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observer, it appears, while walking along by the roadside, saw a large orange-tailed humble-bee, “vigorously towing something heavy after him,” which, on inspection, proved to be “a Clausilia (Cyclostoma ?] the little snail which has an operculum-like plate on the lip;” the latter had fast hold of one of the hind legs of the bee, by which, no doubt, it had been accidentally trodden upon. The smaller operculate-shells, it can hardly be doubted, may possibly be carried in this manner to considerable distances by the large and powerful insects which abound in some parts of the world.
DISPERSAL OF SLUGS.
SLUGs, obviously, are not so well fitted for dispersal as snails, for many are quite naked, and though some possess small external shells, the well-developed snailshell, into which the animal can retire for rest and during periods of adversity, and which, often closable by an epiphragm or operculum, has doubtless largely facilitated the dispersal of shell-bearing kinds, is always absent, and it is clear, therefore, that many of the considerations referred to in the preceding chapter cannot be looked upon as necessarily applicable to the slugs, such creatures being sure to succumb to many of the hardships from which snails may often have escaped in safety. Many slugs, it is notorious, from containing much water, cannot even bear exposure in a dry atmosphere for any length of time, but in many respects, it should be remembered, animals of this kind are much more tenacious of life than might at first be supposed, and this is true of our ordinary absolutely naked sorts, Limax, Arion, etc. The Testacella, which have a small ear-shaped shell near the extremity of their bodies, “snail-slugs" of some authors, are able to protect themselves by forming a kind of cocoon “secreted from their skin and often mixed with earthy and extraneous particles”: the “nest of earth” made by T. maugei in times of drought, it is said, reminds the observer of the cocoon of the puss-moth ; within, the animal lies encysted until moisture, finding its way through the walls, rouses it again to activity. While in the encysted state "a thin white membrane (a development of the mantle) is extended from beneath the shell and stretched over the back and sides of the animal,” checking evaporation, and forming an admirably-designed protective shield. The Testacella's power of retaining life under adverse conditions is well illustrated by observations made by Professor Poulton in 1886:
“Between four and five months ago I found eleven specimens of this slug [Testacella, species not stated] upon a low wall surrounding the garden of a house near the Oxford University Parks, and on the following day I captured eleven more in the same place.
A few of the twenty-two specimens were killed and hardened, and the remainder were put in a box containing earth, in which they buried themselves. In the press of other work the box was neglected, and remained untouched in my laboratory until to-day [October 19th], the earth having quickly dried into a hard cake. To-day I emptied the box, and fully expected to find the slugs dried up dead, but to my surprise I found twelve specimens alive, each encysted
“British Conchology," i. (1862), pp. 143, 147-8.
in a thin transparent capsule formed of the hardened mucous secretion of the animal's skin. The body was contracted, and oval in shape, but it had been so completely protected from evaporation that there was no noticeable reduction in bulk after these hottest months of the year, during which water had been entirely withheld. One or two specimens had died almost immediately after capture, and a few escaped, so that all those which had been exposed to the heat and dryness in the box had become encysted, and survived in apparent health.” 1
Slugs in such a condition at the roots of trees, etc., would be nearly as well fitted for dispersal as snails, at least by fresh-water currents.
Slugs of most sorts and the eggs of some kinds must certainly be carried to short distances within given landareas with some frequency ; various means, organic and inorganic-probably of kinds already suggested-operate no doubt to bring about short involuntary migrations. River agencies must have widely scattered the creatures. Some are arboreal, inhabiting old and decaying trees which, when blown to ground, or washed out of riverbanks, must sometimes be carried off by floods. In our own country we find slugs—the great grey kind, Limax maximus, for instance-living under the bark of old willows by river-sides. At times, too, we find them well concealed in chinks and among débris in the interior of hollow trees, and, like some snails, they probably eat
E. B. Poulton, “Nature,” xxxiv. (1886), p. 618.
their way into the decaying wood, for several specimens of L. maximus, found when removing rubbish and breaking away parts of the wood from a cavity in the trunk of an old hornbeam, subsequently extruded pale yellow pellets composed entirely of “saw-dust." In these situations the creatures would often be well protected, so that they might be carried in safety with floating trunks to great distances. Many kinds, it must not be forgotten, live mostly on the ground, rarely or never ascending trees, and these of course are not likely to be carried with timber, unless they occasionally hide themselves in logs lying upon the ground; they must frequently be swept in numbers from their haunts on grassy or muddy river-margins, but unless buoyed up by some object, they are not likely to be much dispersed by the currents, for I believe they always sink in water, and when carried away by the rushing waters of torrents or floods are hardly likely to be often landed again in a living state. It seems probable, however, that they may sometimes be safely floated upon vegetable-rafts of many sorts. A few years ago, indeed, I had the good fortune to witness the conveyance of a slug by this means, upon a very fragile raft. Walking in the evening from the Lincolnshire coast towards Louth along the banks of the Eau, I saw a large black slug (Arion ater) crawling with out-stretched tentacles upon a mass of “green-scum" which came drifting down with the current at a good way from the bank. This was at no great distance from the sea, and in all probability the animal was ultimately carried out and drowned, but pos