Harry Wallis Kew

“ The habits of the Litiopa are not less worthy of your notice. This is a small Snail, born amid the gulf-weed, where it is destined to pass the whole of its life. The foot, though rather narrow and short, is of the usual character, and, having no extra hold, the Snail is apt to be swept off its weed; but the accident is provided against, for the creature, like a Spider, spins a thread of the viscous fluid that exudes from the foot to check its downward fall, and enable it to regain the pristine site. ”

“ On the foot of the descending animal wave-like appearances are observable; these are the result of muscular action identical with that giving rise to the creature's ordinary locomotion; and we have here a clue to the nature of the progression through the air and of the thread. Whenever a Slug is in motion it discharges mucus, chiefly from the supra-pedal gland below the mouth; in ordinary locomotion it crawls over a film of this mucus which remains behind as a trail, and when in the air it crawls over a similar film, which collapses into a thread as the animal leaves it ”

“ There is thus no special spinning-organ. The thread, however, is of extreme fineness, and is silky when dry; it generally springs up and floats (remaining attached at the point of origin) when the animal alights; but it sometimes becomes attached to the new support, and is left, marking the animal's aerial course. If the animal does not find a new support, or fall, it sometimes turns upon its thread, ascends it, and regains the former support; it creeps up the thread as on a solid body, the slack (with other mucus emitted during the ascent) accumulating at the tail. ”

“ It seems to the present writer that the changes in the creature's specific gravity are largely contributed to by the contraction or extension of the animal itself into or from its shell; and it is probable that the creature, when sufficiently heavy to sink, is usually too much contracted and withdrawn to form a thread. It is interesting to note that Mr. Tye recognizes the fact that here, as in Limax, the thread represents the mucus-trail of ordinary progression, such a trail, though usually invisible in the case of a Limnæid, being always present in the track of the moving animal. ”

“ The animal is ordinarily slightly lighter than water:—a, an individual crawling through the water towards the surface, leaving its locomotory mucus behind in the form of a thread, which retains the animal, and prevents its sudden rise to the surface; b, the animal at the surface, taking in a supply of air—the thread, having been continued as a floating slime-trail, is now attached to the surface; c, the animal returning by descending its thread; another individual is making use of the fixed thread for ascent. ”

“ One doubts, however, whether they derive much advantage from the use of the thread; there is no reason to suppose that they would often be injured if they dropped (as they often do by reason of the imperfection of their spinning) . Falls of a few feet do not appear to be harmful; and the writer regards the Slug's spinning as little more than an accidental circumstance resulting from the possession, for ordinary locomotion, of a continuous supply of tenacious mucus. ”

“ They crawl from the object, and, when the tail parts company with it, the animal is sustained by a thread, which is left by the body at the tail, and is gradually lengthened. Sometimes the animal thus reaches a new support without a fall, but the faculty is imperfect, the animal often falling either without a thread or after making a short one. Large Slugs, when full-grown, are incapable of this kind of locomotion; but small ones, and the young of large kinds, are occasionally capable of making successful descents of surprising length. ”

“ Several of the observations quoted below, it is true, imply descent and suspension; but, as the thread is generally invisible, it is possible that the animals, in some cases, may have been descending, or resting, upon threads already spun and fixed during ascent. The animal's ability to ascend or descend is attributed by Mr. Tye wholly to the condition of the lung-sac; the creatures are lighter than water, he says, when the sac is inflated, and heavier than water when the air of the sac is exhausted or expelled. ”

“ In some cases figures of the foot show conspicuously the long transverse slit-like opening of an anterior pedal gland, whence the mucus of the thread might presumably be derived. In many Pectinibranchs, however, in addition to the anterior gland, a ventral pedal pore exists in the median line of the anterior half of the foot-sole; it forms the opening of a cavity said to be comparable to the byssal-cavity of bivalves, and from it, externally, a well-marked groove often runs to the tip of the tail. ”

“ On June 28th and 29th more weed inhabited by this animal was fished up, and Bélanger again observed the creature's spinning habits. [67] Speculating upon the use of the thread, he remarks that the creature, born, living, and reproducing on floating weed, incessantly tossed with more or less violence by the very deep ocean, would be lost, when detached by a wave, had it not this faculty of spinning a silk which, like a cable, holds it to its habitat. On July 8th further weed was fished up, but the molluscs were now less numerous ”

“ Taylor, [7] that the creatures can spin downward as well as upward threads; and from the observations of these naturalists it certainly appears that when the air in the lung-sac is sufficiently exhausted, the animal is heavy enough, while yet extended from its shell, to descend through the water, making a thread as it goes, and to remain suspended in the water upon a thread thus made. ”

“ During the descent, the flexible leaf was bent with an undulating motion, corresponding with every movement of the Snail, and making it clear that the animal had an attachment to the extremity of the leaf. Proof of the existence of a thread was obtained also by means of an experiment which the observer often repeated with Limnæa stagnalis, L. auricularia, and Amphipeplea glutinosa. ”

“ But suppose the shock has severed their connexion, or that the Litiopa finds it necessary to remove, from a deficiency of food, to a richer pasture, the thread is still made available to recovery or removal. In its fall, accidental or purposed, an air-bubble is emitted, probably from the branchial cavity, which rises slowly through the water, and as the Snail has enveloped it with its slime, this is drawn out into threads as the bubble ascends ”

“ In some notes forwarded to 'Loudon's Magazine' in 1831, Guilding mentions a "Cyclostoma" common in the Virgin Islands, which, having given out a mucous thread, closes the operculum, and swings by the thread when hardened by the air, safe from ants and other enemies. ”

“ This is certainly a curious performance; but, allowing for the reversed condition of specific gravity, it will be noted that it exactly corresponds with the turning and ascent of a thread by Limax; the buoyancy of Physa, of course, keeps the thread taut, just as does the weight of Limax. ”

“ Eulima intermedia, another inhabitant of our coasts, creeps at the surface, and, according to Jeffreys, "it remains suspended in that posture by means of a byssal thread, the operculum then closing the mouth of the shell" ”

“ Mr. Tye, who refers to Woodward, has by mistake attributed the observations to Cerithidea decollata, [82] an animal which does not appear to fix itself by threads, Dr. Gibbons having reported that large numbers seen by him on trunks of marsh trees in Natal were attached, not by threads, but by "a trifle of brittle mucus passing from the lip to the tree," a mode of attachment, as Dr. Gibbons says, resembling that of brackish water Littorinæ. [83]
Planaxidæ. ”

“ E.A. Smith (British Museum) , who has obligingly considered the figure for the writer, thinks that it represents, possibly, Cistula lineolata, a small West Indian shell, of which the Museum acquired unnamed specimens at the sale of Guilding's collection. If this be the case, of course, our spinner belongs not to the present, but to the following family. ”

“ Further, the author even thought it possible that the creatures might thus voluntarily change their positions; a family, he supposes, might find their plant insufficient to feed their increasing numbers ”