Natural History, Mollusca/Pectinibranchiata
ORDER PECTINIBRANCHIATA.
(Comb-gilled Mollusks.)
We have now arrived at the most numerous division of the Gasteropoda, which comprehends nearly the whole of the spiral univalves, and many with simply conical shells. Their distinctive character is the possession of gills composed of numerous leaflets, or fringes, ranged in parallel order, like the teeth of a comb, and attached, in one, two, or three lines, (according to the genus) to the ceiling of the breathing-chamber, a cavity opening by a wide orifice between the edge of the mantle and the body. All the members of the Order respire water, and nearly all are marine.
A pair of tentacles are always present, accompanied by a pair of eyes, often highly organized, carried, sometimes, on spinal footstalks, and sometimes seated, as it were, on the side, or at the base of the tentacles. The mouth takes the form of a proboscis more or less lengthened, and conceals a tongue armed with small recurved hooks, which wear down the hardest bodies by slow and repeated friction. The sexes are always separate.
The shell is in general turbinated, or twisted spirally into a cone more or less regular; the aperture of which is sometimes entire, sometimes notched, sometimes drawn out into a canal. The orifice is in general capable of being closed by an operculum, a horny or shelly disk, attached to the posterior part of the foot of the animal, and fitting the interior of the mouth of the shell when the animal retreats within the cavity.
The most important difference between the animals of this order consists in the presence or absence of a canal, formed by a lengthening of the lung-chamber of the left side, which is carried along a similar canal in the shell, or through a simple notch, to enable the animal to respire without the need of quitting its shelter. The presence or absence of an operculum is also a distinction; and the filaments, fringes, and other ornaments, which are occasionally carried on the head, the foot, and the mantle, afford other characters for the subdivision of this extensive Order.
Family Cypræadæ.
(Cowries.)
Of this extensive group of shells, the majority of which are so exquisitely beautiful that they form the ornaments of cabinets, and the pride of collectors, a single British species is a sufficient warrant for noticing, in this volume, so attractive a family. Most of them are inhabitants of the tropical seas, residing chiefly near the shore, on reefs, and among rolled masses of broken coral; hence archipelagos and smaller groups of islands are peculiarly rich in the lovely Cowry-shells. There the brilliancy and variety of colour displayed both by the shells themselves and by the animals, accord with the glories of those latitudes, where the light and heat of a vertical sun give the greatest stimulus to animal and vegetable life.
The characters of the family are the following: the form is oval, flattened on one side, on which is placed the aperture; this is as long as the shell, narrow, and open at each end: the spire in the adult state is entirely concealed: the outer lip is in general bent inward and thickened. The surface of the shell is often highly polished, with a glassy, or porcelain-like enamel, with no trace of an epidermis, or investing coat of skin.
The animal is large; the mantle is developed into widely-expanded lateral lobes, which are, during activity, turned upward on each side, so as closely to embrace the shell. These lobes are generally gaily coloured, and are often adorned with various fringes and other appendages. The head is furnished with a retractile proboscis, and with a muzzle. The gill-plume is single. The sexes are separate. There is no operculum.
"The difference of aspect," observe Messrs. Forbes and Hanley, "between these mollusks when crawling, with all their beautifully-coloured soft parts exposed, often completely concealing their enamelled shells, and their appearance when, after being seized, they suddenly and instantaneously withdraw their bodies and mantle-lobes, and expose the shell only, is very curious and surprising."[1]
Genus Cypræa.
As specimens of some or other of the numerous species that compose this beautiful genus may be found on almost every mantlepiece, sideboard, and chiffonier, my readers will be at no loss for the means of actual comparison of the following characters by which it is distinguished. The shell is oval, more or less swollen, and flattened on the inferior side: its surface is polished or enamelled, commonly smooth, but sometimes marked with parallel grooves: the aperture is as long as the whole shell, narrow, forming a canal at each extremity; the outer lip is in full age bent inward, and much thickened, and as well as the inner lip, (or that edge of the aperture which faces it,) in almost all the species marked with numerous parallel tooth-like ridges.
The animal has large smooth, or warted mantle-lobes, capable of entirely embracing the shell between them, their edges meeting along its summit. The head is broad, with a retractile proboscis, and long, pointed tentacles, at the bases of which are the prominences which carry the eyes. The jaws are horny, and there is a long ribbon-like tongue, armed with rows of minute-teeth.
Some species appear to have the faculty of changing the colours with which the mantle is vividly adorned. Mr. Stutchbury, who had an opportunity of examining many individuals of C. tigris at the Pearl Islands, has stated that those cowries lived there in very shallow water, and always under rolled masses of madrepore. They never were to be seen exposed to the sun's rays. On lifting one of those masses a tiger-cowry was generally observed with its shell entirely covered by the large mantle, which was mottled with dark colours, the intensity of which the animal seemed to have the power of changing; for the colours varied in the same light and in the same medium, after the manner of the spots on the cephalopodous mollusca, or, to use a more familiar instance, somewhat in the same way that the hues of a turkey-cock's wattle vary.[2]
Mr. Arthur Adams, however, remarks on this statement: "Although I have examined hundreds of Cypræa tigris in a living state, I never saw those changes of colour in the mantle of the animal described by Mr. Stutchbury."[3]
YOUNG COWRY.The form of a cowry-shell is so peculiar, that no one, on first taking it into the hand, would suspect that it is modelled on the same plan as the cones and olives with which it is frequently associated. Yet the structure is essentially the same, and in the youth of the shell the resemblance is manifest, a young cowry being so like an olive as to present no peculiarity worth notice. In the course of growth, however, important changes in the external shape occur, chiefly by the development of the outer lip, and the deposition of the surface-enamel. Mr. Gray defines three stages in the growth of Cypræa exanthema. In the first the shell is generally smooth, of a pale greyish colour, or with three transverse bands, and the upper part of the inner lip is smooth and convex, the lower part flat or concave; the outer lip is thin. The accompanying figures represent this stage.
In the next stage the shell begins to assume more of the character of the genus, the outer lip beginning to be bent in, or rather thickened, and
HALF-GROWN COWRY. the mantle beginning to secrete and to deposit the coat of enamel, which is studded with white spots. The figure displays this state: the spire is sinking behind the elevated lips, which are thickening; while the spotted coat is seen at one side, creeping along over the back of the shell, which it is destined to cover.
At length the thickening of the lips proceeds to such an extent as almost to conceal the spire, and to reduce the aperture to a narrow line, the edges of which are now thickly plaited with the tooth -like ridges so characteristic of the genus. The lobes of the mantle protrude through this aperture, and expanding on each side, have deposited all over the exterior of the shell a coat of glassy enamel, studded with pale round spots, which entirely conceals the transverse bands that were formerly visible across it. The appearance, therefore, is now such as is represented in the following figures, which most of my readers will readily recognise as those of a familiar shell.
The deposition of enamel is the last process of the change, and one to which so much of the beauty of these shells is indebted. It sometimes happens that the glassy coat envelopes, while soft, any accidentally-intruding body adhering to the shell, and, quickly hardening, detains it there. Specimens of cowries are found in collections, on which other shells are fixed, firmly imbedded in the enamel. As such accidents are rare, however, it may be supposed that the frequent use of the mantle-lobes to embrace the shell has a tendency to prevent the adhesion of other shells.
FULL GROWN COWRY.
The earliest stage of life in these animals, as, we believe, in all the Gasteropoda, however diverse their adult condition may be, appears under the form of a nautilus-like shell, the inhabitant of which is furnished with two large-winged lobes, by which it is able to swim freely. Mr. Arthur Adams thus describes the young of one of the cowries:—"While staying at Singapore, I had an opportunity, in conjunction with Dr. Trail of that place, of observing the fry of Cypræa annulus, the species being then in spawn. Several specimens collected by us at low water, were seen to have conglomerated masses of minute transparent shells, adhering to the mantle and other parts of the animal, which masses, when placed in a watch-glass of salt water, under the microscope, became disintegrated, and detached individuals were perceived quitting the rest, and moving in rapid gyrations, with abrupt jerking movements, by means of two rounded flattened alar membranous expansions, reminding one of the motions of some of the Pteropods. When at rest, they joined the principal mass, or adhered, by means of their dilated expansions, to the surface of the watch-glass."[4]
According to the same observer, the minute snail-like shell of the young cowry forms the nucleus of that which afterwards grows, and undergoes the changes in form already described. The young are very active, whirling giddily about through the water, and occasionally adhering to foreign bodies, not by any disk for the purpose, but by means of the dilated expansions of the mantle. In the course of growth, these fleshy expansions become entirely absorbed, and do not ultimately constitute the lobes of the mantle which embrace the shell in the adult.
