ORDER II. DIMYARIA.
Family Arcadæ.
(Arks.)
A well marked character of this family, and one that a glance is sufficient to recognise, is the peculiarity of the hinge. This, whether straight or bent, extends along a large portion of the margin of each valve, and is formed of a great number of minute comb-like teeth arranged in a row, nearly parallel to each other, or slightly diverging. Those of one valve correspond in shape and size to those of the other, with which they accurately interlock. The beaks (umbones) are generally distant from each other when the shell is closed. In form, size, solidity, sculpture, and colour, there is much diversity in the species; but all are covered with an epidermis. Two distinct muscular impressions are strongly marked, which are remote from each other, but are connected by a line running parallel to the border of the valve, which is the pallial impression.
The animal has the mantle freely open, without siphonal tubes, except in a few species. It is partially adherent to the shell; sometimes prolonged backwards. The foot is always considerably developed, deeply grooved, and capable of expanding into a disk, like that of a Gasteropod.
All the species are marine, and inhabit various depths of water from tide-marks outwards. Some of them affix themselves firmly to rocks, by means of a stout byssus.
Genus Arca.
From the square boat-like form of the valves in this genus, resembling that commonly attributed in engravings to the Ark of Noah, these shells have derived their name. The valves are generally equal, much broader than long, and more developed on one side than on the other; four-sided in outline, and usually solid in texture. Their surface is covered with radiating close-set grooves, concealed, however, by a rough, loose, horny epidermis. The hinge is nearly or quite straight; consisting of numerous minute teeth, which are parallel in the centre, but diverge at the sides.
The animal is oblong; the mantle, which, as stated above, is open, is either fringed or simple at its edges. The foot is large, oblong, bent, grooved throughout, and capable of expanding into a disk, with plain, or slightly plaited edges; it is furnished at its base with a gland from which is spun a strong compact byssus. The mouth is surrounded by lips, which are formed by the extremities of the gills.
These Mollusks inhabit all depths of water, but are generally found near the shore; perhaps, however, this is because their peculiar habits render them less accessible in other situations. They chiefly live in holes and fissures of rocks, moored by their powerful byssus. M. Rang remarks that they sometimes adhere by their disk-like foot: and Mr. Swainson states, that when young, they occasionally adhere by means of their fibrous epidermis.
NOAH'S ARK.
The species of this genus are chiefly distributed through the seas of the tropics, where they are very numerous. We have three or four on our own shores, the largest of which is commonly known as the Ark of Noah, though the species properly so named (Arca Noæ) is a native of the Mediterranean. Ours is the Arca tetragona of conchologists. The valves are deep, four-sided, and strongly angular, covered with fine radiating lines, of a warm reddish-brown hue, becoming paler and sometimes white on the front part. The valves sometimes attain a breadth of two inches, with a length of about three-fourths of an inch. It is found commonly all round the British coasts, most abundantly in the south-west, and in the extreme north. "It is taken in crevices of rocks, in chinks of old shells, in the interior of dead shells, and sometimes quite free. When found in fined localities it is often much distorted, but free specimens are very regular."[1] The epidermis in this species often projects beyond the edges of the valves, in the form of a ragged fibrous fringe.
Family Mytilidæ.
(Mussels.)
The members of this family are almost all marine, though one genus (Dreissena) inhabits the rivers of Eastern Europe, and is now found plentifully in some of the navigable streams of this country, having been introduced, as is supposed, adhering to timber, or to the bottoms of ships. The distinctive characters of the family, as enumerated by M. de Blainville, are as follows:—
The shell is regular, with the valves equal but much lengthened, and produced on one side. The hinge is destitute of teeth; the ligament dorsal and linear. The texture of the shell is sometimes thin and horny, and it is often covered with an epidermis. There are two muscular impressions, the hinder of which is considerably larger than the other.
In the animal we find the mantle adhering towards the borders, but open throughout its inferior portion, with a distinct anal tube, but the branchial one rudimentary, and indicated only by a thickening of the posterior borders of the mantle. The foot is narrow and tongue-shaped, grooved down its centre, and furnished with a byssus backwards at its base. In some of the genera, as Lithodomus a foreign group which burrow in the hardest rocks, the faculty of spinning is confined to the early life of the animal, but in others a strong and copious byssus is formed at pleasure through life.
Genus Mytilus.
The name of this familiar shell-fish is derived from the obvious resemblance which its shell bears to a mouse, the united beaks representing the nose. The Greeks used the term Mύς for both the shell and the quadruped, and both the Latin term Mytilus and our own Mussel are derived from that appellation.
The genus is characterised by a shell very unequal-sided, but equal-valved, somewhat triangular, running off into pointed terminal beaks, the valves swelling, the surface covered with a horny skin; the hinge, though sometimes notched, has no true teeth; the narrow ligament is internal.
