The majority of the Land Mollusca are probably more sensitive than is usually believed. The humidity of the air must affect the surface of their skin to a considerable extent. Every one has noticed how the snails ‘come out’ on a damp evening, especially after rain. As a rule, they wait till rain is over, probably objecting to the patter of the drops upon their delicate tentacles. Snails kept in captivity under a bell-glass are acutely sensitive of a damp atmosphere, and will bestir themselves after rain just as if they were in the open air. Certain Helices which are accustomed to live in moist places, will find their way to water, if removed from their usual haunts. A case is recorded[33] of a specimen of H. arbustorum, kept in a kitchen, which used to find its way directly under the cold water tap, and appeared to enjoy the luxury of a douche. How delicately the conditions of life are balanced in some of these creatures is seen in the case of Omalonyx, a genus akin to Succinea, which is found in Brazil and the northern parts of South America. It lives creeping on plants which overhang the margin of water, but perishes equally, if placed in the water itself, or removed to a distance from it for any length of time.[34]

Endurance of Heat and Cold.—The Mollusca are capable, at least as far as some species are concerned, of enduring severe extremes both of cold and heat. The most northern pulmonate yet observed is a fresh-water species, Physa (Aplecta) hypnorum L. This hardy mollusc, whose shell is so fragile as to need most careful handling, has been noticed on the peninsula of Taimyr, North Siberia, in 73° 30’ N. lat, a region whose mean annual temperature is below 10° F. with a range of from 40° F. in July to -30° F. in January.

It is well known that the Limnaeidae, and probably most fresh-water Mollusca of sub-temperate regions, can continue to live not merely under, but enveloped in ice, and themselves frozen hard. Garnier relates[35] that, during the winter of 1829–30, some large Limnaea auricularia, which had been placed in a small basin, were frozen into a solid mass, experiencing a cold of -2° F. He supposed they were dead, but, to his surprise, when the basin thawed, the Limnaea gradually revived. Paludina vivipara and Anodonta anatina have been known to resist a temperature of 23° F., and the former has produced young shortly after being thawed out of the ice.[36] As far north as Bodø in Norway (67° 37’ N. lat., well within the Arctic circle) there are found no less than fourteen species of terrestrial Mollusca, among them being Balea perversa and Clausilia rugosa.[37]

Vitrina is one of our most hardy molluscs, and may be observed crawling on bright mornings over the frost-covered leaves of a wood or copse. V. glacialis is said by Charpentier to live in the Alps at a height where the stones are covered with snow from nine to ten months of the year. Many of the Hyaliniae are very hardy. Arion, in spite of having no external shell to protect it, is apparently less affected by the cold than Helix, and does not commence hibernation till a later period in the autumn. The operculate land Mollusca, in spite of the protection which their operculum may be supposed to afford, are exceedingly sensitive to cold, and the whole group is without doubt a product of tropical or semi-tropical regions (see map at frontispiece). A species of Helicina which inhabits the southern States of North America has been known to be almost exterminated from certain districts by the occurrence of an unusually severe winter.

One of the highest altitudes at which a land shell is known to live appears to be the Liti Pass (Himalayas, 14,000 ft.). At this enormous altitude, two species of Buliminus (arcuatus Hutt. and nivicola Bens.) live on juniper bushes among patches of snow. An Anadenus is said to have been found in a similar locality at 15,000 ft., while Limnaea Hookeri has been taken from over 16,400 ft. in Landour. In the Andes of Peru and Bolivia, five species of Bulimulus, one of Pupa, and one of Limax occur at an elevation of 10,500 to 15,000 ft. Several fresh-water Mollusca inhabit Lake Titicaca, which stands at a height of 12,550 ft. in the Bolivian table-land.

In certain parts of the desert of Algeria, where there is not a trace of vegetation to be seen, and the temperature at mid-day is 110° F., the ground is sometimes so covered with Helix lactea as to appear perfectly white. Dr. F. H. H. Guillemard has told me that he noticed, in somewhat similar surroundings between Fez and Tangier, H. pisana in such extraordinary abundance that they hung from the low scrub in bunches the size of a man’s two fists. It is singular that Mollusca should live, and not only live, but flourish, in localities apparently so unpromising. Shells which occur in the Algerian Sahara are actually larger and altogether finer than the ordinary European form of the same species. In order to protect themselves to some extent against the scorching heat and consequent evaporation, desert species are frequently modified in one of two ways; the shell becomes either white or a light dusky brown, as in the familiar Helix desertorum, or else it gains immensely in thickness. Specimens of H. pomatia, recently procured from Fez, are of extraordinary thickness as compared with forms from our own chalk downs of Kent and Surrey.

Fresh-water Mollusca are frequently found inhabiting hot springs. Thus Neritina fluviatilis lives at Bagnères de Bigorre in water at about 68° F. In another hot spring in the eastern Pyrenees a Bithynia lives at a temperature of over 73° F.; while Blainville mentions another case of a Bithynia living in water at 122° F.

