Natural History, Mollusca/Pulmonifera
ORDER PULMONIFERA.
(Lung-breathing Mollusks.)
This extensive Order contains Mollusca which differ from all the rest of the Class, by breathing atmospheric air. There is an orifice situated on the right side, beneath the margin of the mantle, which opens into a chamber lined with a delicate net-work of vessels. This lined cavity is analogous to the lungs of vertebrate animals, and its muscular floor is said to perform alternate motions, answering to those of the diaphragm, by which the lungs are filled and emptied by turns.
Many of the species inhabit fresh waters; but the greater number are denizens of the land, requiring, however, a damp atmosphere, to preserve them in health and vigour. The close, humid forests of tropical countries sustain the terrestrial species in immense number and variety: the shells of many of these—distinguished by their curious forms, by their large size, by their rich and brilliant colours, often arranged in finely-contrasted bands, and by their delicately sculptured surfaces,—are among the most precious ornaments of conchological cabinets. The aquatic species form, notwithstanding the element in which they live, no exception to the leading character of the Order; they also breathe air, which they obtain by coming periodically to the surface.
In general, the subjects of this Order are protected by an ample spiral shell; but in some this appendage is very minute, concealed within the substance of the mantle, or altogether wanting. Our own country furnishes examples of each of these kinds. All the native species are comprised in five families,— Cyclostomadæ, Auriculadæ, Limneadæ, Helicidæ, and Limacidæ.
Family Cyclostomadæ.
This is an extensive family, though represented in Britain by but a single species. The great majority of its members are elegantly formed and beautifully sculptured shells from warm countries. The shell is spiral, ample in its dimensions, with a circular aperture, generally surrounded in the adult with a frill-like, shelly expansion; it is closed by a spiral, shelly operculum.
The animal has a broad foot, divided longitudinally; a central spiral body, enveloped in a single-edged mantle.
THE ELEGANT CYCLOSTOMA. There are two tentacles, which are lengthened and contractile, with an eye placed on the outer side of the base of each. The sexes are distinct. All the species are terrestrial, and are generally found on trees, on the leaves of which they feed. The only British species (Cyclostoma elegans) is a little shell, about half an inch in length, and rather less in width, of a grey or purplish yellow hue, often marked with two rows of dark spots. The spire is composed of five rounded whorls, marked with numerous close-set, raised lines, running spirally, with finer longitudinal ones between them. There is an umbilicus behind the pillar; the operculum is hard, horny externally, and marked with a single, depressed, spiral line.
This Mollusk lives on vegetable matter, like the snails, and is found in damp places, on a chalky soil. It is not very generally distributed, but is said to be abundant on the warm chalk hills, covered with brushwood, at Caversham, near Reading, in Berkshire. It is common also in the Isle of Portland, where I have seen it numerous in April, crawling on the twigs of shrubs, with the operculum carried behind, in a curious manner.
Family Auriculadæ.
The characters of this family are the following :— The animal has a lengthened foot, a lengthened ringed muzzle, two somewhat cylindrical tentacles, with eyes near their bases on the inner side. The body is spiral, placed on the centre of the foot, and invested with a thin mantle, with a thickened edge. There is an ample spiral shell, the pillar of which is plaited at all periods of its life.
Mr. Gray observes of these Mollusca, which are feebly represented in Britain by some three or four small species comprised in three genera, that they appear, by their habit and character, to be exactly intermediate between the land and the fresh-water Univalve Mollusca. They have the sessile eyes of the Pond-snails, placed behind instead of in front of the tentacles, and the subcylindrical tentacles of the Land-snails; but the tentacles are not retractile under the skin of the neck. In the same manner, the Carychia and the Acmea are terrestrial, living in damp moss; the Conovuli live in the mud at the mouths of rivers, or in the sea: they seldom leave salt, or at least brackish water. There are some foreign species which live in ponds, and have all the habits of our Pond-snails, only that their pillar is more distinctly plaited.