The animal is lengthened and oval, the lobes of the mantle are simple or fringed, united behind in a single point, so as to form an anal siphon. The mouth is rather large, furnished with two pairs of soft lips. The foot is slender, strap-shaped, grooved, carrying at its base a silky byssus of considerable strength.
The Edible Mussel (Mytilus edulis) is the only British species, but is too abundant and too well known to need description. Sold in every town as an article of food, its three-sided shells, black without and blue within, and the bearded animal, both in its raw and cooked state, are familiar to every one.
Dr. Knapp of Edinburgh has communicated to Messrs. Forbes and Hanley a very interesting account of the quantities of this animal destroyed annually in the neighbourhood of that city. "As an article of food," he states, "there cannot be used fewer than ten bushels per week in Edinburgh and
MUSSEL.
Leith, say for forty weeks in the year; in all, 400 bushels annually. Each bushel of mussels, when shelled and freed from all refuse, will probably contain from three to four pints of the animals, or about 900 or 1,000, according to their size. Taking the latter number, there will be consumed in Edinburgh and Leigh about 400,000 mussels. This is a mere trifle compared to the enormous number used as bait for all sorts of fish, especially haddocks, cod, ling, halibut, plaice, skate, whiting, &c. In Newhaven alone there are four large deep-sea fishing-boats, which generally go out three times a-week, and fish for about thirty weeks in the year, excluding Sundays and bad weather. Each of these large boats carries eight men, with eight lines of 800 yards in length, which, at a low calculation, take 1,200 mussels to bait each time they are used; so that each large boat will use 28,800 mussels per week, equal to 864,000 per annum. But there are about sixteen other smaller boats, which go out daily, or rather at 12 o'clock every night, for about the same number of weeks in the year. Each carries four men, with four lines 800 yards long. Their consumption of mussels will come to 3,456,000. The total consumption of mussels for bait annually in Newhaven alone may be reckoned at 4,320,000. As there are nearly as many used at Musselburgh, and Fisherrow, Buckhaven, Elie, Anstruther, Pittenweem, Crail, and other places on the Frith of Forth, we may calculate that thirty or forty millions of mussels are used for bait alone by the fishermen of this district each year. Numbers come from the river Eden-on-Fife, and are sold at 25s. per cask. The best mussels at Newhaven are got directly north of the Pier, in three fathoms water, and are sold at 8d. per basket, each containing nearly a bushel. The beds are private property, and some of them having been injudiciously or avariciously exhausted, the number of mussels in the Forth has decreased, and the price increased, within the last ten years."
The eminent authors of the work just cited furnish us with some other particulars of interest.
"Mussels are kept in many places in artificial beds, to be used when required for bait. At Anstruther, in Fifeshire, we have seen these 'mussel gardens,' as they are called—little plots of seashore between tide-marks, edged in by stones, and held as private property. In Northumberland, Mr. Alder states, the fishermen build up piles of stones among the rocks, to keep their mussels safe."[2]
The Freshwater Mussel (Unio) has long been known as one of those shell-fish that produce pearls; but those procured from the present species are commonly small, ill-coloured, and of little value, though they have been at various times much sought for. The passage in Camden's "Britannia," about the pearls in Cumberland, evidently refers to these. "Higher up the little river that runs into the sea, . . . the shell-fish, having by a kind of irregular motion taken in the dew, which they are extremely fond of, are impregnated and produce pearls, or to use the poet's phrase, baccæ cochleæ, shell-berries, which the inhabitants, when the tide is out, search for, and our jewellers buy of the poor for a trifle, and sell again at a very great price." The attempt to account for the origin of pearls by the drinking in of dew-drops, my readers may probably think more poetical than philosophical. A very curious account of a recent pearl-fishery in North Wales is given by a correspondent in "Loudon's Magazine of Natural History" for 1830. The writer has confounded the Mytilus edulis with the Unio. To the latter only his remark on pearls "found up the river" applies. His account is as follows, with some slight omissions:— "The pearl-mussel is found in abundance in the river Conway, in North Wales, and is collected by many of the natives, who obtain their livelihood entirely by their industry in procuring the pearls. When the tide is out, they go in several boats to the bar at the mouth of the river, with their sacks, and gather as many shells as they can before the return of the tide. The mussels are then put into a large kettle, over a fire, to be opened, and the fish taken out singly from the shells with the fingers and put into a tub, into which one of the fishers goes barefoot and stamps upon them until they are reduced to a sort of pulp. They next pour in water to separate the fishy substance, which they call solach, from the more heavy parts, consisting of sand, small pebbles, and the pearls, which settle at the bottom. After numerous washings, until the fishy part is entirely removed, the sediment, if I may so term it, is put out to dry, and each pearl separated on a large wooden platter, one at a time, with a feather; and when a sufficient quantity is obtained, they are taken to the overseer, who pays the fisher so much an ounce for them. The price varies from eighteenpence to four shillings: there are a number of persons who live by this alone, and