Hibernation and Aestivation.—As autumn begins to draw on, and the first frosts to nip vegetation, terrestrial species retire beneath stones, into cracks in old walls, holes in tree trunks, deep fissures in rocks, and nooks and crannies of every kind, or else bury themselves deeply in the earth or in moss and heaps of leaves. They thus commence their period of hibernation, which varies in length according to the duration of winter. Frequently masses of Helices may be found attached to one another, probably not so much for the sake of warmth, for their temperature is but low, as to share the comforts of a cosy retreat in common. Slugs generally hibernate alone, excavating a sort of nest in the earth, in which they encyst themselves, contracting their bodies until they are almost round, and secreting a covering of their own slime. The Helices usually close up the mouth of their shell by the formation of a membranous or chalky epiphragm, which will be further described below. Both snails and slugs take care to be in good condition at the time their winter sleep begins, and for this reason the former are said to be most esteemed by foreign epicures if captured just at this period.[38]

During hibernation, the action of the heart in land Pulmonata ceases almost entirely. This appears to be directly due to the effect of cold. Mr. C. Ashford has related[39] some interesting experiments made upon H. hortensis and Hyal. cellaria, with the view of ascertaining the effect of cold upon their pulsations. His observations may be tabulated as follows:—

At low temperatures the character, as well as the number of the pulsations changed; they became imperfect and intermittent, although exceptionally at 31° F. a H. rufescens gave five or six pulsations a minute, very full and deliberate. The result of taking the Hyalinia suddenly into the heat of a greenhouse was to bring on palpitations. Further experiments resulted in evidence of a similar kind. Hyal. radiatula, placed upon a deal table in a room, showed 52 pulsations per minute at 62° F. Placed upon the palm of the hand, the action soon rose to 108. Hyal. alliaria, similarly treated, rose from 72 pulsations to 110. Floated upon water, the action of the heart of the latter suddenly fell to 29.

Fresh-water Pulmonata do not appear to hibernate. Unio and Anodonta, however, bury themselves more deeply in the mud, and Dreissensia casts off its byssus and retires under the mud in deeper water.[40] Limnaea and Planorbis have often been noticed to crawl about under the lower surface of a thick coating of ice. In periods of prolonged drought, when the water in the ponds dries up, the majority of genera bury themselves in the mud. I have known Limnaea peregra bury itself three inches deep, when surprised by a sudden fall of the water in the ditch on Coe Fen, behind Peterhouse, Cambridge. Physa hypnorum frequents by preference ditches which dry up in summer, as does also Planorbis spirorbis, the latter often forming a sort of epiphragm against evaporation. Ancylus has been observed to spend the whole winter out of water, and P. spirorbis has been noticed alive after four months’ desiccation.[41]

True aestivation, however, occurs mainly in the tropics, where there is no winter, but only a period when it is not quite so hot as the rest of the year, or on a coast like the Mediterranean, which is subject to sudden and severe heat. This period is usually rainless, and the heat is therefore a dry heat. At this season, which may last for three or four months, most of the land Mollusca enter upon a period of inaction, either burying themselves deeply in the ground, or else permanently attaching themselves to the stalks of grass and other herbage, or the under sides of rocks. For instance, the large and beautifully painted Orthalicus, Corona, and Porphyrobaphe, which inhabit Brazil, Ecuador, and eastern Peru, bury themselves deeply in the ground during the dry season, while in the rains they climb to the topmost branches of the great forest trees.[42] Thus it may well happen that a visitor to a tropical island, Ceylon for instance, or one of the Greater Antilles, if he times his visit to coincide with the rainless season, may be grievously disappointed at what seems its unaccountable poverty in land Mollusca. But as soon as the weather breaks, and the moisture penetrates their retreats, every bush and every stone, in favoured localities, will be alive with interesting species.

The Epiphragm.—A considerable number of the land Pulmonata (and a very few of the fresh-water) possess the power of closing the aperture of their shell by means of what is known as an epiphragm or covering of hardened mucus. This epiphragm is habitually formed by certain species during hibernation or aestivation, or even during shorter periods of inactivity and retirement, the object being, either to check evaporation of the moisture of the body, or to secure the animal against the cold by retaining a thin layer of slightly warm air immediately within the aperture of the shell.

The epiphragm differs widely in character in different species, sometimes (Clausilia, Pupa, Planorbis) consisting of the merest pellicle of transparent membrane, while at others (Helix aperta, H. pomatia) it is a thick chalky substance, with a considerable admixture of carbonate of lime, with the consistency of a hardened layer of plaster of Paris. Within these extremes every variety of thickness, solidity, and transparency occurs. During long hibernation several epiphragms are not unfrequently formed by the same individual snail, one within the other, at gradually lessening distances. The epiphragm thus performs, to a certain extent, the part of an operculum, but it must be remembered that it differs radically from an operculum physiologically, in being only a temporary secretion, while the operculum is actually a living part of the animal.

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.

Limax flavus, which is fond of inhabiting the vicinity of cellars, makes its presence most disagreeable by attacking articles of food, and especially by insinuating itself into vessels containing meal and flour.[57] It is particularly partial to cream.

