MOVEMENTS, ETC., OF FRESH-WATER J'LANARIA^S. 551 



lated or injured at the anterior end. This movement almost 

 always occurs when the head is cut off, and may usually be 

 induced in such decapitated specimens for a considerable 

 period after the operation by stimulating the anterior end. 

 The backward crawling" is not so rapid as the same move- 

 ment forward. The reason for this appears to be that the 

 posterior end is unable to take so firm a hold upon the 

 bottom as does the anterior end. The backward crawling* is 

 usually not very long continued, the animal soon coming to 

 rest. The inability of the animal to glide in a backward 

 direction should, of course, be noted in this connection. 

 Strong^ chemical stimulation of the anterior end will cause 

 the backward movement to appear in some cases. Light, so 

 far as I have observed, "will not, nor will the electric current. 

 There is considerable variation as to the appearance of this 

 backward crawling. Some individuals cannot be induced to 

 do it at all, or only in a very slight degree, while others will 

 crawl backward for considerable distances after injury to the 

 anterior end. It appears to be a complex of reflexes which 

 under normal circumstances is inhibited, and only appears 

 in any pronounced way under comparatively rare condi- 

 tions. It is not, as might be expected, a method ordinarily 

 used by the organism to get out of danger. Tliis is one of 

 the cases quite frequently met where an organism lias among 

 its available assets, so to speak, a reaction which is well 

 adapted to a certain end, but of which use is not nuide at all, 

 or but very little. 



2. Stimuli which induce Crawling. — It may be said 

 in general that almost any strong stimulus applied to the 

 posterior portion of the organism causes the forward crawl- 

 ing movement to appear. Mechanical and strong chemical 

 stimuli applied in this region will do this. Light, either of 

 ordinary intensity, or of such high intensity as tluit from an 

 arc light, so far as 1 have observed, will not cause the ci-awl- 

 ing movement. The electric current does cause it, but 

 greatly diminishes the rate. Any operative treatment — as, for 

 example, cutting the body in two in tlie middle — almost in- 