One of the species (C. moneta) possesses an interest, as forming a recognised currency in some parts of Africa, and of further India. Their value in Bengal is said to be as follows: 3,500 cowries are equal to one rupee, or about 2s. 3d. sterling. They are procured chiefly from the Maldives, and the coast of Congo. After the spring-tides, women collect the sea-sand in baskets; the cowries are then picked out, and heaped up in the sun; the animals soon dry up, and the shells, being cleaned, are ready for the money-market.
CYPRÆA EUROPÆA.Many of my readers are doubtless familiar with our little native cowry (Cypræa Europæa). It varies in size, from that of a split pea to that of a large horse-bean. It is elegantly marked all over with transverse ridges. These ridges are porcellanous white, and the alternate furrows between are purplish, or flesh colour. The larger specimens commonly display three spots of dark brown, arranged lengthwise. But probably few are aware how very elegant a creature it is when tenanted by its living inhabitant, and crawling at ease in clear water. The foot, on which it glides with a slow but smooth motion over the surface of the rock on which it habitually dwells, or, if you please, on the bottom of the saucer of sea-water in which you are examining it, is a broad expansion spreading out to twice the superficies of the base of the shell. Above this is the fleshy mantle, which is so turned up as closely to invest the shell, conforming to its shape, and even fitting into the grooves between the ridges. This mantle can be protruded at the will of the animal, so far that the two sides meet along the top of the shell, and completely cover it; or can be completely retracted within the wrinkled lips beneath; and it is capable of all gradations of extension between these limits. From the front of the shell protrudes the head, armed with two straight and lengthened tentacles, answering in function and appearance to the upper part of horns in a snail; except that the little black points which constitute the visual organs are not in this case placed at the tips, but on a little prominence on the outside of the base of each tentacle. Above and between these, which diverge at a considerable angle, projects the proboscis, a rather thick, fleshy tube, formed by a flat lamina, with its edges bent round so as to meet along the under side. The interior of this proboscis is lined with delicate cilia, by whose constant vibrations a current of water is drawn into the tube, and poured over the surface of the gills for the purpose of respiration. This current may be readily perceived by any one who will take the trouble to watch with a pocket-lens, as I have been this moment doing, a Cowry crawling up the side of a phial filled with seawater. By placing the vessel between your eye and the light, and fixing your attention on the front of the proboscis, you will presently perceive the minute particles of floating matter {always held in suspension even in clear water) drawn in various directions towards the tube, with a motion which increases in velocity as they approach, and at length rapidly sucked in and disappearing one after another within. It is an interesting sight to see, and one that cannot be looked on without delight and admiration at this beautiful contrivance of divine wisdom, for the incessant breathing of the respiratory organs, in water charged with vivifying oxygen.
Let us now look at the vivid hues of all these organs. The foot, which expands to so great a length and breadth behind the shell, is of a buff, or pale orange-ground colour, delicately striated with longitudinal undulating veins of yellowish white. The mantle which embraces the shell is of a pellucid olive, thickly mottled and spotted with black, and studded with glands protruding through its substance of light yellow; and it is edged with a narrow border of red. The proboscis is vermilion-red, varying in brilliancy in different individuals. The tentacula are of a paler tint, of the same colour, speckled with yellow.
Such, then, is the beauty of the animal which inhabits this familiar and plain little shell; a beauty, of which those who know it only in cabinets can hardly form an idea; while as one gazes on it placidly gliding along, one cannot avoid an emotion of surprise that such an amplitude of organs can be folded within the narrow compass of the shell, and protruded through so contracted an aperture.
Family Muricidæ.
(Whelks, &c.)
The Rock-shells and the Whelks, with their numerous allies, have commonly been considered as constituting two families, the Muricidæ and the Buccinidæ, or, to use the terms of Professor De Blainville, the Siphonostomata and the Entomostomata. But by Messrs. Forbes and Hanley all these mollusca are united in one family, under the name first mentioned; and this appears to be their true relation to each other; for even De Blainville confesses that his two families differ evidently very little, whether in the soft parts or the shell.
The species which, thus united, are very numerous, have the following characters in common: The shell is very variable in form, but always distinctly spiral, often turreted, with an aperture varying in size from excessive width to excessive narrowness, but always provided with a canal, which is sometimes produced into a long gutter, at others is contracted to a mere notch.
The animals are distinguished by a spiral body, with the foot, which is shorter than the shell, rounded in front. The mantle is furnished in front of the breathing-chamber with a long canal, always uncovered, which is used as an organ of prehension. The head is crescent-shaped, with a protrusile, proboscis-like mouth, whence is unfolded a ribbon-shaped tongue, armed with teeth arranged in triple rows, of three in a row. The breathing apparatus consists of two unequal plumes, the leaflets of which are arranged like the teeth of a comb. An operculum is for the most part present, horny in texture, marked with lines which show the layers of progressive growth, but varying much in form, according to the different genera.
Some of the more prominent, or more interesting of the genera composing this important group, I shall slightly notice; and first of all, that fine series of shells which gives name to the family, the genus Murex. Many of the species are remarkable for their brilliancy of colouring, and for the beauty and singularity of the forms which they assume. The siphonal canal is sometimes greatly lengthened, as in the species called the Woodcock's-head (M. haustellum), and is occasionally beset, as well as other parts of the shell, with long shelly spines, as the Thorny Woodcock (M. tennispina): often the progressive stages of growth are marked by beautiful shelly foliations, as in the magnificently-hued Royal Murex (M. regius) of South America.
It was from various species of this family, but preeminently from those of the genus Murex, that the ancients obtained the purple dye which made Tyre the "crowning city." This rich hue was of great costliness; its beauty has been celebrated by poets and historians, and the very finest kinds were reserved for the hangings of temples, and the robes of kings and priests. In the reign of Augustus, double-dyed purple wool was sold for about 36l. sterling per lb. But as wealth would not hesitate at any price to obtain that which was fashionable, laws were enacted, rendering it penal for any one but the emperor to wear cloth of this sort.
The observations of Mr. Wilde have thrown a confirmatory light on the accounts handed down to us by ancient writers of the mode of procuring the dye. This gentleman, when visiting the ruins of Tyre in 1838, found on the shore a number of round holes cut in the solid rock, varying in size from that of an ordinary metal-pot to that of a large boiler. In these cavities, and scattered on the beach around, lay large quantities of shells, broken, apparently, by design, but subsequently agglutinated together. It was evident that the shells had been collected in quantities, and deposited in the cavities in order to be pounded in the very mode described by Pliny, for the purpose of extracting the purple dye contained in the animal. The broken shells proved, on examination, to be all of one species, Murex trunculus, which was known to have yielded the Tyrian purple, and recent specimens of the same species were found on the adjacent beach.
In this family are placed the largest of univalve shells, such as the Tritonium, of which one species, richly clouded with brown and red like tortoiseshell, is sometimes found two feet in length; and the genus Cassis, well known as Helmet-shells, of triangular form and ponderous structure. All these are highly ornamented, especially the massive kinds from the West Indies and the Indian Ocean. The use of the helmets for the cutting of cameos has been noticed in a former page of this volume; but some statistical details on the same subject may not be uninteresting to my readers. They were communicated by Mr. J. E. Gray to the Society of Arts, in 1847.