Slugs will sometimes bite their captor’s hands. Mr. Kew relates that a Limax agrestis, on being stopped with the finger, while endeavouring to escape from the attack of a large Arion, attempted to bite fiercely, the rasping action of its radula being plainly felt. According to the same authority, probably all the slugs will rasp the skin of the finger, if it is held out to them, and continue to do so for a considerable time, without however actually drawing blood.[58] While Mr. Gain was handling a large Arion ater, it at once seized one of the folds of skin between the fingers of the hand on which it was placed; after the action of the radula had been allowed to continue for about a minute, the skin was seen to be abraded.[59] Another specimen of Arion ater, carried in the hand for a long time enclosed in a dock leaf, began to rasp the skin. The operation was permitted until it became too painful to bear. Examination with a lens showed the skin almost rasped away, and the place remained tender and sore, like a slight burn, for several days.[60]

Helix pisana, if freshly caught, and placed in a box with other species, will set to work and devour them within twenty-four hours. The present writer has noticed it, in this position, attack and kill large specimens of H. ericetorum, cleaning them completely out, and inserting its elongated body into the top whorls of its unfortunate victims in a most remarkable manner. Amongst a large number of species bred in captivity by Miss F. M. Hele,[61] was Hyalinia Draparnaldi. In the first summer the young offspring were fed on cabbage, coltsfoot, and broadleafed docks. They would not hibernate even in the severest frosts, and, no outdoor food being available, were fed on chopped beef. This, Miss Hele thinks, must have degenerated their appetites, for in the following spring and summer they constantly devoured each other.

Zonites algirus feeds on decayed fruit and vegetables, and on stinking flesh.[62] Achatina panthera has been known to eat meat, other snails (when dead), vegetables, and paper.[63] The common Stenogyra decollata of the South of Europe has a very bad character for flesh-eating habits, when kept in captivity. Mr. Binney[64] kept a number for a long time as scavengers, to clean the shells of other snails. As soon as a living Helix was placed in a box with them, one would attack it, introduce itself into the upper whorls, and completely remove the animal. One day a number of Succinea ovalis were left with them for a short time, and disappeared entirely! The Stenogyra had eaten shell as well as animal. This view of Stenogyra is quite confirmed by Miss Hele, who has bred them in thousands. “I can keep,” she writes,[65] “no small Helix or Bulimus with them, for they at once kill and eat them. They will also eat raw meat.”

Even the common Limnaea stagnalis, which is usually regarded as strictly herbivorous, will sometimes betake itself, apparently by preference, to a diet of flesh. Karl Semper frequently observed the Limnaeae in his aquarium suddenly attack healthy living specimens of the common large water newt (Triton taeniatus), overcome them, and devour them, although there was plenty of their favourite vegetable food growing within easy reach.[66] The same species has also been noticed to devour its own ova, and the larvae of Dytiscus. Limnaea peregra has been detected capturing and partially devouring minnows in an aquarium, when deprived of other food, and Dr. Jeffreys has seen the same species attack its own relatives under similar circumstances, piercing the spire at its thinnest point near to the apex.[67] L. stagnalis, kept in an aquarium, has succeeded in overpowering and partially devouring healthy specimens of the common stickleback.[68]

Powers of Intelligence, Homing, and finding Food.—It is not easy to discover whether land Mollusca possess any faculties which correspond to what we call intelligence, as distinct from their capacities for smell, sight, taste, and hearing. Darwin mentions[69] a remarkable case, communicated to him by Mr. Lonsdale. A couple of Helix pomatia, one of which was sickly, were placed in a small and ill-provided garden. The stronger of the two soon disappeared over the wall into the next garden, which was well furnished with food. It was concluded that the snail had deserted its weakly mate, but after twenty-four hours it returned, and apparently communicated the results of its expedition, for after a short time both started off along the same track, and disappeared over the wall. According to Dr. W. H. Dall,[70] a young girl who possessed a remarkable power over animals succeeded in training a snail (H. albolabris) to come out of its lurking-place at her call. If placed in a room, it would shrink into its shell at the sound of any other voice, but it would always start off in the direction of hers.

Snails and slugs possess to a considerable extent the faculty of ‘homing,’ or returning to the same hiding-place day after day, after their night excursions in search of food. Mr. C. Ashford once marked with a dab of white paint seven Helix aspersa found lurking under a broken flagstone; at 10 P.M. the same evening three had disappeared on the forage; the next morning all were ‘at home.’ The following night at 10 P.M. five were gone out, two being discovered with some difficulty ‘in a small jungle’ six feet away; the next morning six out of the seven were safely beneath the flagstone. According to the same authority, Helix aspersa will find its way across a cinder-path (which it specially detests) to get to its favourite food, and will return by the same way to its old quarters, although it could easily have found new lodgings nearer the food-supply. A snail has been observed to occupy a hole in the brick wall of a kitchen-garden about four feet from the ground. Leaning against the wall, and immediately under the hole, was a piece of wood, the lower end of which rested in a bed of herbs. For months the snail employed this ladder between its food and its home, coming down as soon as it was dark, and retiring to rest during the day.

In greenhouses a slug will forage night after night—as gardeners know to their cost—over the same beat, and will always return to the same hiding-place. Limax flavus has been noticed crawling with great regularity to a sink from a hole near the water-pipe, and keeping to a well-marked circular track. In all probability the scent, either of the desired object of food, or of the creature’s own trail, plays a considerable part in keeping it to the same outward and homeward track, or at least in guiding it back to its hiding-place. Yet even scent is occasionally at fault, for on one occasion a Limax flavus was accustomed to make nightly excursions to some basins of cream, which were kept in a cool cellar. When the basins were removed to a distant shelf, the creature was found the next morning ‘wandering disconsolately’ about in the place where the basins had formerly stood.[71]

A remarkable case of the power of smell, combined with great perseverance on the part of a Helix, is recorded by Furtado.[72] He noticed a Helix aspersa lodged between a column on a verandah and a flower-pot containing a young banana plant, and threw it away into a little court below, and six or seven yards distant. Next morning the snail was in precisely the same place on the flower-pot. Again he threw it away, to the same distance, and determined to notice what happened. Next morning at nine o’clock, the snail was resting on the rail of a staircase leading up to the verandah from the court; in the evening it started again, quickening its space as it advanced, eventually attacking the banana in precisely the same place where it had been gnawed before.

