312 Ke7v; Slime -Threads of Planar ian- Worms. 



the length of the fully extended animal. Writing of the 

 Bipalium, the observer points out that an interruption in the 

 continuity of the support on which the animal is crawling, if 

 of less distance than the length of the animal, offers no barrier 

 to its progress ; where the support ends the anterior part of the 

 body is extended into the air in search of a new support, and 

 when such is found, the anterior extremity comes into contact 

 with and proceeds to occupy it, the remainder of the body 

 following in a straight line. When the posterior extremity 

 leaves the old support it is sustained by the locomotory mucus, 

 and is thus drawn through the air exactly in the same line as 

 the foregoing part. The mucus, left behind in the form of a 

 thread, stretches from point to point, and bridges the interruption. 

 As in the case of descent, this thread has the appearance of 

 being produced by the posterior extremity ; but, in reality, as 

 Lehnert says, it is commenced, as the uninterrupted slime-layer 

 of the sole, the moment the anterior extremity touches the new 

 support. The thread, which soon dries, frequently remains in 

 position for days ; but the animal has not been seen to use it 

 again as a permanent bridge.* Similar threads, as appears 

 probable from an observation by Sir W. Elliot, are available 

 for progression across the surface of narrow channels of water. 

 When a bottle in which some Bipalium lunatum were kept was 

 placed in the middle of a basin of water, the creatures ' crawled 

 out till they encountered the surface of the fluid, which at first 

 they seemed unwilling to face ; but, at length, after many 

 attempts, and much apparent repugnance, some of the larger 

 ones pushed forward their heads, retaining their hold upon the 

 cork by the tail alone, till they touched the edge of the basin on 

 which the disc-like head was firmly fixed, and the body was 

 drawn after it.'f Some of the creatures, in this manner, 

 escaped, presumably without dipping into the water, the body 

 doubtless passing over a floating bridge-thread — a continuation 

 of the ordinary slime-trail. It thus appears that with this 

 Bipalium a channel of water less in width than the animal's 

 length need not prohibit the creature's onward motion; the same 

 is the case, no doubt, with many planarians ; and it is interest- 

 ing to note that Geodesmus bilineatus was found by Lehnert to 

 be able, by means of its locomotory mucus, to extend its body 

 upon water. If no new support was within reach, the Geodesmus 



* Lehnert, I.e. 

 f Elliot, I.e. 



Naturalist, 