Mr. Gray observed that numerous attempts have been made to substitute various materials, such as porcelain and glass, for the ancient cameos (which were cut in onyxes and other precious stones); but their great inferiority has caused them to be neglected. The best and now most-used substitutes are shells, several kinds of which afford the necessary difference of colour, and at the same time are soft enough to be worked with ease, and hard enough to resist wear. The shells used are those of the flesh-eating univalves, which are peculiar as being formed of three layers of calcareous matter, the layers being perpendicular laminæ placed side by side.
The cameo-cutter selects those shells which have the three layers composed of different colours, as they afford him the means of relieving his work; but the kinds now employed, and which experience has taught him are best for his purpose, are the Bull's-mouth (Cassis rufa) from the Indian seas, the Black Helmet (C. Madagascariensis), a West Indian shell, the Horned Helmet (C. cornuta), from Madagascar, and the Queen Conch (Strombus gigas), a native of the West Indies. The first two are the best shells.
After detailing the peculiarities of these shells, Mr. Gray proceeded to give an account of the progress of the art, which was confined to Italy until within the last twenty years, at which period an Italian commenced the making of cameos in Paris; and now about three hundred persons are employed in this branch of trade in that city. The number of shells used annually, thirty years ago, was about three hundred, the whole of which were sent from England, the value of each shell in Rome being 30s. The increase of the trade is shown by the following account of the number of shells used in France in the preceding year:—
| 80,000 | Bull's-mouth, | valued at | £6,400 |
| 8,000 | Black Helmet | " | 1,800 |
| 500 | Horned Helmet | " | 60 |
| 12,000 | Queen Conch | " | 700 |
| 100,500 | shells. | Total Value | £8,960 |
The average value of the large cameos made in Paris is about six francs each, giving a sterling value of £32,000, and the value of the small cameos is about £8,000, giving a total value of the cameos produced in Paris for one year of £40,000, while in England, not more than six persons are employed in this trade.[5]
Those glories of a conchological cabinet, the Harp-shells (Harpa), are also members of the present family. The number of species is but small, and they are all found in the Indian Seas, especially around the Mauritius and neighbouring islands, whence the finest specimens of the common species, as well as of the more valuable Many-ribbed Harp, are procured for the European collectors. The animal is brightly coloured, and beautiful. The fishing for these shells is carried on at low water, with a small rake, to which a net is attached, on sand-banks, at night and at sunrise, when the Harps principally roam about to feed. They have been known to take the bait on the fishing-lines laid for Olive-shells.
"The shells when in fine condition are great favourites with collectors, and indeed a drawer of fine Harps, in all the freshness of their beauty, is a sight worth seeing. Care should be taken to keep them with their mouths downwards, and from the sun and light, or their brilliant colours will soon fade."[6]
WHELK.
I must not omit to mention also, the more homely, but at the same time, more useful Spindles and Whelks of our own shores. The former genus (Fusus) is remarkable for the size of its members. F. antiquus, the largest of British univalves, being frequently found six and occasionally seven inches in length. This shell is used by the inhabitants of the Shetland Isles as a lamp, suspended horizontally by a cord, the ample cavity being made to hold the oil, and the wick projecting from the canal. The Whelk (Buccimum undatum), as many of my readers well know, is extensively sold on stalls in the streets of London. Hard, indigestible, and unwholesome as it is, there are multitudes of the poorer classes to whom it is a delicacy; it is simply boiled, and seasoned with vinegar and pepper. With our ancestors it seems to have found a place at tables of more pretension, for Dr. Johnston mentions, that at the enthronization feast of William Warham, Archbishop of Canterbury, in 1504, no fewer than 8,000 Whelks were supplied, at five shillings a thousand. Whelks are caught in creels or pots, baited and sunk in shallow water.
The genus, however, which I select specially to exemplify the family is the following:—
Genus Purpura.
It was included by Linnæus and his followers under the great genus Buccinum, but has now been separated to include a considerable number of species having the following characters:—
The shell is oval, with the spine usually much shorter than the aperture, which, in most of the species, is very wide; the surface is sculptured spirally, often forming fringed edges, or rows of knobs: the outer lip is rarely thickened, but is commonly notched; the inner lip is ill defined, covered with a glassy enamel; the pillar is broad, flattened, and sometimes hollowed; a short, strongly notched canal is present, and a horny operculum.
The animal has a broad flattened head, with two tentacles, the bases of which are thickened by the union with them of the eye-stalks; a reticulate proboscis, a long tongue, armed with teeth placed three in a row, of which the middle one is three-pointed, and the outer ones hooked. The mantle is produced into a short siphon; the foot is ovate, notched in front and obtuse behind.
Nearly a hundred species of the genus are found in the warmer seas, some of them of large size, and almost all with a very wide-spread aperture and short spire. Our own common Dog-Winkle (Purpura lapillus) approaches more nearly the form of Buccinum: it is an exceedingly variable shell in size, colour, and sculpture; its most common appearance is white or pale yellow, sometimes banded with light or dark brown, and sometimes wholly of a deep chocolate hue. The figure on page 38 will enable my readers to recognise it, especially as it is one of the most abundant of our native shells, occurring by thousands on every rocky shore.
I have already described in the introductory chapter of this volume, the purple secretion possessed by this mollusk, and the mode of applying it; the dye is common to the genus, and in a greater or less degree to many genera of the same family.
The Dog-Winkle is to be found attached to rocks and stones between tide-marks, and few who behold it sluggishly clinging to its hold, would suppose that it is as ravenous and ferocious a tyrant among its fellow mollusks, as the lion or leopard among the flocks. Yet abundant evidence exists to show that it habitually preys upon other shell-fish, both univalves and bivalves. Mr. Hanley has "seen a Purpura devour a Periwinkle in the course of an afternoon, when placed in the same vessel of sea-water, sucking its prey as it were out of the shell, after placing the orifice of its own body-case against that of its victim."
From Mr. Stevenson's interesting account of the erection of the Bell-Rock Lighthouse, we learn that the valves of the Mussel are no defence against the Dog-Winkle.
"When the workmen," says this gentleman, "first landed upon the Bell-Rock, limpets of a very large size were common, but were soon picked up for bait. As the limpets disappeared, we endeavoured to plant a colony of mussels from beds at the mouth of the river Eden, of a larger size than those which seem to be natural to the rock. These larger mussels were likely to have been useful to the workmen, and might have been especially so to the light-keepers, the future inhabitants of the rock, to whom that delicate fish would have afforded a fresh meal, as well as a better bait than the limpet; but the mussels were soon observed to open and die in great numbers. For some time this was ascribed to the effects of the violent surge of the sea, but the Buccinum lapillus [Purpura] having greatly increased, it was ascertained that it had proved a successful enemy to the mussel. The Buccinum being furnished with a proboscis capable of boring, was observed to perforate a small hole in the shell, and thus to suck out the finer parts of the body of the mussel; the valves of course opened, and the remainder of the fish was washed away by the sea. The perforated hole is generally upon the thinnest part of the shell, and is perfectly circular, of a champhered form, being wider towards the outward side, and so perfectly smooth and regular as to have all the appearance of the most beautiful work of an expert artist. It became a matter extremely desirable to preserve the mussels, and it seemed practicable to extirpate the Buccinum. But after we had picked up and destroyed many barrels of them, their extirpation was at length given up as a hopeless task. The mussels were thus abandoned as their prey, and in the course of the third year's operations, so successful had the ravages of the Buccinum been, that not a single mussel of a large size was to be found upon the rock, and even the small kind which bred there, are now chiefly confined to the extreme points of the rock, where it would seem their enemy cannot so easily follow them."