For further instances of the power of smell in snails, see chap. vii.

Slugs have been known to make their way into bee-hives, presumably for the sake of the honey.[73] ‘Sugaring’ the trees at night for moths will often attract a surprising concourse of slugs. Sometimes a particular plant in a greenhouse will become the object of the slugs’ persistent attacks, and they will neglect every other food in order to obtain it. Farfugium grande is one of these favourite foods, “the young leaves and shoots being always eaten in preference to all other plants growing in the houses; where no Farfugiums were kept the slugs nibbled indiscriminately at many kinds.”[74] The flowers of orchidaceous plants exercise a special attraction over slugs, which appear to have some means of discovering when the plants are in bloom. “I have often observed,” says Mr. T. Baines, “that a slug will travel over the surface of a pot in which is growing a Dendrobium nobile, a Cattleya, Vanda, or similar upright plant for a score of times without ever attempting to ascend into the head of the plant unless it is in bloom, in which case they are certain to find their way straight to the flowers; after which they will descend, and return to some favourite hiding-place, often at the opposite end of the house.”[75] Mr. R. Warner has “actually seen many little slugs suspending themselves by slime-threads from the rafters and descending on the spikes of the beautiful Odontoglossum alexandrae; and thus many spikes, thickly wadded round with cotton wool (which the slugs could not travel over), and growing in pots surrounded by water, had been lost.”[76] Perhaps the most singular instance of a liking for a particular food is that related by Mr. E. Step.[77] In a London publishing house, slugs were observed, during a period of nearly twelve months, to have fed almost nightly on the colouring matter in certain bookcovers, and though the trails were often seen over the shelves, and cabbage and lettuce leaves laid down to tempt the creatures, they continued their depredations with impunity for the time above mentioned.

Limnaea peregra has been observed feeding on old fish-heads thrown into a dirty stream, and a large gathering of Limnaea stagnalis has been noticed feeding upon an old newspaper in a pond on Chislehurst Common, ‘so that for the space of about a square foot nothing else could be seen.’[78]

Tenacity of Life.—Land Mollusca have been known to exhibit, under unusual conditions, remarkable tenacity of life. Some of the most noteworthy and best authenticated instances of this faculty may be here mentioned.

The well-known story of the British Museum snail is thus related by Mr. Baird.[79] On the 25th March 1846 two specimens of Helix desertorum, collected by Charles Lamb, Esq., in Egypt some time previously, were fixed upon tablets and placed in the collection among the other Mollusca of the Museum. There they remained fast gummed to the tablet. About the 15th March 1850, having occasion to examine some shells in the same case, Mr. Baird noticed a recently formed epiphragm over the mouth of one of these snails. On removing the snails from the tablet and placing them in tepid water, one of them came out of its shell, and the next day ate some cabbage leaf. A month or two afterwards it began repairing the lip of its shell, which was broken when it was first affixed to the tablet.

While resident in Porto Santo, from 27th April to 4th May 1848, Mr. S. P. Woodward[80] collected a number of Helices and sorted them out into separate pill-boxes. On returning home, these boxes were placed in empty drawers in an insect cabinet, and on 19th October 1850, nearly two and a half years afterwards, many of them were found to be still alive. A whole bagful of H. turricula, collected on the Ilheo de Cima on 24th April 1849, were all alive at the above-mentioned date.

In September 1858 Mr. Bryce Wright sent[81] to the British Museum two specimens of H. desertorum which had been dormant for four years. They were originally collected in Egypt by a Mr. Vernèdi, who, in May 1854, while stopping at one of the stations in the desert, found a heap of thorn-bushes lying in a corner of the building, rather thickly studded with the snails. He picked off fifteen or twenty specimens, which he carried home and locked up in a drawer, where they remained undisturbed until he gave two to Mr. Wright in September 1858.

In June 1855 Dr. Woodward placed specimens of H. candidissima and H. aperta in a glass box, to test their tenacity of life; he writes of their being still alive in April 1859.

Mr. R. E. C. Stearns records[82] a case of Buliminus pallidior and H. Veatchii from Cerros I. living without food from 1859 to March 1865.

H. Aucapitaine mentions[83] a case of H. lactea found in calcinated ground in a part of the Sahara heated to 122° F., where no rain was said to have fallen for five years. The specimen revived after being enclosed in a bottle for three and a half years.

In August 1863, Mr. W. J. Sterland[84] put specimens of H. nemoralis in a box and afterwards placed the box in his cabinet; in November 1866 one specimen was discovered to be alive.

