the next. In this in- 

 terval the body as a whole keeps moving ahead as a result of 

 the ciliary action ; that is, the gliding movement continues 

 during the crawling, so that the latter may be regarded as 

 an additional movement for the purpose of advancing the 

 animal faster than the gliding alone can do it. The crawling 

 may take place, however, with the ciliary beat entirely 

 stopped. 



The duration of the crawlin<>- movement after it is induced 