The family may be represented by Conovulus denticulatus, an oblong, spiral shell, rarely exceeding half an inch in length, of a brown or purplish hue. Its last whorl is long and compressed; the pillar is grooved, with several spiral plates; the throat is also grooved. A remarkable character of this shell is, that the pillar extends no farther than the upper part of the last whorl, the upper whorls being destitute of any pillar or internal spiral division. This character is common to most species of the family, and forms, as Mr. Gray observes, one of its best technical distinctions. It is attributed to the animal's absorbing the partitions which separate the upper whorls, and thus converting the spire into a single cavity.
THE DENTICULATED CONOVULUS.
(Nat. size and magnified.)This little Mollusk is by no means common; it has been found in the marshes near Faversham, at the roots of rushes. It is said also to inhabit the clefts of rocks, near high water-mark, as well as the mud left bare by the tide, at the mouths of rivers. The animal feeds, according to M. Bouchard Chantreux, on the detritus of marine plants and rotten wood. It lays twelve or thirteen eggs in the months of June and September, united by a viscid matter into a small mass, which is fixed under the more humid stones. The eggs are globular, yellowish, and quite diaphanous: they are hatched about the fifteenth day, and the animals reach their full size about the end of the second year. They do not hybernate.
Family Limneadæ.
(Pond-Snails)
The Fresh-water Snails were scattered by Linnæus and his followers among various marine and terrestrial genera, on account of the diversity which is found in the shape and appearance of their shells. Since more attention has been paid by conchologists to the structure of the animal inhabiting a given shell, the close similarity which subsists between them has prompted their union into one family, and that one of the most natural of all those into which the Mollusca are divided. They are distinguished by the following characters:—
The animals have a lengthened foot, a spiral body, a short, broad muzzle, two large tentacles, triangular and compressed, or awl-shaped, with the eyes near their outer bases. The tongue is furnished with rows of hooked teeth. The mantle, which is ample, has a thin edge, and is protected by a shell of exceedingly variable form, being spiral, turreted, discoid, or simply conical. Those which are spiral are sometimes regular, and sometimes reversed. The colour is generally pale brown, uniform in hue, and the surface is closed with a hard olive skin, technically called the periostraca, or that which is around the shell. They are destitute of an operculum.
The habits of these Mollusca are as identical as their structure. They inhabit lakes, ponds, and ditches of fresh water; are found, but less commonly, in rivers, and still more rarely in brackish water. They crawl on the mud at the bottom, or on the stems and leaves of aquatic plants, always coming to the surface, at intervals, to take in a fresh supply of air into the lung chamber for respiration. They may frequently be seen floating, at the surface of still water, by the expanded foot, the shell being downward and submerged.
They lay their eggs in round or oval masses of consistent jelly, each mass containing a number of eggs, varying from three to upwards of a hundred, according to the genus. The masses are attached to plants or stones beneath the surface, and are hatched in about a fortnight after they are deposited.
The Pond-snails are very numerous, and widely distributed, species being found in almost all parts of the world. Twenty-four are reckoned by Messrs. Forbes and Hanley as British.
"It had been supposed that the shells of fluviatile Mollusca could be distinguished from those of the terrestrial kinds, by the edge of the mouth of the shell never being furnished with a thickened internal rib, and not being in the slightest degree reflexed, and that the animal never closes it with an epiphragm; however, further examination has shown that when the Pond-snails and the Whirl-shells are left nearly dry by the evaporation of the water, either by the heat, or by dryness of the weather in winter, these animals assume the character of terrestrial Mollusca, thicken and reflect their mouth, and form an epiphragm to prevent themselves from being destroyed by the drying up of the fluid necessary for their respiration and life."[1]
Genus Limneus.
In this genus the shell is ovate, oblong, or turreted, thin and horny, and transparent. The aperture is ovate or expanded, the margin entire, and the pillar marked with a single oblique plait running into the axis.