Gaskoin relates[85] a case in which specimens of H. lactea were purchased from a dealer in whose drawer they had been for two years. This dealer had them from a merchant at Mogador, who had kept them for more than that time under similar conditions. One of these shells on being immersed in water revived, and in April 1849 was placed quite alone under a bell jar with earth and food. In the end of the following October about thirty young H. lactea were found crawling on the glass.

Mr. R. D. Darbishire bought[86] some H. aperta in the market at Nice on 18th February 1885. Two specimens of these, placed with wool in a paper box, were alive in December 1888. This is a very remarkable case, H. aperta not being, like H. desertorum, H. lactea, H. Veatchii and Bul. pallidior, a desert snail, and therefore not accustomed to fasting at all.

Age of Snails.—It would appear, from the existing evidence, which is not too plentiful, that five years is about the average age of the common garden snail. Mr. Gain has published[87] some interesting observations on the life of a specimen from the cradle to the grave, which may be exhibited in a tabular form.

According to Clessin, the duration of life in Vitrina is one year, Cyclas 2 years; Hyalinia, Succinea, Limnaea, Planorbis, and Ancylus are full grown in 2 to 3 years, Helix and Paludina in 2 to 4, and Anodonta in 12 to 14. Hazay finds[88] that the duration of life in Hyalinia is 2 years, in Helix pomatia 6 to 8, in Helix candicans 2 to 3, in Paludina 8 to 10, in Limnaea and Planorbis 3 to 4.

Growth of the Shell.—Mr. E. J. Lowe, many years ago, conducted[89] some interesting experiments on the growth of snails. The facts arrived at were—

(1) The shells of Helicidae increase but little for a considerable period, never arriving at maturity before the animal has once become dormant.

(2) Shells do not grow whilst the animal itself remains dormant.

(3) The growth of shells is very rapid when it does take place.

(4) Most species bury themselves in the ground to increase the dimensions of their shells.

Six recently hatched H. pomatia were placed in a box and regularly fed on lettuce and cabbage leaves from August until December, when they buried themselves in the soil for winter; at this period they had gradually increased in dimensions to the size of H. hispida. On the 1st April following, the box was placed in the garden, and on the 3rd the Helices reappeared on the surface, being no larger in size than they were in December. Although regularly fed up to 20th June, they were not perceptibly larger, but on that day five of them disappeared, having buried themselves, with the mouth of the shell downwards, in the soil. After ten days they reappeared, having in that short time grown so rapidly as to be equal in size to H. pisana. On the 15th July they again buried themselves, and reappeared on 1st August, having again increased in size. For three months from this date they did not become perceptibly larger; on 2nd November food was withheld for the winter and they became dormant.

A similar experiment, with similar results, was carried on with a number of H. aspersa, hatched on 20th June. During the summer they grew but little, buried themselves on 10th October with the head upwards, and rose to the surface again on 5th April, not having grown during the winter. In May they buried themselves with the head downwards, and appeared again in a week double the size; this went on at about fortnightly intervals until 18th July, when they were almost fully grown.

Helix nemoralis, H. virgata, H. caperata, and H. hispida bury themselves to grow; H. rotundata burrows into decayed wood; Hyalinia radiatula appears to remain on decaying blades of grass; Pupa umbilicata, Clausilia rugosa, and Buliminus obscurus bury their heads only.

The observations of Mr. W. E. Collinge[90] do not at all agree with those of Mr. Lowe, with regard to the mode in which land Mollusca enlarge their shells. He bred and reared most of the commoner forms of Helix and also Clausilia rugosa, but never saw them bury any part of their shell when enlarging it. While admitting that they may increase their shells when in holes or burrows of earthworms, he thinks that the process of burying would seriously interfere with the action of the mantle during deposition, and in many cases damage the membranaceous film before the calcareous portion was deposited. Mr. Collinge has found the following species under the surface in winter: Arion ater (3–4 in.), Agriolimax agrestis, (6–8 in.), Hyalinia cellaria and H. alliaria (6–8 in.), Hyalinia glabra (5 in.), Helix aspersa (5–6 in.), H. rufescens (4–6 in.), H. rotundata (4–5 in.), H. hispida (7 in.), Buliminus obscurus (4–6 in.), B. montanus[91] (24 in.), and the following in summer, Hyalinia cellaria and alliaria (6–8 in.), Helix rotundata (4–5 in.), Balea perversa (6–8 in.), Cyclostoma elegans (3–4 in.). The same author has found the following species of fresh-water Mollusca living in hard dry mud: Sphaerium corneum (3–14 in.), S. rivicola (5–6 in.), S. lacustre (10–14 in.), all the British species of Pisidium (4–12 in.), Limnaea truncatula (18 in., a single specimen). All our species of Unio, Anodonta, Bithynia, and Paludina bury themselves habitually in fine or thick wet mud, to a depth of from 4 to 14 inches.

