The actual mode of formation of the epiphragm would seem to differ in different species. According to Fischer,[43] the mollusc withdraws into its shell, completely blocking all passage of air into the interior, and closing the pulmonary orifice. Then, from the middle part of the foot, which is held exactly at the same plane as the aperture, is slowly secreted a transparent pellicle, which gradually thickens, and in certain species becomes calcareous. Dr. Binney, who kept a large number of Helix hortensis in confinement, had frequently an opportunity of noticing the manner in which the epiphragm was formed.[44] The aperture of the shell being upward, and the collar of the animal having been brought to a level with it, a quantity of gelatinous matter is thrown out [? where from]. The pulmonary orifice is then opened, and a portion of the air within suddenly ejected, with such force as to separate the viscid matter from the collar, and to project it, like a bubble of air, from the aperture. The animal then quickly withdraws farther into the shell, and the pressure of the external air forces back the vesicle to a level with the aperture, when it hardens and forms the epiphragm. In some of the European species in which the gelatinous secretion contains more carbonate of lime, solidification seems to take place at the moment when the air is expelled, and the epiphragm in these is in consequence strongly convex.
Thread-spinning.—A considerable number of fresh-water Mollusca possess the power of stretching a thread, which is no more than an exceedingly elongated piece of mucus, to the surface of the water, and of using it as a means of locomotion. This thread bears no analogy whatever to the fibrous byssus of certain bivalves, being formed in an entirely different manner, without the need of a special gland.
The threads are ‘spun’ by several species of Limnaea, Physa, and Planorbis, by Bithynia tentaculata, and several of the Cycladidae. They are anchored to the surface by a minute concavity at the upper end, which appears to act like a small boat in keeping the thread steady. The longest threads are those of the Physae, which have been noticed to attain a length, in confinement, of 14 inches. They are always spun in the ascent, and as a rule, when the animal descends, it rolls the thread up and carries it down as it goes. A single thread is never spun on the descent, but occasionally, when a thread has become more or less of a permanence, it becomes stronger by the addition of more mucus each time it is used, whether for ascending or descending purposes. Cyclas cornea appears to be an exception to the rule that threads are only spun on the ascent. This species, which is particularly fond of crawling along the under surface of the water, has been noticed to spin a thread half an inch in length while on the surface, and to hang suspended from it for a considerable time.
What the exact use of the thread may be, must to a certain extent be matter of conjecture. The Limnaeidae are, in the great majority of cases, compelled to make periodic visits to the surface in order to inspire oxygen. It is also a favourite habit with them to float just under the surface, or crawl about on its under side, perhaps in pursuit of tiny vegetable organisms. Whatever may be the object of an excursion to the surface, a taut thread will obviously be a nearer way up than any other which is likely to present itself; indeed, without this thread-spinning power, which insures a tolerably rapid arrival at the surface, the animal might find itself asphyxiated, or at least seriously inconvenienced, before it could succeed in taking in the desired supply of oxygen. With the Cycladidae, which do not breathe air, such an explanation is out of place; in their case the thread seems to be a convenient means of resting in one position in the intervals of the periods of active exercise to which several of the species are so much addicted.