LAKE MUD SHELL. The animal has a short broad foot; the head and snout, and the tentacles, are also all characterised by peculiar breadth; the two eyes are placed in front of the base of the last-named organs. The mantle has an even edge sometimes reflected upon the shell, but never covering it. The tongue is armed with many transverse rows of short, hooked teeth.
The largest British species is the Lake Mud Shell (Limneus stagnalis), which attains a length of two inches, and a diameter of one. In the large rivers of Eastern Europe it grows to a much larger size. The shell is thin and brittle, of a greyish hue, often covered with an extraneous coat; the spire is composed of about seven whorls, tapering to a fine point; the last whorl is striated lengthwise, and generally crossed by transverse raised lines, giving it an angular appearance; this whorl is large, and often greatly swollen. The pillar fold is strongly marked, almost forming an umbilicus. The animal is of a yellowish hue, paler beneath.
In stagnant and slow moving waters this species is not uncommon; and, on a warm summer's day, numbers of this and other species may be seen traversing the mud, climbing the aquatic plants, or mounting to the top, and floating on the unruffled surface in a reverse position. The principle by which an animal, heavier than water, is thus enabled to float, is not very easy of explanation; a parallel to it, however, is afforded by the familiar experiment of carefully laying a needle on still water, where it will float as long as it remains unshaken. The swimming body must, of course, be considered as in contact with the incumbent air, the cohesive power of which to the body, and among its own particles, is probably sufficient to overcome the force of gravity. But the Pond-snail not only floats, but swims at the surface, traversing its pool with a smooth, gliding motion, in an undulating line. M. de Quatrefages is of opinion that the progression of Mollusks, in this reversed position on the surface of the water, cannot be made by any muscular action of the foot; and he ascribes the motion to the action of the vibratile cilia, which cover the entire body as well as the sole of the foot. Dr. Johnston,[2] however, sets in opposition to this opinion the fact, that an Eolis crossing a basin can at once stop and remain there for any time, though, during all this period of rest, the cilia are in as active a state as when the creature was in motion. I would add, also, as adverse to the opinion of the French naturalist, that in the Eolis and other floating species I have distinctly observed the action of the foot muscles; the animal, indeed, literally crawls on the under surface of the stratum of air, just as if it were a plate of glass.
The curious habit which these Water-snails have of rising perpendicularly through the water, and the still more curious power of spinning a thread, by the help of which this feat is accomplished, have been already described in the earlier pages of this work. I shall merely add to these particulars of their history, that they lay in summer large oval masses of clear jelly, which they affix to the stalks and leaves of submerged plants. Each mass contains from 100 to 130 eggs, which are hatched in sixteen days.
Family Helicidæ.
(Snails.)
This is an immense family. Between sixty and seventy species belonging to it are enumerated as natives of the British isles, and those which inhabit foreign countries are far more numerous. The technical characters may be thus described. The head and tentacles are capable of being wholly withdrawn into the body, in which state they are covered by the infolded skin as with a sheath; the end of the tail tapers to a point, and is destitute of a gland. The lung chamber is generally in front of the body, with the breathing hole at its hinder part. The body is spiral, the mantle thin, with a thickened edge lining the inside of an external shell.
The indigenous members of this extensive family are familiar to every one. Some of them are destructive and voracious tenants of our gardens; others occur to us in our summer walks, swarming by scores on the banks and hedgerows; and not a few attract our admiration by their clean globose shells, and by the beauty of the colours, frequently disposed in spiral bands, with which they are adorned. But the beauty of our native species is far surpassed by many of those from the tropics, many of which, especially those of the genus Bulimus, shine in the most gorgeous colours, crimson, scarlet, orange, yellow, green and blue; and many of them are of gigantic size.