This burying propensity on the part of Mollusca has been known to play its part in detecting fraud. When my friend Mr. E. L. Layard was administering justice in Ceylon, a native landowner on a small scale complained to him of the conduct of his neighbour, who had, during his absence from home, diverted a small watercourse, which ran between their holdings, in such a way as to filch a certain portion of the land. The offender had filled up and obliterated the ancient course of the stream, and protested that it had never run but in its present bed. Mr. Layard promptly had a trench sunk across what was said to be the old course, and the discovery of numerous living Ampullaria, buried in the mud, confirmed the story of one of the litigants and confounded the other.[92]

Depositing and Hatching of Eggs: Self-fertilisation.—There appears to be no doubt that Helices, when once impregnated, can lay successive batches of eggs, and possibly can continue laying for several years, without a further act of union. A specimen of Helix aspersa was noticed in company with another on 5th August; on 9th August it laid eggs in the soil, and early in the following summer it laid a second batch of eggs, although its companion had been removed directly after its first introduction. An Arion received from a distance laid 30 eggs on 5th September, and 70 more on the 23rd of the same month, although quite isolated during the whole time.[93] By far the most remarkable case of the kind is related by Gaskoin.[94] A specimen of Helix lactea was kept in a drawer for about two years, and then in another drawer for about two years more. It was then taken out, and placed in water, when it revived, and was placed alone under a bell jar with earth and food. Six months after, about 30 young H. lactea were found crawling on the glass, the act of oviposition not having been observed.

The observations of Mr. F. W. Wotton,[95] with regard to the fertilisation and egg-laying of Arion ater, are of extreme interest and value. A pair of this species, kept in captivity, united on 10th September 1889, the act lasting about 25 minutes. From that date until the eggs were laid, the animals looked sickly, dull of colour, with a somewhat dry skin. Eggs were deposited in batches, one, which we will call A, beginning three days before B. On 10th October A laid 80 eggs; on the 16th, 110; on the 25th, 77; on 8th November, 82; and on 17th November, 47; making a total of 396. Specimen B, which began on 13th October, three days after A, made up for the delay by laying 246 eggs in 40 hours; on 26th October it laid 9, on 10th November, 121; and on 30th November, 101; a total of 477. These eggs weighed 624 to the ounce, and, in excluding the batch of 246, B parted with ⅜ of its own weight in 40 hours, while the whole number laid were rather over ¾ of its own weight!

While depositing the eggs, the slug remained throughout in the same position on the surface of the ground, with the head drawn up underneath the mantle, which was lifted just above the reproductive orifice. When taken into the hand, it went on laying eggs without interruption or agitation of any kind. After it had finished laying it ate half a raw potato and then took a bath, remaining submerged for more than an hour. Bathing is a favourite pastime at all periods. Specimens, says Mr. Wotton, have survived a compulsory bath, with total submersion, of nearly three days’ duration.

Mr. Wotton’s account of the hatching of the eggs is equally interesting. It is noticeable that the eggs of one batch do not hatch by any means simultaneously; several days frequently intervene. The average period is about 60 days, a damp and warm situation bringing out the young in 40 days, while cold and dryness extended the time to 74 days, extremes of any kind proving fatal. Of the batch of eggs laid by B on 30th November, the first 2 were hatched on the following 16th January, and 2 more on the 17th; others, from 10 to 20, followed suit on the succeeding 5 days, until 82 in all were hatched, the remaining 19 being unproductive.[96]

By placing the egg on a looking-glass the act of exclusion can be perfectly observed. For several days the inmate can be seen in motion, until at last a small crack appears in the surface of the shell: this gradually enlarges, until the baby slug is able to crawl out, although it not unfrequently backs into the shell again, as if unwilling to risk itself in the world. When it once begins to crawl freely, it buries itself in the ground for 4 or 5 days without food, after which time it emerges, nearly double its original size. At exclusion, the average length is 9 mm., increasing to 56 mm. after the end of 5 months. Full growth is attained about the middle of the second year, and nearly all die at the end of this year or the beginning of the next. Death from exhaustion frequently occurs after parturition. Death from suffocation is sometimes the result of the formation of small blisters on the margin of the respiratory aperture. The attacks of an internal parasite cause death in a singular way. The upper tentacles swell at the base in such a way as to prevent their extrusion; digestive troubles follow, with rigidity and loss of moisture, and death ensues in 2 or 3 days.

Mr. Wotton isolated newly-hatched specimens, with the view of experimenting on their power of self-fertilisation, if the opportunity of fertilising and being fertilised by others was denied them. One of these, after remaining in absolute solitude for 10½ months, began to lay, scantily at first (11th January, 2; 25th January, 2; 11th February, 2), but more abundantly afterwards (3rd April, 60; 15th and 16th, 70; 29th, 53, etc.), the eggs being hatched out in 42–48 days. The precautions taken seem to have been absolutely satisfactory, and the fact of the power of self-fertilisation appears established as far as Arion ater is concerned.

Braun took young individuals of Limnaea auricularia on the day they were hatched out, and placed them singly in separate vessels with differing amounts of water. This was on 15th June 1887. In August 1888 specimen A had only produced a little spawn, out of which three young were hatched; specimen B had produced four pieces of spawn of different sizes, all of which were hatched; specimen C, which happened to be living with three Planorbis, produced five pieces of spawn distinctly Limnaeidan, but nothing is recorded of their hatching. Self-impregnation, therefore, with a fruitful result, appears established for this species of Limnaea.[97]