The mode in which the Purpura actually performs the operation, has been described by Mr. Spence Bate from observation. "The Whelk," he observes, attacked the Mussel, but it bored where there was no epidermis. I pulled it off, and turned the Mussel upside down (the other valve having more epidermis upon it), but in a short time I returned, and found that the Whelk had turned over the Mussel, and had resumed its operation at its old bore. This I did twice or thrice, with the same result. Giving up the idea of its boring at any other point, I next thought I should like to see how it managed to devour its prey. For this purpose I divided the muscles of the Mussel, so that the valves parted, so as to enable me to observe the work of gormandizing as it proceeded, but to my surprise, the animal gave up all idea of boring, when there was an easier method of obtaining food, and so passed its proboscis between the valves. I think this shows that the Whelk, when it attacks its prey, seeks out for the part most suitable for its operation, and I believe invariably chooses a point from which the epidermis has been removed previously. A section of the bore, taken during the operation, shows that it is convex, and contradicts the received notion of the operation being performed bj the action of the ribbon, which being in the centre of the proboscis, would perforce wear the middle of the bore deepest, but this is not the case. The animal makes no movement of a rotatory kind or otherwise, during the operation. It takes about two days to get through the shell, when it eats about two-thirds of a moderate-sized mussel, which seems to satisfy hunger for about three weeks."[7]
The curious inquirer after the many natural objects which the receding tide reveals, may occasionally see in the spring months as he peeps into the crevices of the rocks, a number of little urn-like bodies crowded together, and standing erect from the rock. They are about the size of a grain of wheat, to which they bear no small resemblance, being of a yellow colour, but tinged as they approach maturity with reddish-purple. These are the egg-capsules of the Dog-Winkle. Réaumur states, that the purple dye is obtained from these vesicles with less trouble than from the animal; an assertion which by no means agrees with my experience. The membrane of which they are composed is very tough, but if we cut it open we find that each contains many infant mollusks, all inclosed, as the period of birth draws nigh, in their tiny shells.
Mr.Peach, who bred the animals from the capsule, observed that the latter change form as the included young ones ripen, the apex of the cup becoming thinner and more convex. He found that so long a time as four months elapsed before the vesicle opened, and then the included whelklings did not quit their cradle all at once, but took their time in coming out, according to their individual dispositions; doubtless, the quick-minded and more curious commencing their travels first, whilst those of slow and studious constitutions would remain as long as a fortnight before resolving to see the world, which with young Purpuræ is no very dangerous adventure, since the neighbouring barnacles enable them to look about with safety, before making a long journey from their birthplace.[8]
Family Velutinidæ.
A small and unimportant group is indicated by this name, represented in Britain by two genera, each consisting of two species. They have a shell, the aperture of which is very broad and open, and the spire minute; in texture it is thin, sometimes pellucid, and sometimes even membranaceous. In one genus it is entirely included within the substance of the mantle, as in Pleurobranchus: in the other it is external, but partially invested by the edges of the mantle, and covered with a skin (epidermis).
The animal is large, with a short broad head, furnished with two tentacles, and eyes at the exterior of their bases. There are two gill-plumes. The operculum is wanting.
Our most common species is Velutina lævigata, reckoned by Linnæus among the snails, and long supposed to be a fresh-water mollusk; it is, however, exclusively marine, and generally inhabits deep water. It is an open-mouthed shell, about half an inch in diameter, of a dull reddish hue, covered with a furry epidermis. Messrs. Forbes and Hanley distinguish it from its fellow species by affirming that it is not membranaceous, but this does not agree with my own experience; the specimens that have fallen under my notice having been quite flexible and membranaceous, especially near the margin. Nothing is known of its habits.
Family Naticadæ.
(Naticks.)
The shell in this family is globose, with the spire minute and scarcely raised; the surface is generally smooth, and often covered with a porcelain-like polish; the aperture is large and semi-circular; the pillar is always thick and solid, and its exposed part, constituting what is technically known as the inner lip, is often very broad.
The animal is large in proportion to the shell, yet capable of being wholly withdrawn into it. The mouth is not extended into a proboscis, but is concealed beneath a broad hood or veil. When the eyes can be recognised, they are placed at the bases of the tentacles. The mantle is entire, that is, its edges are not cut into filaments. An operculum is always present, sometimes horny in texture, sometimes shelly, but invariably closing the wide aperture of the shell.
Though the genera comprised in this family are few, the constituent species are numerous, and widely scattered in geographical distribution. For the most part they are marine, but some inhabit rivers and lakes of fresh water; and M. Lesson affirms of one species which is found in Australia, that it lives abundantly on trees. Some of the Neritinœ are ascertained, on indubitable authority, to live indifferently in the sea and in fresh-water.
Genus Natica.
Handsome globose shells, with a smooth surface as if varnished, and with a very wide aperture, compose this genus, which is technically distinguished by having the inner lip smooth, not depressed; the umbilicus open, with a central gibbous ridge or prominence, and the operculum formed of shelly substance.
The animal is large, slimy, and slug-like, with the foot so enormously developed as to communicate to the creature a most remarkable aspect when seen crawling. It is a long oval plate of soft flesh, commonly squared in front, and rounded or notched behind. The forepart, where it is widest, forms a thick sort of shield, somewhat eared, which is turned up on the front of the shell, partially covering it, and hiding the whole head of the animal, except the tentacles, which project from behind it. The hind part of the foot, and the sides also, partially envelope the shell, of which only the summit is seen, apparently embedded in an immense mass of white flesh. The operculum, notwithstanding its large size, is not visible when the animal is in motion, being hidden by the extremity of the shell.
When one beholds the great volume of fleshy substance which the animal displays when crawling, we can scarcely believe that it can possibly be withdrawn into the shell; yet this is easily effected, and that so completely, that the closely-fitting operculum is seen, shutting the aperture considerably within the margin. It is performed, however, if I may judge from my own experience of several British species, with some difficulty, and by a succession of efforts; and the animal when once forced by annoyance to retire, is often ill-disposed to protrude again, at least for a considerable time.
NATICA.
The species of the genus are widely distributed, and are most numerous within the tropics. Several of them are of large size. Seven or eight species are found on the British shores, of which the finest is Natica monilifera.
In this large and handsome shell, the form is globose, nearly as broad as long; it is strong but not solid, smooth and glossy, though with a few fine sunken lines, which mark the progressive growth. The ground-colour is whitish, sometimes tinged with flesh-red, or with bay, and each whorl is marked along its upper margin with a line of oblique but parallel dashes of rich chestnut.
The animal, in such specimens as I have seen, is almost wholly of a puree milk-white hue. It is, as already stated, very large, the long oval foot extending far beyond the outline of the shell on every side, and partly inclosing it. Messrs. Forbes and Hanley state that this is a local species, found, however, on most parts of our coast where sand is plentiful. I have taken it by dredging in Weymouth Bay, where it seems a common species, together with N. Alderi. Both of these animals have a curious habit, which I have not seen noticed. They are said to live buried in sand, but I believe this is only to a partial extent. When put into an aquarium with a sandy bottom, they soon begin to crawl just beneath the surface of the sand, the foot alone being immersed in it; and this organ as it slowly moves along, deposits and leaves behind a broad belt of clear mucus, of slight density. The progress of the creature through the fine soft sand, is very curious to witness.
In places where this Natica is common, the dredge not unfrequently brings up its spawn-masses. Professor Harvey in his delightful "Sea-Side Book," thus speaks of them.—"These egg-clusters are really very curious and elegantly formed objects, which must often have attracted the notice of a rambler, who felt puzzled to know what they were. They are firmly gelatinous, or of the consistence of gristle, transparent, or nearly so, slightly coated with fine sand, and in shape resemble the hoof of an animal. When dry, they look not unlike pieces of thin Scotch oaten bread. The surface is marked with little hexagonal spaces, which define the eggs. But what is most to be admired in the structure, is the form of the curves which the hoof-like body assumes, which fit it for lying on loose sand, without becoming deeply buried in it."[9] I am inclined to believe that the original form of this egg-mass is that of a very broad ribbon, attached by one edge to a rock, like that of a Doris, in a spiral curve, the upper edge of the ribbon leaning outwards a little, so as to resemble a cup, and that the likeness to a horsehoof which Professor Harvey alludes to, and which I have myself observed, is seen only when the ribbon is torn away by the dredge, and turned upside-down. This curious mass of eggs was considered to be a zoophyte by the earlier naturalists, and was characterised as such by Gmelin, under the name of Flustra arenosa. Its true nature was first suspected by Mr. Boys, and was fully proved by Mr. Hogg, in 1833, who hatched the Natica from it.[10]
The species of this genus are all carnivorous, boring holes into other shells and devouring their contents. Dr. Gould asserts that they are very voracious, and play a conspicuous part in devouring the dead fish and other animals which are thrown up by the tide. The small circular holes with which bivalve shells are often drilled, are the work of these snails, and made by them to gain an entrance to the animal apparently so well secured against such a foe. The foot can be expanded so as to envelope completely the objects on which they prey, for a long retention of it in its grasp is necessary, from the slowness with which they work their auger or spiniferous tongue.[11]
Family Pyramidelladæ.