All the members of the family are voracious vegetable feeders; many of them devour indiscriminately the leaves of various plants, especially such as are tender either from youth, or from incipient decay. To aid them in crushing down the vegetable fibre, the mouth, which is situated on the under part of the head, is armed with a cutting instrument of beautiful contrivance. The upper one of the two fleshy contractile lips is armed with a broad horny plate, the lower edge of which is free, very sharp, and slightly curved, forming in fact a knife, admirably adapted to divide the leaves and soft parts of vegetables, when they are pressed by the action of the lips against its cutting edge. The floor of the mouth is provided with a small cartilaginous tongue, covered with delicate transverse striæ, and so disposed that by its movements it is well calculated to assist in propelling the food into the œsophagus.
A moist state of the atmosphere combined with a certain degree of warmth, though not essential to life, is necessary to the healthy performance of its functions in the Snails. On the approach of winter in cold or temperate climates, they hide themselves in protected situations, where they construct, in a manner presently to be described, a tight chamber, within which each individual sleeps away the cold season in a torpid state. During the summer, a continuance of dry weather will induce a similar retirement and a similar torpidity, though more brief in its duration.
The great majority of the species deposit a number of eggs glued together into a mass, and concealed under rubbish, the bark of decaying trees, dead leaves, or moss, or beneath the surface of the ground. Those of the Garden Snail (Helix aspersa) are soft, semi-transparent, and about as large as small peas; those of many foreign species are oval, and are enclosed in a firm, white, calcareous shell, like those of birds. Some of these are of considerable size. That of the magnificent Bulimus hœmastoma, from the West Indies, is as large as a blackbird's egg, and that of Bulimus ovalis from Brazil still larger. The latter species has produced eggs in England. A specimen had been presented to the Horticultural Society, and was kept in their conservatory at Chiswick. At first it appeared rather sickly; but after it had been kept in the hot-house for some time, it recovered, and began to move about. Mr. Booth, who was on the spot, says—"It cannot now be correctly ascertained when it produced the first egg, but it was very shortly after its arrival—I should think about the beginning of November. This egg was sent by the desire of Mr. Sabine to the Zoological Society. About the same time this year (1829) it produced a second egg, and three weeks afterwards a third; the latter was unfortunately broken by the animal itself, but the former is still in preservation. It fed upon lettuces, and the tender leaves of cabbages; the former seemed to be its favourite food. Sometimes it would devour two large lettuces, and then remain for days afterwards without touching food, or moving from its place, except when cold water was sprinkled upon it. During the day, it was usually in a dormant state in the shade; but towards the evening, when the house was moist and warm, it would spread itself out, and move from one part to another. It seemed to like moisture, and I have no doubt that it might have been preserved for years, if it had not been accidentally killed. On Saturday last, it was at the end of the house where the fire comes in, and ventured too far upon the hot bricks after they had been watered; in the morning, it was found fixed to them quite dead."[3]
Genus Helix
The animal in this well-known genus has a lengthened, depressed foot, and a large produced central spiral body, covered with an ample shell. The form of the shell is generally more or less globose, but sometimes depressed or flattened; the mouth is large and rounded, but the swelling of the last whorl intruding into it renders the interior of the aperture crescent-shaped; the mouth is strengthened by an internal thickened rib, and its margin, when the animal is full grown, is turned outwards.
Mr. Gray, in his "Land and Fresh-water Shells of the British Islands," has enumerated twenty-six species of this genus as natives of this country and its adjacent isles. By far the finest of these is the Edible Snail (Helix pomatia), which inhabits woods and hedges in chalk districts, in the southern and midland counties of England. The shell of this fine species attains a diameter of two inches, and a height of the same; the last whorl is very large and globose, and all are strongly striated across with close-set lines; the colour is commonly pale brown, with four spiral bands of dark brown; the interior is tinged with violet.
The animal is of a pale greyish-brown, the body studded with warts, the tentacles are long, the foot dilated, marked with impressed lines, forming a network.