Reproduction of Lost Parts.—When deprived of their tentacles, eyes, or portions of the foot, Mollusca do not seem to suffer severely, and generally reproduce the lost parts in a short time. If, however, one of the ganglia is injured, they perish. Certain of the Mollusca possess the curious property of being able to amputate certain parts at will. When Prophysaon, a species of Californian slug, is annoyed by being handled, an indented line appears at a point about two-thirds of the length from the head, the line deepens, and eventually the tail is shaken completely off. Sometimes the Prophysaon only threatens this spontaneous dismemberment; this line appears (always exactly in the same place), but it thinks better of it, and the indentation proceeds no further.[98] According to Gundlach,[99] Helix imperator and H. crenilabris, two large species from Cuba, possess the same property, which is said to be also characteristic of the sub-genus Stenopus (W. Indies). Amongst marine species, Harpa ventricosa and Solen siliqua have been observed to act in a similar way, Harpa apparently cutting off the end of the foot by pressure of the shell. Karl Semper, in commenting on the same property in species of Helicarion from the Philippines (which whisk their tail up and down with almost convulsive rapidity, until it drops off), considers[100] it greatly to the advantage of the mollusc, since any predacious bird which attempted to seize it, but only secured a fragment of tail, would probably be discouraged from a second attack, especially as the Helicarion would meanwhile have had time to conceal itself among the foliage.

Strength and Muscular Force.—The muscular strength of snails is surprisingly great. Sandford relates[101] an experiment on a Helix aspersa, weighing ¼ oz. He found it could drag vertically a weight of 2¼ oz., or nine times its own weight. Another snail, weighing ⅓ oz., was able to drag in a horizontal direction along a smooth table twelve reels of cotton, a pair of scissors, a screwdriver, a key, and a knife, weighing in all no less than 17 oz., or more than fifty times its own weight. This latter experiment was much the same as asking a man of 12 stone to pull a load of over 3¾ tons.

If a snail be placed on a piece of glass and made to crawl, it will be seen that a series of waves appear to pursue one another along the under surface of the foot, travelling from back to front in the direction in which the animal is moving. Simroth has shown that the sole of the foot is covered with a dense network of muscular fibres, those which run longitudinally being chiefly instrumental in producing the undulatory motion. By means of these muscles the sole is first elongated in front, and then shortened behind to an equal extent. Thus a snail slides, not on the ground, but on its own mucus, which it deposits mechanically, and which serves the purpose of lubricating the ground on which it travels. It has been calculated that an averaged sized snail of moderate pace progresses at the rate of about a mile in 16 days 14 hours.[102]

Sudden Appearance of Mollusca.—It is very remarkable to notice how suddenly Pulmonata seem to appear in certain districts where they have not been noticed before. This sudden appearance is more common in the case of fresh-water than of land Mollusca, and there can be little doubt that, wherever a new pond happens to be formed, unless there is something in its situation or nature which is absolutely hostile to molluscan life, Mollusca are certain to be found in it sooner or later. “Some 23 years ago,” writes Mr. W. Nelson,[103] “I was in the habit of collecting shells in a small pond near to the Black Hills, Leeds. At that time the only molluscan forms found there were a dwarf form of Sphaerium lacustre, Pisidium pusillum, Planorbis nautileus, and Limnaea peregra. About 10 years ago I resumed my visits to the locality, and found, in addition to the species already enumerated, Planorbis corneus. These were the only species found there until this spring [1883], when, during one of my frequent visits, I was surprised to find Physa fontinalis and Planorbis vortex were added to the growing list of species. Later on Pl. carinatus, Limnaea stagnalis, and Ancylus lacustris turned up; and during June, Pl. contortus was found in this small but prolific pond.” Limnaea glutinosa is prominent for these remarkable appearances and disappearances. In 1822 this species suddenly appeared in some small gravel pits at Bottisham, Cambs., in such numbers that they might have been scooped out by handfuls. After that year they did not appear numerous, and after three or four seasons they gradually disappeared.[104] Physa (Aplecta) hypnorum is noted in a similar way. In February 1852, for instance, after a wet month, the water stood in small puddles about 3 feet by 2 in a particular part of Bottisham Park which was sometimes a little swampy, though usually quite dry. One of these puddles was found to contain immense numbers of the Aplecta, which up to that time had not been noted as occurring in Cambridgeshire at all.[105] In a few days the species entirely disappeared and was never again noticed in the locality.[106]

Writing to the Zoological Society of London from New Caledonia, Mr. E. L. Layard remarks:[107] “The West Indian species Stenogyra octona has suddenly turned up here in thousands; how introduced, none can tell. They are on a coffee estate at Kanala on the east coast. I have made inquiries, and cannot find that the planter ever had seed coffee from the West Indies. All he planted came from Bombay, and it would be interesting to find out whether the species has appeared there also.”

Sometimes a very small event is sufficient to disturb the natural equilibrium of a locality, and to become the cause either of the introduction or of the destruction of a species. In 1883 a colony of Helix sericea occupied a portion of a hedge bottom twenty yards long near Newark. It scarcely occurred outside this limit, but within it was very plentiful, living in company with H. nemoralis, H. hortensis, H. hispida, H. rotundata, Hyalinia cellaria and Hy. nitidula, and Cochlicopa lubrica. In 1888 the hedge was well trimmed, but the bottom was not touched, and the next year a long and careful search was required to find even six specimens of the sericea.[108]

