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JOURNAL OF CONCHOLOGY, VOL. ID, NO. 5, JANUARY, I902. 



locomotion. The same " waves " are observable ^vhen a slug cra\vls 

 on a piece of glass, and notwithstanding their course from behind 

 forwards, they are sometimes supposed to be vermicular undulations 

 by which the animal is propelled ; but, far from this being the case, 

 there is in reality no change of form externally, the creature's locomo- 

 tion being effected and the " waves " produced by constant elongations 

 of the foot in front and consequent shortenings soon afterwards to the 

 same extent behind.^ 



The identity of action in spinning and in crawling is a point to 

 which I wish to call attention. 



The manner in which the thread is produced has often been mis- 

 understood, and confusion has prevailed as to the source of the slime 

 of which it is formed. It is necessary to bear in mind that whenever 

 a slug is in motion it emits, mainly from the supra-pedal gland between 

 the mouth and the foot, a continuous supply of slime, which spreads 

 over the foot and is left on the surface over which the animal is passing, 

 in the form of the iridescent trail with which everyone is familiar ; thus 

 the foot is never in contact with the object on which the animal is 



said to crawl, for the creature moves in- 

 variably upon an interposed film of slime. 

 The writer does not know that all the 

 slime of the trail comes from the supra- 

 pedal gland, for there are mucus-glands in 

 the whole skin, and those of the foot per- 

 haps contribute to the trail ; the bulk of 

 the material, however, is derived from 

 the gland named, and the point to be 

 emphasized is that the suspensory thread 

 is derived in the same manner, and repre- 

 sents in every respect the continuous trail 

 of ordinary progression. When the crea- 

 ture crawls over a solid substance, the 

 foot slides upon a film which is attached 

 to the floor, and so remains as a trail ; 

 when it descends the foot slides upon 

 an exactly similar film, which is unat- 

 tached, and which when left behind 

 collapses into a thread. Thus, in spin- 

 ning, the animal crawls through the air 

 upon a free film, just as it crawls upon 

 an attached film over a solid object. The 

 thread is merely a continuation of the 



Fig. 7. 

 (Diagrammatic). 

 The slug has crawled down a win- 

 dow-frame and is descending from the 

 sill. 'J he line ae represents the con- 

 tinuous, very thin slime- trailand thread. 

 The trail bridges an angle at B, and at 

 c is continued as the thread by which 

 the animal is retained. The slime is 

 mainly derived at e, is crawled upon by 

 the animal, and is left behind by it at d. 



I Scharff (quoting Simroth), /oitrn. of Conch., vol. 5, 1887, p. 239 ; Marey, Animal Mechanism, 

 ed. 2, 1874, p. 105 ; Main, Zool, Journ., vol. 3, 1828, p. 599. 