This is an extensive group of small, and for the most part, minute shells, which often display much beauty to the close observer; their forms being in general elegantly turreted, their surfaces smooth, often polished, or ornamented with the most beautiful and elaborate sculpture. The aperture is entire, and not lengthened into a canal; the pillar or inner lip is often plaited.
The animals are furnished with a retractile proboscis, and with tentacles of varying form, with the eyes not set on footstalks, but immersed in the bases. The tongue is remarkable for being unarmed with teeth.
As the fossil remains of a former world present the extinct forms of this family in great numbers and variety, the group is one of much interest to the geologist.
Genus Stylifer.
I select the genus Stylifer to illustrate the family, because of its singular form and still more curious economy, rather than for its abundance in these latitudes, for it is represented on the British coast by a single species, and that of very rare occurrence.
The shell is somewhat globose, with the tip slender, and projecting in the form of a little point or style; its surface is smooth and polished; its whorls are numerous. There is no operculum.
The animal has slender tentacles, and eyes immersed at their bases; the mantle has been described as thick, fleshy, cup-shaped, enveloping the last whorl of the shell, but Mr. Adams from observation on the living animal, informs us that it is entirely enclosed and covered by the thin shell, while the foot is narrow, slender, and very much produced beyond the head in front.
STYLIFER.The habits of this interesting mollusk are most singular, for it is found to live parasitically upon the animals of the class Echinodermata (Star-fishes and Sea-Urchins). Three species are known, one of which (Stylifer Turtoni) is a rare inhabitant of the British seas. Dr. Turton, however, its discoverer, found no fewer than a dozen attached to the spines of Echinus sphæra, dredged in Torbay. It has since been found in several localities, as on the coasts of Northumberland, Durham, and Cork, always under similar circumstances. Mr. Alder states, that it occurs on young Sea-Urchins.
A fine species was discovered in the Indian Archipelago by Mr. Hugh Cuming, and named S. astericola. He found this elegant parasite burrowing in different parts of the oral disc of Asterias solaris. It was almost hidden from sight, so deeply does the animal penetrate into the substance of the star-fish, in which it makes itself a comfortable cyst (or cell) for itself, and wherein it most probably turns by aid of its rudimentary foot. All the specimens infested with these testaceous mollusks appeared to be in the best health, though there is reason to believe that they feed upon the juices of the Star-fish. Mr. Broderip observes that Stylifer (with that instinct of self-preservation which is imparted to all parasites whose existence depends upon that of their nidus) appears, like the larvæ of the ichneumon tribes among insects, to avoid the vital parts; for in no instance did Mr. Cuming find it imbedded anywhere save in the rays, though some of the individuals had penetrated at their base, and very near the disc. When extracted, the older shells have the appearance of a milky-clouded glass bubble: the younger shells Mr. Broderip found of an unclouded transparency.
Family Scalariadæ.
(Wentletraps.)
Though very limited in numbers, this family is interesting to the conchologist as containing some species of singular form and remarkable beauty, one of which was formerly valued so highly as to command enormous prices, and to have acquired the name of the Precious Wentletrap. "In this family a spiral shell with an entire aperture is combined with an animal whose head is not produced into a muzzle, but furnished with a retractile trunk. The sexes are distinctly separated. The eyes are immersed at the external bases of subulate (awl-shaped) tentacles." The dentition of the ribbon-like tongue is very peculiar; there is no central tooth differing from the rest, but all are alike, arranged in transverse rows, and forming simple claw-like hooks. The animals are probably predaceous like the Whelks, &c. Most of the species, which amount to nearly a hundred, inhabit the seas of warm climates, though a few are found on our own shores.
Genus Scalaria.
The shell is spiral, consisting of many whorls, assuming a turreted form, ornamented with many elevated ribs, which cross the whorls in the same direction as the length of the shell; the aperture is rounded, with the lip thickened and entire. There is a horny operculum.
The animal has an angularly lunate head, with two long pointed tentacles, and eyes immersed at their bases; the mouth, which opens below, has a retractile proboscis; the mantle is rudimentary; the foot is triangular, grooved beneath, and furnished in front with a fold.
In the principal section of the genus, confined to the warmer parts of the globe, the whorls or turns of the spire do not touch each other in any direction: these are designated true Wentletraps. In this division is placed the shell above alluded to, the Precious Wentletrap or Royal Staircase (Scalaria pretiosa), a large shell, twisted into a loose, untouching spiral, of a pale yellow hue, ornamented with ribs of pure white. This is always a prized addition to a cabinet, for it is undoubtedly a shell of extraordinary beauty; but the value which was attached to it in former years can only be considered as a phase of insanity, analogous to the well-known tulip mania, and other fantasies of a like kind. In 1753, at the sale of Commodore Lisle's shells at Longford's, four Wentletraps were sold for seventy-five pounds twelve shillings: viz. one not quite perfect, for sixteen guineas; a very fine and perfect one for eighteen guineas; one for sixteen guineas; and one for twenty-three pounds two shillings.[12]
But higher prices than these have been given. That in Mr. Bullock's museum, supposed to be the largest known, brought at his sale the sum of 27l., and was estimated in 1815 at double that value; and there is a tradition that a specimen was sold in France for 2,400 livres, or 100 louis!
COMMON WENTLETRAP.
Another section, known as false Wentletraps, have the whorls contiguous; and many of these species are European. Some of them secrete a purple liquor, as has already been noticed in these pages respecting our commonest native species, S. communis. This shell is turreted, usually about an inch and a quarter in length, of a pale bay or drab hue, with prominent ribs, spotted with purple. The animal is blackish-grey speckled with white.
The Wentletraps inhabit rather deep water, and affect a sandy or muddy bottom: hence they are obtained only by dredging. The species just described has been procured at various parts of our coast, but principally on the shores of Devonshire.
Family Cerithiadæ.
This group, as defined by our latest malacologists, includes shells which at first sight appear to be very dissimilar, as the slender turreted Cerithium and the broad-lipped Pelican's foot. The genera are "remarkable for the muzzle-shaped heads and corresponding features of organization of the animals which construct them." They seem to constitute a group intermediate between those comb-gilled Gasteropoda which have entire mouths, and those which are furnished with siphons, partaking of and mingling many of the characters of both.
Genus Aporrhais.
A thick, massive, many-whorled shell marks this genus, subject to much alteration in form as it advances in age. In youth the aperture is simple, slightly angular, with a moderate canal; in adult age the canal becomes lengthened, and the outer margin of the shell is produced into a wide wing-like expansion, the edge of which projects in diverging lobes or finger-like processes.
The animal has a long muzzle; cylindrical tentacles, with the eyes placed on prominences at their bases; the mantle digitated, loose, with a rudimentary siphon; the foot short, angular in front, and obtuse behind; the branchial plume single; the operculum horny.
PELICAN'S FOOT.
We have two native species, called the Cormorant's foot (A. pes carbonis), and the Pelican's foot (A. pes pelicani). Of these the former is exceedingly rare, the latter very common. The name in both cases is derived from the wing-like expansion of the adult shell resembling the webbed foot of a sea-bird. The Pelican's foot is the larger shell, measuring commonly about two inches in length, and nearly an inch and a half in width, when full grown. Its colour is yellowish, with clouds and spots of chestnut brown. The animal is yellowish-white, marked with scarlet spots, especially about the head and on the tentacles. It is commonly brought up by the dredge, particularly from a gravelly bottom. In captivity it is uninteresting, as it remains sluggish and inactive, obstinately keeping its body contracted within the shell. Though I have kept many specimens, I have never seen one crawl.