Showers of Shells.—Helix virgata, H. caperata, and Cochlicella acuta sometimes occur on downs near our sea-coasts in such extraordinary profusion, that their sudden appearance out of their hiding-places at the roots of the herbage after a shower of rain has led to the belief, amongst credulous people, that they have actually descended with the rain. There seems, however, no reason to doubt that Mollusca may be caught up by whirlwinds into the air and subsequently deposited at some considerable distance from their original habitat, in the same way as frogs and fishes. A very recent instance of such a phenomenon occurred[109] at Paderborn, in Westphalia, where, on 9th August 1892, a yellowish cloud suddenly attracted attention from its colour and the rapidity of its motion. In a few moments it burst, with thunder and a torrential rain, and immediately afterwards the pavements were found to be covered with numbers of Anodonta anatina, all of which had the shell broken by the violence of the fall. It was clearly established that the shells could not have been washed into the streets from any adjacent river or pond, and their true origin was probably indicated when it was found that the funnel-shaped cloud which burst over the town had passed across the one piece of water near Paderborn, which was known to contain the Anodonta in abundance.

Cases of Singular Habitat.—Mollusca sometimes accustom themselves to living in very strange localities, besides the extremes of heat and cold mentioned above (pp. 23–24). In the year 1852, when some large waterpipes in the City Road, near St. Luke’s Hospital, were being taken up for repairs, they were found to be inhabited in considerable numbers by Neritina fluviatilis and a species of Limnaea.[110] Dreissensia polymorpha has been found in a similar situation in Oxford Street, and also in Hamburg, and has even been known to block the pipes and cisterns of private houses. In an engine cistern at Burnley, 60 feet above the canal from which the water was pumped into the cistern, were found the following species: Sphaerium corneum, S. lacustre; Valvata piscinalis, Bithynia tentaculata; Limnaea peregra, very like Succinea in form and texture; Planorbis albus, P. corneus, P. nitidus, P. glaber, and thousands of P. dilatatus, much larger than the forms in the canal below, a fact probably due to the equable temperature of the water in the cistern all the year round.[111] In certain parts of southern Algeria the fresh-water genera Melania and Melanopsis inhabit abundantly waters so surcharged with salt that the marine Cardium edule has actually become extinct from excess of brine. The common Mytilus edulis is sometimes found within the branchial chamber and attached to the abdomen of crabs (Carcinus maenas), which are obliged to carry about a burden of which they are powerless to rid themselves (see p. 78). A variety of the common Limnaea peregra lives in the hot water of some of the geysers of Iceland, and has accordingly been named geisericola.

Underground Snails.—Not only do many of the land Mollusca aestivate, or hibernate, as the case may be, beneath the surface of the soil, but a certain number of species live permanently underground, like the mole, and scarcely ever appear in the light of day. Our own little Caecilianella acicula lives habitually from 1 to 3 feet below ground, appearing to prefer the vicinity of graveyards. Testacella, the carnivorous slug, scarcely ever appears on the surface during the day, except when driven by excessive rain, and even then it lurks awhile under some protecting cover of leafage. There is a curious little Helix (tristis Pfr.), peculiar to Corsica, which is of distinctly subterranean habits. It lives in drifted sand above high-water mark, always at the roots of Genista Saltzmanni, at a depth which varies with the temperature and dryness of the air. In hot and very dry weather it buries itself nearly 2 feet below the surface, only coming up during rain, and burying itself again immediately the rain is over. Like a Solen, it often has a hole above its burrow, by which it communicates with the air above, so as to avoid being stifled in the sand. The animal, in spite of its dry habitat, is singularly soft and succulent, and exudes a very glutinous mucus. It probably descends in its burrow until it arrives at the humid stratum, the persistence of which is due to the capillarity of the sand.[112] I am assured by Mr. E. L. Layard that precisely similar underground habits are characteristic of Coeliaxis Layardi, which lives exclusively in sand at the roots of scrub and coarse grass at East London.

Rock-boring Snails.—Cases have sometimes been recorded, from which it would appear that certain species of snails possess the power of excavating holes in rocks to serve as hiding-places. At Les Bois des Roches, ten miles from Boulogne, occur a number of solid calcareous rocks scattered about in the wood. The sides of the rocks which face N.E. and E. are covered with multitudes of funnel-shaped holes, 1½ inch in diameter at the opening and contracting suddenly within to ½ inch. Sometimes the holes are 6 inches deep, and terminate, after considerable windings, in a cup-shaped cavity. Helix hortensis inhabits these holes, and has been observed to excavate them at the rate of ½ inch each hibernation, choosing always the side of the rock which is sheltered from the prevailing rains. It does not form an epiphragm, but protrudes part of its body against the rock. That the snails secrete an acid which acts as a solvent seems probable from the fact that red litmus paper, on being applied to the place where the foot has been, becomes stained with violet.[113] Helix aspersa is said to excavate holes 10 to 12 cm. deep at Constantine,[114] and H. Mazzullii is recorded as perforating limestone at Palermo.[115]

Snails as Barometers.—An American writer of more than thirty years ago[116] gave his experience of Helices as weather-prophets. According to him, H. alternata is never seen abroad except shortly before rain; it then climbs on the bark of trees, and stations itself on leaves. Helix clausa, H. ligera, H. pennsylvanica, and H. elevata climb trees two days before rain, if it is to be abundant and continuous. Succinea does the same, and its body is yellow before rain and bluish after it. Several of the Helices assume a sombre colour after rain, when their bodies are exceedingly humid; after the humidity has passed off they resume a clearer and lighter tint.