KEW : ON THE MUCUS-THREADS OF LAND-SLUGS. 163 



this slug ascend its thread ; I have heard also of a similar observation 

 from Mr. A. E. Manger. Zykoff's notes refer to j-oung Arion ater\ 

 the creatures, which lived under a bell-glass, suspended themselves from 

 the roof, and some, without touching the bottom, reascended by their 

 threads. I have observed short ascents by Agr. agrestis and Agr. 

 hevis, and in Part II. have described and figured the proceedings 

 of a young Arion siil?/asius, which reversed its position when seven 

 inches from its support, crept up the thread, and replaced itself upon 

 the support. One gathers from Harte that the ascent is commonh' 

 practised, but this does not agree with my experience. It is said by 

 the same author that if a finger moistened with saliva be held beneath 

 the animals they will ascend upon touching it. It is certain that they 

 may occasionally be made to return by this or similar means ; Saun- 

 ders' slug commenced its retreat after its tentacles had been examined 

 with a lens, and some of those observed by W.R. ascended after being 

 touched on the head ; the writer, however, though once or twice suc- 

 cessful, has often tried devices of this kind unsuccessfully, as well as 

 others, such as placing saucers of water beneath the animals, clouding 

 them with tobacco-smoke, etc. As regards the manner in which the 

 reversal of position and ascent are affected, Saunders says that his slug 

 got about its retreat "by curving the head portion of the body upwards 

 and inwards, until it touched the hinder portion of the body, a part of 

 which it actually climbed up, and then took to the suspending-thread"; 

 W.R., also, says that the animals "climb up their own bodies to the 

 thread," and this is precisely what I saw in Arion subfuscus. By reach- 

 ing upwards this animal brought the fore-part of its foot into contact 

 with its hinder parts, and advanced, by creeping upon its own body, 

 until it reached the thread upon which it immediately applied its foot, 

 and ascended, slowly but without apparent difficulty, until the support 

 was reached. The locomotion is of the kind employed upon a solid 

 body, a new film of slime being no doubt deposited upon the thread ; 

 the foot remains nearly flat, but is a little folded anteriorly. An accu- 

 mulation of mucus (the slack of the thread together with the new film) 

 soon appears dorsally at the tail. 



Turning to the question of the utility of the spinning faculty, we find 

 that Harte (23, 26) who held the opinion that slugs do not suspend 

 themselves voluntarily, supposed the thread to be useful, not for de- 

 scent, but for enabling individuals which by mischance slip from their 

 supports, to recover themselves, by ascending the thread, and thus 

 avoid falling into water, etc. It is not likely, however, that slugs often 

 lose their footing accidentally, and even if this were the case it is im- 

 probable that the ability to ascend the thread would often enable them 

 to avoid water or other dangers. One author (39) mentions that a 

 descending slug suddenly dropped on reaching water ; and failure to 