EDIBLE SNAIL.Among ourselves these animals are occasionally eaten, and, when properly cooked, are said to be not unpleasant to the taste. Lister tells us how they were dressed in his time. "They are boiled in spring water, and when seasoned with oil, salt, and pepper, make a dainty dish." But on the Continent, as I have before intimated, snails have been from the earliest times an admired luxury. "The ancient Romans kept these animals in what were called cochlearia, or snail-stews. These were generally formed under rocks or eminences, the bottoms of which were watered by lakes or rivers; and, if a natural dew or moisture was not found, an artificial one was formed by bringing into the place a pipe, bored full of holes like a watering-pot, through which it was continually sprinkled. The snails required little attendance or food, supplying themselves, in a great measure, as they crawled about the sides or floor of their habitation. To fatten them, however, they were fed with bran and sodden lees of wine.
"These snails are at this day much admired in some parts of the Continent, and are not always used from economical motives; for at Vienna, but a few years ago, seven of them were charged the same price in the inns as a plate of veal or beef. The usual modes of preparing them for the table are by boiling, frying them in butter, or sometimes stuffing them with force-meat; but, in what manner soever they are dressed, their sliminess always remains.
"The greatest numbers, and the finest snails, are brought from Suabia.
"Dr. Townson was shown at Erlau a snailery, which the proprietor informed him was constructed on an improved plan. In our island, he says, this might have had the denomination of a patent snailery, or philosophical snail-sty. It consisted only of a large hole, two or three feet deep, dug in the ground, having a wooden house as a cover. The animals were fed on the refuse of the garden, which was thrown to them."[4]In the latter part of summer, the Edible Snail lays beneath the surface of the earth from sixty to eighty eggs, which are of a globular shape, covered with a white leathery skin, and about as large as dried peas. In from twenty to thirty days, according to the state of the weather, the young snails are hatched, each enclosed in a delicate shell, of a single whorl. A period of thirteen months from the time of hatching suffices, according to M. Bouchard Chantreux, to bring the animal to its full growth.
The name pomatia, derived from the word πωμα, which signifies a lid, refers to the curious covering with which the animal closes the mouth of its shell, to exclude the air during its residence in winter quarters. All the circumstances connected with its hybernation are so interesting that I shall describe them at length from a memoir on the subject by M. Gaspard, condensed in the Zoological Journal, with some valuable notes by Professor Bell.
M. Gaspard remarks that in our temperate climate, as soon as the first autumnal chills are felt, about the commencement of October generally. Helix pomatia becomes indolent, loses its appetite, and associates in considerable numbers on hillocks, the banks of ditches, in thickets, hedges, and such places. In a day or two the animals cease feeding, expel the last contents of the intestines, and then hide themselves under moss, grass, dead leaves, or the like rubbish. Here each forms for itself, with the anterior part of its muscular foot, a cavity sufficiently large to contain at least its shell. This cavity it enlarges and excavates by turning itself round on every side, then raising itself against the sides of the cavity, and at last against the roof formed of moss or leaves, or a small quantity of earth, brought there by its motions. When it has succeeded in bringing the aperture of the shell to nearly a horizontal position, it stops. The foot is soon contracted within the shell; the snail then expands, so as completely to cover it, the collar of the mantle, which is at this period very white; and then inspires a quantity of air, after which it closes the respiratory hole. When this is done, a fine transparent membrane is formed with its mucus, and interposed between the mantle and any extraneous substances lying above. The mantle then secretes a quantity of very white fluid over its whole surface, which sets uniformly, like plaster of Paris, instantly forming a continuous covering about half a line thick. When this is hardened, the animal separates its mantle from it by another and stronger mucous secretion; and after a few hours, expelling a portion of the air it had previously inspired, it is enabled to shrink a little farther into the shell. It now forms another lamina of mucus, expires more air, and thus retires farther into the shell. In this way sometimes a fourth, fifth, and even sixth partition are formed, with intermediate cells, filled with air.