Family Turbinidæ.
(Winkles.)
An immense assemblage of species, some of which are of large size and great beauty, is comprised in this family. The animal is spiral, with the sides occasionally ornamented with tentacular appendages differing in number and form; the head is protruded somewhat in the form of a proboscis, furnished with slender thread-like tentacles; the latter carry at their bases a pair of eyes, usually raised on footstalks; the mouth has no tooth on the lip, but is provided with a ribbon-shaped tongue of great length, rolled up spirally when not in use, and carried in the cavity of the body. A furrow passes across the foot near its front border; the gills consist of two fringes.
The shell is thick and solid, often more or less pearly on the inside, forming a spiral cone, with the opening round or slightly depressed. There is an operculum, which is calcareous (shelly) in some species, horny in others; in the latter the spiral formation is visible on the outside, in the former on the inside.
All the members of this family are believed to be vegetable feeders, subsisting on the sea-weeds, the substance of which they rasp down by the action of their rough tongue. Yet the large and beautiful Phasianella bulimoides, an Australian species assigned to this family, is said by MM. Quoy and Gaimard to be taken in nets baited with flesh, and let down into the sea.
Genus Littorina.
The shell in this common and well-known genus is spiral with but few whorls, generally more or less oval in form, and thick and solid in substance. The spire is sometimes pointed, as in the common Periwinkle (L. littorea); sometimes obtuse or round, as in the equally common Yellow Winkle (L. neritoides). The aperture is round and entire; the outer lip is sharp-edged, thickened within. The operculum is horny and elastic; its spire consists of a few turns, rapidly enlarging, with a central nucleus.
In most of our seaport towns, and in many of our inland cities, the Periwinkle is sufficiently familiar, from its being so commonly sold by measure as human food. The animals are found by thousands on rocks at low-water, or on the mud left exposed by the receding tide in harbours and estuaries; they are collected by the children of fishermen, boiled, and hawked about the streets at a low price. They are eaten not infrequently by persons above the lowest grade of society, not from necessity, but from choice; though to most uneducated palates they are coarse, tough, and indigestible.
The Periwinkles are able to bear long-continued exposure to the air with impunity. The species just mentioned may be observed adhering to the rocks by hundreds under a hot sun, and that for hours together; but a smaller kind (L. rudis),—which may be recognised by its being frequently found white, pale-green, yellow, and orange in colour,—habitually resides in hollows of rocks that are elevated many feet above the range of water. But their gills are constantly moistened by a minute quantity of sea-water contained in the cavity of the body; and this is prevented from evaporation, partly by the close adhesion of the margin of the shell to the rock, and partly by the tightly-fitting operculum.
Indeed there is reason to believe that this and some other species of the genus spend the winter in the air, hybernating, like the Snails. "Mr. Gray found that many individuals of L.petræa and some of L. rudis, were in this condition, during his stay at Dawlish. They were attached to the rocks several feet above the reach of the highest autumnal tides; the foot was entirely retracted; and a membranous film was spread between the rock and the edges of the outer lip of the shell; the gills were only moist, the branchial sac being destitute of that considerable quantity of water which exists in it in those of the same species which are adherent to it by their expanded foot. In this torpid condition the individuals observed by Mr. Gray continued during the whole of his stay, which lasted for more than a week. On removing several of them, and placing them in sea-water, they recovered in a few minutes their full activity."[13]
In Sweden the common people affect to prognosticate the weather by the position of the Periwinkles; when these ascend the rocks, it is considered as a sure sign that a storm is near, as their instinct leads them to place themselves out of the reach of the dashing of the waves; on the contrary, when they descend upon the sands it is supposed to indicate calm weather. I much doubt, however, the fact of any such connexion between the habits of the Winkle and the state of the atmosphere, and still more, the philosophy of the reason assigned for the habit.
The plant-eating Gasteropoda are said to "lay their eggs merely enveloped in a mass of jelly, just firm enough to retain its form in the water, and which, deposited on the fronds of sea-weed, or on the surface of rocks and stones, adheres to them with tenacity. The form of the mass is roundish, oval, or oblong, and it may be more complex in some. The ova are always immersed in the mass, which forms a common bed to the whole; but besides this, each egg (or at most three or four eggs) has its own proper globule of jelly, contained within a skin or pellicle of the greatest tenuity, and which isolates it from the rest."[14]
SPAWN OF PERIWINKLE,
(magnified).
The accompanying figure, which is copied from Dr. Johnston, represents the mass of spawn laid by the common Periwinkle, in which each of the infant animals is seen enclosed in its proper globule, and covered already with a shell of a single whorl.
PERIWINKLE.
The Periwinkle (L. littorea) is subject to much variation in form, colour, and markings; it is generally about an inch in height, globose, very thick, with the margin thin; the colour is russet brown, or olive, sometimes yellowish, with spiral bands of black. The ground colour occasionally becomes a rich orange or scarlet, with or without black bands; and these varieties are very handsome.
Family Paludinadæ.
(Marsh Snails.)
This is a group of fresh-water Mollusks, so closely resembling the Periwinkles in many of their characters as to have been confounded with them by Cuvier and other zoologists. Those characters are as follows:—
The shell is conical, spiral, thin in texture, covered with an olive-coloured skin; the orifice is ovate, entire, but angular behind. The animal has a muzzle-shaped head, and long, slender tentacles, with eyes seated at the outer side of their bases; the gills are always enclosed in the breathing cavity. An operculum is always present, generally horny, but sometimes shelly, formed of concentric laminæ ranged round a nucleus commonly placed in or near the centre.
Unlike the great majority of the Comb-gilled Mollusca, the members of this family are confined to fresh waters, inhabiting lakes, streams, and marshes. They are distributed all over the world; and occur in a fossil state as far back as the oolitic series. The fine globose shells which are found in the rivers of tropical countries, known as Apple-snails (Ampullaria), are but slightly separated from this family.
Genus Bithinia.
In addition to the family characters, those which distinguish this small group are, that the operculum has a thick shelly coat on the inner surface, and has the nucleus nearly central; and that the aperture of the shell has a slightly thickened rib, along the interior of the margin.
BITHINIA TENTACULATA,
(magnified).
Two species of this genus are found in the streams and ditches of this country, the more common of which is the Tentacled Bithinia (Bithinia tentaculata). It is about half an inch in length; the shell is often covered with a blackish foul coat; the spire is composed of five whorls, the lowest of which is swollen. The animal is purplish black, with brilliant yellow specks.
This genus, like the Mollusca generally, produces eggs, while its fellow-genus Paludina is viviparous. The mode in which the eggs are laid is curious, and has been particularly described by M. Bouchard Chantreaux. According to this careful observer, the Tentacled Bithinia lays from May to August.
Family Neritidæ.
(Nerits.)
A single British species represents a family which, in warmer climates, plays a conspicuous part along the shores of the ocean, and in their rivers and lakes. The members have solid shells, more or less oval, the last whorl so greatly developed as to occupy by far the principal portion, the aperture very open, somewhat crescent-shaped, with an expanded and flattened inner lip.
The animals have broad muzzle-shaped heads, with awl-shaped tentacles, and eyes on short footstalks; the foot is somewhat three-sided, without any lateral filaments. An operculum is always found, which is spiral, semi-oval, and furnished with two internal processes on its front edge, forming a sort of hinge on the sharp edge of the inner lip of the shell. Dr. J. E. Gray thinks that this peculiar structure of the operculum "makes this family more closely resemble the bivalve shells: the processes appearing to answer the same purpose (that of keeping the two parts in their proper situation) as the teeth of the hinges in the bivalves."
Genus Neritina.
The shell in this genus is semi-oval, without any perforation; the inner lip is slightly toothed, sharp-edged; the surface is smooth, or striated, or spinous, covered with a skin.
