KEW : ON THE MUCUS-THREADS OF I.AND-SLUGS. l6l 



trail found upon the object from which the animal glides, a fact illus- 

 trated in fig. 7. A slug which has crawled down the frame of a window 

 is shown descending from the sill; the trail and thread (of extreme fine- 

 ness in nature) are represented by the line ae; the slime, mainly derived 

 at E, is crawled upon by the animal, and is left by the tail at D,asa thread 

 DC when the animal passes through the air, and as a trail ca when it 

 passes over a solid body. The trail bridges an angle in the form of a 

 short thread v>. When a slug leaves an object the trail hitherto de- 

 posited upon it serves, when contact ceases, to retain the animal in the 

 air ; during the descent the tail is seen leaving the slime in the form of 

 a film, but this immediately collapses into a thread, and soon after- 

 wards dries and becomes silky. The thread is lengthened, primarily, 

 by the continued crawling action of the mollusc, combined with con- 

 stant emission of slime anteriorly. By means of the regularly repeated 

 anterior elongations of the foot, the animal occupies the new slime as 

 fast as it is emitted, and the conse([uent posterior shortenings of the 

 foot, of course, leave similar quantities free at the tail. The process 

 is accelerated by the weight of the animal, for the film of slime after 

 being left by the tail and while collapsing is thus elongated. 



We see, then, that these animals though genuine thread-makers 

 have no special spinning-organ. The supra-pedal gland can hardly 

 be looked upon as a spinneret, for it is situate near the head, and its 

 secretion is not formed into a thread until passed by the hinder extre- 

 mity of the body. The caudal mucus-pore of Arionids, at one time 

 said to be a spinning-organ, has nothing to do with the production of 

 thread. 



The animals' ability to produce slime in sufficient quantity for the 

 formation of several feet of thread has been remarked upon with sur- 

 prise ; but the quantity required is not greater than that necessary for 

 ordinary creeping ; and the animals are known to make nocturnal 

 journeys of very considerable length. The inseparable connection of 

 a continuous slime-film Avith locomotion, however, though a matter of 

 common observation, is certainly remarkable; and is, perhaps, inter- 

 esting for comparison with the constant discharge during locomotion 

 of thread-forming secretions by certain arthropods, e.g:, some insect- 

 larv£e and spiders. 



The extreme fineness of the slug's thread is its most noteworthy 

 character. When a slug which has been descending for some minutes 

 falls, the thread is caught up and made to float by the slightest breath; 

 it remains attached, of course, at the point of origin, yet difficulty is 

 experienced in catching a specimen of it, an object moved towards 

 it causing the filament to fly away in a most tantalizing manner. It 

 seems incredible that such a thread serves for the suspension of a 

 comparatively heavy slug : Agr. ngj-esfis, one of the best spinners, 



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