Such is M. Gaspard's account; but Mr. Bell remarks that it does not completely explain the manner in which the excavation is formed. "It is not by the pressure of the foot," says the last-named zoologist, "and the turning round of the shell, that this is principally effected. A large quantity of very viscid mucus is secreted on the under surface of the foot, to which a layer of earth or dead leaves adheres: this is turned on one side, and a fresh secretion being thrown out, the layer of earth mixed with mucus is left. The animal then takes another layer of earth on the bottom of the foot, turns it also to the part where he intends to form the wall of his habitation, and leaves it in the same manner, repeating the process until the cavity is sufficiently large, and thus making the sides smooth, even, and compact. In forming the dome or arch of the form, a similar method is used, the foot collecting on its under surface a quantity of earth; and the animal turning it upwards, leaves it by throwing out fresh mucus, and this is repeated until a perfect roof is formed. As I have very often watched this curious process, I am certain of the facts. On removing very carefully a portion of the roof soon after its completion, I was enabled to see the formation of the operculum. In about an hour, or even less, after the hybernaculum is covered in, the whole surface of the collar of the mantle instantaneously pours out the calcareous secretion in considerable quantity. This is at first as fluid as thick cream, but very soon acquires exactly the consistence of bird-lime, being excessively adhesive and tenacious, and in about an hour after it is poured out, it is perfectly solid."
M. Gaspard states that the labour of each individual continues for about two or three days, but that the whole of the month of October is occupied by the general closing of the shells of the species. He adds that, about the beginning of April, the hybernation ceases. "The mode by which their escape from confinement is effected is simple and easily comprehended. The air which is contained in the different cells, and which had been expired on the animal withdrawing itself farther and farther into the shell, after the formation of the operculum, is again inspired, and each separate membranous position broken by the pressure of the hinder parts of the foot, projected through the mantle. When it arrives at the calcareous operculum, the animal, making a last effort, bursts and detaches its most obtuse angle; then, insinuating by little and little the edge of the foot between the shell and the operculum, it forces the latter off, or breaks it away."[5]
Family Limacidæ.
(Slugs.)
In general, the animals of this family resemble those of the preceding. The body, however, is lengthened and slender, attached to the foot by its whole length, instead of rising into a spire. The mantle is generally small, not nearly covering the body. The shell is minute and rudimentary, sometimes concealed within the substance of the mantle, and sometimes altogether wanting.
In general the Slugs are, like the Snails, herbivorous; but the curious genus Testacella, of which a species has been found in the neighbourhood of London, feeds almost exclusively on earth-worms.
Genus Limax.
Our common Slugs, but too familiarly known, have a lengthened body, with a granular surface, keeled behind. The mantle is small, covering like a shield the fore part of the body. Its substance encloses a small, flat, transparent, oval shell, somewhat resembling the human nail.
Slugs proper are widely distributed, species being found in various parts of both hemispheres; but the temperate regions of the northern hemisphere are principally troubled with them. The common Grey Slug (Limax agrestis) commits great ravages every year in our fields and gardens, notwithstanding the number of birds which make this species their prey; and various means have been devised to get rid of the pest, or at least to diminish its intensity. Quicklime, soot, coal-ashes, and saw-dust, are in turn sprinkled on the ground around tender plants; but the effects of these and similar substances depend upon their dryness, and the inability of the Slug to crawl upon powdery substances. The first shower of rain, however, and even the dews of night, break the spell.
This species varies much in size and colour, being by turns white, pale reddish-grey, and nearly black; but it may be easily distinguished by its body being furrowed with interrupted lines, with a short keel, which is always placed obliquely. The mantle is comparatively large, marked with circular lines; the enclosed shell is very minute, thick, hard, and irregular.
The Grey Slug is very prolific, continuing to lay its groups of eggs from April to the end of November, and depositing from thirty to seventy eggs at each time. The young increase in size rapidly, and reach their full growth in three months from their birth. When irritated, the Slug pours out from the whole surface of its body a copious white mucus, of the consistence of thick white cream, which dries into a white membrane.