The animal has two slender tentacles; a rather short foot, three-sided in outline, with the angles rounded. The tongue is armed with rows of teeth, differing much in form and size. The operculum is semi-oval, slightly shelly, with a sharp, flexible outer edge, and a tooth on its lower margin.
Many species are assigned to this genus, the greater number of which inhabit fresh-water rivers, especially of tropical countries, where they adhere to stones with considerable tenacity. Yet there is a species which inhabits one of the great North American rivers, through a range of two hundred miles, from the mouth, where the water is quite salt, to beyond the reach of the tide, where it is perfectly fresh. Another species is found only in the sea.
When the animals have arrived at their full size, they have the power of absorbing the shelly matter of the divisions which separate the whorls of the spire, so as to allow more room for the spiral body, without increasing the size of the shell. This reduction of substance is accomplished without endangering the strength of the shell, as only a very small part of the whorl is exposed on the outside.
NERITINA FLUVIATILIS.Our single native species (Neritina fluviatilis) is about three-eighths of an inch long, and two-eighths broad; usually of a purplish hue, banded or chequered with spots of white; but the colouring of different specimens varies exceedingly. The animal is white, with the head and neck black. It is found chiefly in slowrunning streams, adhering to stones. The shells are
often covered and disguised by irregular incrustations of calcareous matter, deposited by the water, which may serve as a protection to the animals, causing them to take the appearance of rough stones or masses of earth. This species, like all its fellows, displays very little of its body when crawling.
Family Trochidæ.
(Top-shells.)
An extensive group of very fine shells is included under the above appellation, many of which are of considerable size, of very regular and elegant shape, and of exquisite beauty of colour, and sometimes of sculpture. Our own shores possess many species, among which are some of the finest of our univalve shells, and specimens distinguished by all of these characteristics. Yet the finest species are, as usual, exotic, and tropical; for the great Pearly Top (Trochus pica) of the West Indies, the Imperial Sun (Imperator imperialis) of Australia, and the Perspective Staircase (Solarium perspedivum) of the Indian Seas, belong to this family.
The shell in this large group varies considerably in form, but is always spiral; the spire sometimes is drawn out to great length, at other times so much depressed as to be nearly flat; but it always forms a large portion of the shell. The aperture is entire, without notch or canal, as the animals are destitute of a siphon.
The animal has a head terminating in a broad muzzle, and often ornamented with head-lobes; side-lobes greatly developed, and furnished with lappets and tentacular filaments, a pair of true tentacles, and eyes fixed on footstalks behind their bases; the gill-plume is single; and an operculum is always present, spiral in structure, commonly circular in shape, and either horny or shelly in texture.
Genus Trochus.
Even a glance at the British species of this genus would show the great diversity that subsists in the external form of the shell, from the regular pointed cone, in which the whorls do not break the uniformity of the outline, as in the beautiful T. granulatus and T. striatus, &c., to the tubercled, almost hemispheric form of T. magus, in which the swollen and knobbed whorls project like a winding staircase round a pictorial "Tower of Babel."
The aperture is entire, usually angular, and approaching to a four-sided figure, and opens on a plane which is oblique with respect to the axis of the spire. The interior of the shell is pearly. The animal is considerably developed; it is furnished with a pair of tapering tentacles, and two eyes set at the ends of stout footstalks. Behind these, on each side, is a large lappet, which merges into a broad wing-like expansion of the mantle, bearing commonly three, sometimes more, tentacular filaments, which are probably delicate organs of touch. The foot is oblong, more or less lengthened, carrying on its posterior part an operculum, which is composed of many spires, of horny texture.
Of the sixteen species which are enumerated as inhabitants of the British seas, one of the most beautiful, and certainly the largest, is the Granulated Top (Trochus granulatus). It is esteemed a local and rather rare shell, being confined to our southern shores and the Irish Sea. I find it quite common in Weymouth Bay, in from eight to twenty fathoms. It not unusually attains a height and a breadth of an inch and a quarter, and has been taken of an inch and a half. Its shape is elegantly conical, with the base rounded; the whorls scarcely break the regularity of the outline; they are sculptured with spiral raised lines, each of which is composed of a number of minute rounded knobs, like a string of beads. The texture of the shell is thin and rather fragile; its ordinary tint is a pale flesh-colour, or yellowish white, with a few scattered dashes or short streaks of purple, that run parallel with the spiral lines.
GRANULATED TROCHUS.
The animal is large and handsome, with the lobes and wing-like appendages much developed. It is white, speckled with brown, especially about the head.
All the species of the genus feed on marine vegetables: I have watched with pleasure the little Grey Top (T. cinerarius), the commonest species of our weed-clad rocks, rasping down with the teeth of its ribbon-like tongue the minute Confervœ that grow on the inside of my glass vases. With a pocket lens it is easy to see the process, which I can compare to nothing else than the mode in which a cow licks up, as it were, the grass, as she moves along, by successive sweeps of her tongue.
Family Fissurelladæ.
(Keyhole-shells.)
In external form and appearance, the shells which compose this group bear the closest resemblance to the Limpets (Patelladæ). All of them, however, have the peculiarity of an orifice in the shell, either at the summit of the cone, or in the form of a slit at the front edge.
The characters of the included animals distinguish them at once; they have well-developed heads, with short muzzles, and tapering tentacles, at the outer bases of which the eyes stand on short footstalks. Beneath the mantle on each side is a series of short tentacular filaments, similar in character to those of the Trochidæ. There are two gill-plumes, which are large, pectinated, and equal; they are placed in an ample cavity, which communicates with the aperture of the shell, whether this be situated on the summit, or in the front margin.
All the mollusks of this family are marine, and are distributed through the seas of most parts of the world; but principally those of warm climates.
Genus Fissurella.
From the usual form of the orifice in the shell of this genus, which resembles that of a key-hole, the species are familiarly known by the appellation of Keyhole Limpets. It is placed at the summit of the shell, which is very regularly conical, and not otherwise to be distinguished from that of a Patella. In the very young condition the orifice is placed more forward. The animal has been already sufficiently described.
FISSURELLA RETICULATA.
We have but a single native species, the Netted Keyhole Limpet (Fissurella reticulata). It attains an inch in length, is of an oval form, and of a dirty white hue, with dusky rays. Its surface is sculptured with a raised network, formed by radiating and circular ridges. The animal is large, and of a yellow colour, varying in shade from cream-white to a rich deep orange; the latter hue being seen only in those specimens that are found adhering to shells incrusted with a scarlet sponge. It is obtained only on our western and southern coasts.
Family Calyptræadæ.
Another group of Limpet-like shells are associated under this title, represented poorly in the British seas, but numerous and much varied in their details in those of the tropics. They are commonly more or less circular in outline, rising into a cone, the tip of which sometimes is produced into a spire, which falls over. In the interior of the shell, there is in some of the genera a variously shaped shelly plate, which is quite wanting in others.
The animal has a distinct head furnished with tentacles, and eyes placed at their bases; the muzzle is not produced into a proboscis. The tongue is armed with teeth, arranged in rows of seven each, the central one differing in form from the others. The gill plume is single, and the foot is unfurnished with lateral filaments.
Some of the genera, at least, sit on and hatch their eggs. According to Audouin and Milne Edwards, the parent Calyptræa disposes them under her belly, and preserves them as it were imprisoned, between the foot and the foreign body to which she adheres, her patelloid shell thus serving not only to cover and protect herself, but as a shield to her offspring. These eggs are oval bodies of a yellow colour, enclosed in membranous capsules, which are elliptical, flattened, translucid, and filled with an albuminous matter. The number of these little capsules varies from six to ten; they are connected among themselves by a footstalk, so as to represent a sort of rosette; each of them contains from eight to ten eggs. It appears that the young Calyptrææ are developed under this sort of maternal roof, and do not quit it until they are in a condition to affix themselves, and are provided with a shell sufficiently hard to protect their own bodies."[15]
Our other native genus of this family is said to manifest an instinct similar to this.
Genus Pileopsis.