The power of suspension by a thread is also possessed by certain of the Cyclostomatidae, by some Cerithidea, several Rissoa and other marine genera, prominent among which is Litiopa bombyx, whose name expresses its power of anchoring itself to the Sargasso weed by a silken thread of mucus. Several species of slugs are known to be able to let themselves down by threads from the branches of trees. Limax arborum is especially noted for this property, and has been observed suspended in pairs during the breeding time. According to Binney, all the American species of Limax, besides those of Tebennophorus, possess this singular property. Limax arborum appears to be the only slug which has been noticed to ascend, as well as descend, its thread. It has also been observed[45] that when this species is gorged with food, its slime is thin and watery, and unable to sustain its weight, but that after the process of digestion has been performed, the mucus again becomes thick and tenacious. It appears therefore that when the animal is hungry and most in need of the power of making distant excursions in search of food, its condition enables it to do so, but that when no such necessity is pressing, the thread-forming mucus is not secreted, or is perhaps held in suspense while the glands assist in lubricating the food before digestion.[46]
Food of Land and Fresh-water Mollusca.—Arion ater, the great black slug, although normally frugivorous, is unquestionably carnivorous as well, feeding on all sorts of animal matter, whether decaying, freshly killed, or even in a living state. It is frequently noticed feeding on earthworms; kept in captivity, it will eat raw beef; it does not disdain the carcases of its own dead brethren. An old man near Berwick-on-Tweed, going out one morning to mow grass, found a black slug devouring, as he supposed, a dead mouse. Being of an inquisitive turn, and wishing to ascertain if it were really thus engaged, he drew the mouse a little back. When he returned in the evening, the mouse was reduced almost to a skeleton, and the slug was still there.[47] Indeed it would seem almost difficult to name anything which Arion ater will not eat. Dr. Gray mentions[48] a case of a specimen which devoured sand recently taken from the beach, which contained just enough animal matter to render it luminous when trodden on in the dark; after a little time the faeces of the slug were composed of pure sand, united together by a little mucus. A specimen kept two days in captivity was turned out on a newspaper, and commenced at once to devour it. The same specimen ate dead bodies of five other species of slugs, a dead Unio, pupae of Adimonia tanaceti, part of the abdomen of a dragon-fly, and Pears’ soap, the latter reluctantly.[49]
According to Simroth[50] and Scharff[51] the food of several of our British slugs, e.g. Limax maximus, L. flavus, Arion subfuscus, A. intermedius, consists of non-chlorophyllaceous substances only, while anything containing chlorophyll is as a rule refused. On the other hand L. agrestis and Amalia carinata feed almost entirely on green food, and are most destructive in gardens. The latter species lives several inches under ground during the day, and comes to the surface only at night. It is largely responsible for the disappearance of bulbs, to which it is extremely partial. L. marginatus (= arborum Bouch.) feeds exclusively on lichens, and in captivity absolutely refuses green leaves and a flesh diet. It follows therefore, if these observations are correct, that the popular notions about slugs must be revised, and that while we continue to exterminate from our gardens those species which have a taste for chlorophyll, we ought to spare, if not encourage those whose tastes lie in the opposite direction.
Limax agrestis has been seen devouring the crushed remains of Arion ater. Five specimens of the same species were once noticed busily devouring a May-fly each, and this in the middle of a large meadow, where it may be presumed there was no lack of green food. The capture and eating of insects by Mollusca seems very remarkable, but this story does not stand alone. Mr. T. Vernon Wollaston once enclosed in a bottle at least three dozen specimens of Coleoptera together with 4 Helix cantiana, 5 H. hispida, and 1 H. virgata, together with an abundant supply of fresh leaves and grass. About a fortnight afterwards, on the bottle being opened, it was found that every single specimen of the Coleoptera had been devoured by the snails.[52] Amalia marginata in captivity has been fed upon the larvae of Euchelia jacobaeae, eating three in two hours.[53]
Limax maximus (Fig. [19]) has been seen frequently to make its way into a dairy and feed on raw beef.[54] Individuals kept in confinement are guilty of cannibalism. Mr. W. A. Gain kept three specimens in a box together, and found one of them two-thirds eaten, “the tail left clean cut off, reminding one of that portion of a fish on a fishmonger’s stall.” That starvation did not prompt the crime was proved by the fact that during the preceding night the slug had been supplied with, and had eaten, a considerable quantity of its favourite food. On two other occasions the same observer found one of his slugs deprived of its slime and a portion of its skin, and in a dying condition.[55] An adult L. maximus, kept for thirty-three days in captivity with a young Arion ater, attacked it frequently, denuded it of its slime, and gnawed numerous small pieces of skin off the body and mantle.[56] The present writer has found no better bait for this species on a warm summer night than the bodies of its brethren which were slain on the night preceding; it will also devour dead Helix aspersa. Mr. Gain considers it a very dainty feeder, preferring fungi to all other foods, and apparently doing no harm in the garden.
