3°7 



ON THE SLIME-THREADS OF PLANARIAN-WORMS. 



H. WALLIS KEW, F.Z.S., 

 London ; formerly of Louth, Lincolnshire. 



Certain Mollusca-Gastropoda are well known to make slime- 

 threads by which they descend through the air or water. 



The thread — in land-slugs, air-breathing- water-snails, and 

 sea-slugs — is not the product of a special spinning-organ, but 

 consists of ordinary locomotory mucus, and is, in fact, a con- 

 tinuation of the slime trail which the creature always leaves in 

 its path. 



The animals with which we are at present concerned, the 

 Planarians or Flat-Worms of the order Turbellaria, though 

 far removed from the Mollusca and immeasurably lower in the 

 scale, are found, somewhat oddly, to make a surprisingly similar 

 use of slime-threads — the comparison being- with the naked 

 Mollusca-Gastropoda rather than with those bearing- shells. 



The land-planarians (terrestrial Turbellaria-Tricladida), re- 

 quiring- much shade and moisture, move from their retreats 

 chiefly at night, creeping - like slugs on a flattened ventral 

 surface. Progression is effected by the action of cilia, aided by 

 muscular undulations which proceed from the anterior extremity 

 backwards, not from behind forwards like the locomotory wave- 

 like appearances of Mollusca-Gastropoda ; and thus the land- 

 planarian's locomotion, though superficially like that of a slug, 

 is really of different nature, being truly vermicular and having 

 nothing to do with the highly characteristic Gastropodan mode 

 of progression. In both cases, however, the creature emits a 

 continuous film of locomotory mucus, the land-planarian, like the 

 slug, leaving a slime-trail wherever it goes; and this trail, in the 

 planarian as well as in the mollusc, may be continued as a 

 thread, capable, for a time, of suspending the animal as it 

 descends from its support.* The land-planarian, however, 

 crawling from an object and not finding a new resting place, 

 soon falls — the thread, less reliable even than that of the 

 mollusc, quickly giving way, and never, as far as is known, 

 sen ing tor long descents Jike those occasionally accomplished 

 by slugs. 



* It is the same, probably, in the case of other trail-leaving creatures, 

 e.#., Xemertines : see Mcintosh, Monograph of British Annelids: Xemer- 

 teans, 1873-4, PP- °- '77- 



1900 October i. 