The Fools-cap Limpets, as the mollusks of this genus are called, have the shell shaped like a somewhat high cone, with the summit a little produced, and turned over backwards. The surface is commonly marked with lines (striæ), and covered with a horny skin, which is sometimes invested with a short velvety down. The interior has no plate or partition of any kind. The place of the attachment of the muscle is marked by a horseshoe impression. The animal has been already described.
The only British species is commonly known by the appellation of Torbay Bonnet (Pileopsis Hungaricus); it also bears the names of Fools-cap Limpet, Cap of Liberty, and Hungarian Bonnet, all of which designations, as well as both of its scientific ones, have an obvious reference to its form. It is a rather large shell, being frequently more than an inch and a half in diameter, and an inch in height. Its substance is rather thin, though strong, and somewhat translucent; its colour is a delicate pink, or flesh-white, though this is concealed, especially around the lower part, by an olive-coloured skin, covered with shaggy down. The interior of the shell is delicately smooth, and of the same roseate hue as the exterior.
TORBAY BONNET.
The animal is usually pale yellow, with a pink mantle bordered with a fine orange-coloured fringe. The head, which is large and swollen, is tinged with brown. Though generally distributed, the Fools-cap must be considered a rare shell. Torbay, as one of its familiar names indicates, is the locality in which it occurs in greatest abundance. I have had several specimens brought to me from Weymouth Bay, and the West Bay of Portland. Messrs. Forbes and Hanley state that it "chiefly inhabits rocky ground, and oyster and scallop banks, adhering to shells living in various depths of water, from fifteen to as deep as eighty fathoms, and extending its range to considerable distances from land. It is finest in from fifteen to twenty-five fathoms, and usually small in very deep water."[16]
It has been proved, says M. Deshayes,[17] by observation, that the animals of this genus are still more sedentary than the true Limpets, for there may be seen in certain individuals of the P. Hungaricus irregularities, proceeding from the body on which it has lived when young, continuing exactly the same to adult age; irregularities whose traces may be observed on the lines of growth, and which prove, in the opinion of M. Deshayes, that during its whole life the animal has never changed its place. But, probably, M. Deshayes was not aware of those interesting facts which have been observed in the habits of the true Limpets, already described in these pages; for, if a similar habit of roaming for food, and returning with precision to the exact spot which it has chosen as its home, be common to the Foolscaps also, it appears to me that the phenomena alluded to by the eminent French zoologist would be sufficiently accounted for without his hypothesis.
Family Dentaliadæ.
(Tusk-shells.)
On many a pebbly beach upon our coasts there is frequently found, among other shells washed up by the sea, one which bears the closest resemblance to an elephant's tusk in miniature. It is the representative of a family, comprising but a single genus, which is interesting because its characters, as well those of the shell as of the animal, manifest a decided approach to those of another great division of organized beings, the Articulata. Indeed it was formerly considered by the best zoologists to be a genus of Annelida, allied to those which form the shelly tubes so commonly seen on submerged shells and stones, known as the family Serpuladæ; and this position was assigned to the tusk-shells by Cuvier, even in his last edition of the "Règne Animal"
The shell, in this family, is a tube more or less curved, wider at one extremity than at the other, and open at both ends. It is, in fact, a very lengthened cone, with an open apex, as in Fissurella, a genus with which it has been considered to have some alliance.
The animal is of the same form as the shell, and presents not only in this respect, but also in many details of its structure, peculiarities which distinguish it from all the rest of the Gasteropoda. According to Mr. Clark, of Exmouth, to whose elaborate and skilful dissections our knowledge of the anatomy of the Mollusca is so much indebted, the gills are two symmetrical organs, hanging from the sides of the animal, a little behind its middle. The heart is placed at the front of the gill-cavity; a peculiarity of position which is dependent on the curious fact that the water to be respired flows into the gills from the posterior aperture of the shell. The front orifice is occupied and stopped by the thick collar of the mantle, through the centre of which the tip of the foot protrudes.
DENTALIUM,
(natural size)
without the shell.The mantle invests the body like a tube; but if this be slit down the back, a minute head is discovered near the middle of the body, furnished with horny jaws, and bearing on each side a large tuft of filaments, which are considered to be salivary glands. There are neither eyes nor tentacles.
The animal is attached to its shell only at its posterior end; the fore parts are capable of being protruded or withdrawn at the pleasure of the animal. When fully expanded the foot is thrust out in the form of a thickened and pointed tongue, surrounded by a trumpet-like lobe; the whole of which has been compared to the expanded corolla of a flower, with a very thick and pointed pistil.
Genus Dentalium.
As the family includes but this one genus, it is needless to repeat the characters by which it is distinguished. The species are rather numerous; M. Deshayes enumerates twenty-three living and thirty-four fossil, and several have been added to both lists since he wrote. Their geographical distribution is extensive, few seas being without some representatives of the genus; and they range from deep water to within tide-marks. In their habits they are carnivorous, feeding on those minute animals with chambered shells called Foraminifera, as well as on small bivalves. Mr. Clark has found species of as many as eleven distinct genera in the pouches on the two sides of the mouth, or in the stomach, of our commonest Tusk-shell (Dentalium Tarentinum). The same acute naturalist has the following observations on the affinities of the genus, already slightly alluded to:—
"The symmetrical, subventral position of the branchiæ, the posterior flow of water to them, and the resemblance of the foot to that of some of the bivalves, combined with the similar character of its action, appear in a striking manner to show its connexion with the Conchifera: whilst by its œsophageal cerebral ganglions, and completeness of the circulation, it has established its claims as a Gasteropod. There are also traces of alliance with some of the inferior classes. The red blood and vermiform configuration of the posterior part of the animal show some of the characters of the Annelida."
Two species of the genus are recognised as British, D. entalis, the Smooth Tusk, and D. Tarentinum, the Grooved Tusk. Of these the former is common around the shores of Scotland and the north of England; the latter on our southern coasts. Though possessing much resemblance to each other, they may be distinguished by the small end of the latter being finely grooved lengthwise, while the former is quite smooth and shining.
DENTALIUM TARENTINUM,
(magnified).
The Smooth Tusk, here represented, grows to a length of nearly two inches, with a diameter of about a quarter of an inch at the larger end. It is opaque, of a shining white surface, like porcelain, never tinged (as its southern congener usually is) with pink, and never marked with sculpture, but often with rings indicating the progress of growth.
It is a deep-water species, living "buried in sand or sandy mud, in from ten to one hundred fathoms;" but obtained most abundantly in from forty to seventy fathoms.
Dr. Gray remarks that the apices of the British specimens often appear to be either broken off, or to have fallen off of themselves, like the tips of those shells called decollated, as some of the Helicidæ; and he adds that when the tip is broken, the animal forms a slight tube within, which is more or less produced beyond the tip. Specimens in this condition have been described as distinct species. A parasitical worm, of the class Echinodermata, the Sipunculus Bernhardus, is frequently found inhabiting old and dead shells of the Dentalium, the mouth of which it closes, except a minute orifice, with a sort of mortar made with sand. This parasite has sometimes been mistaken for the original inhabitant of the shell.
- ↑ Brit. Mollusca, iii. 493.
- ↑ Zool. Journ. iv. 163.
- ↑ Zoology of Samarang, part iii. p. 24.
- ↑ Zool. of Samarang, Part III. p. 23.
- ↑ Athenæum, May 1, 1847.
- ↑ Penn. Cyclop.; art. Entomostomata.
- ↑ Forbes and Hanley's Br.Moll.iii.385.
- ↑ Cited in Forbes and Hanley, iii.384.
- ↑ Sea-Side Book, p.33.
- ↑ Linn. Trans. 1833.
- ↑ Invertebr. of Massachusetts, 232.
- ↑ Da Costa's Elements, 204.
- ↑ Proc. Zool. Soc. iii. 116.
- ↑ Johnston's Introd. to Conch. 351.
- ↑ Litt. de la France, i. 133.
- ↑ Br. Moll. ii. 461.
- ↑ Annales du Muséum.