where there is a small family to gather the shells and pick out the fish, it is preferable to any other daily labour. The pearls are generally a dirty white, sometimes blue, but never, I believe, green or reddish. I cannot with accuracy say how many ounces are taken to the overseer each week, though I might say there are some scores. But what makes this fishery more singular, is the mystery which hangs over it. At present it is a perfect monopoly, and there is but the one who buys them up, that knows what becomes of them afterwards. It has been carried on in this manner for many years, and as such a thing, if made public, might prove more beneficial to the neighbouring poor, by causing a higher price to be given for the pearls, it would be more so if any of your numerous correspondents could throw some light on this interesting subject. There have been some curious and fanciful surmises, which may not be thought worth mentioning. Some suppose that the pearls are sent abroad to be manufactured into seed pearls; others, more gravely, that they are exported to India to be dissolved in the sherbet of the nabobs! However, at present it is a mystery; and, notwithstanding the pains taken and the expense incurred by some liberal gentlemen in endeavouring to find out the secret, it is as great a mystery as ever. The huts which have been erected for the convenience of boiling the fish, are on the extremity of the marsh, about a mile north of the town of Conway. The pearls are seldom found here much larger than the enclosed specimens, though about twelve miles up the river they have been found occasionally as large as a moderate-sized pea, and have been sold for a guinea the couple, but they are very rarely met with. When I say that the price varies from eighteenpence to four shillings, I do not mean to say that they are valued according to their size, for the large and small pearls are all sold together: but some years ago they were as high as four shillings, now they are only two shillings per ounce."
The ill effects produced by eating the mussel, at least at certain times, and on certain constitutions, have been before alluded to. (See ante, p. 43.) Such cases, however, are rare, and, as Pennant remarks, for one who is affected by eating mussels, a hundred remain uninjured. In general, they are simply boiled with us; but probably they might be made much more savoury by a different mode of cooking. Captain King, in speaking of a large species (M. choros) found abundantly on some islands on the Pacific coast of South America, observes:—"The manner in which the natives of these islands, both Indians and descendants of foreigners, cook shell-fish is similar to that used for baking in the South Sea Islands, and in some parts of the coast of New Holland. A hole is dug in the ground, in which large smooth stones are laid, and upon them a fire is kindled. When they are sufficiently heated, the ashes are cleared away, the shell-fish heaped upon the stones, and covered first with leaves or straw, and then with earth. The fish thus baked are exceedingly tender and good, and this mode of cooking them is superior to any other, as they retain within the shell all their own juiciness."
Family Unionidæ
(Freshwater Mussels.)
From the true Mussels the transition is easy to those of this family. They are distinguished chiefly by the structure of the foot of the animal, which is greatly developed in dimensions, and, at least in general, has not the power of spinning a byssus. Mr. Anthony, however, a conchologist of the United States, where the great abundance of these Mollusks in the large rivers affords peculiar facilities for observation, has stated that under peculiar circumstances certain species do spin a byssus, both in the young and adult conditions.[3]
The shell is lengthened oblong, equal-valved, unequal-sided, though in a much less degree than the family last noticed. The interior is pearly, often very brilliant; the exterior is invested by a horny epidermis, which, though generally black or olive, is sometimes richly coloured. The hinge varies greatly, being sometimes destitute of teeth, and sometimes furnished with long ones. The muscular impressions are numerous, the ligament and cartilages are external.
The animal has the mantle free all round, except behind the hinder edges, forming, when in conjunction, two tubes, the larger of which is guarded by pointed and tooth-like tentacles. The foot is large, compressed, somewhat four-sided.
These Mollusks live at the bottoms of rivers and lakes, plunged perpendicularly into the mud, with the terminations of the siphons just exposed. They sink deeper when disturbed. Sometimes they are found under the shelter of stones in rivers with rocky bottoms.
The distribution of the family is peculiar. Very few are found in the Eastern Hemisphere, but all the rivers of America swarm with them. More than two hundred species are enumerated as inhabiting the United States.
The sexes of these animals are distinct, and may be distinguished by the shell in the female being more swollen than that of the male.
Genus Unio.
The condition of the hinge affords the distinctive character in this family. The genus Unio has it furnished with a short crested anterior tooth, and a lengthened posterior lateral one in the right valve, shutting between two similar teeth in the opposite.
The shells of this genus, which are frequently large and richly pearly in the interior, are frequently used by artists for containing their colours. One of our British species is hence named Unio pictorum, or the Painter's Unio. Many of them yield pearls, which are generally superior to those of the marine Mussels. Our finest native species, hence named U. margaritiferus, is one of these. It is about five inches long, and half as broad, somewhat kidney-shaped, covered with a black epidermis, usually worn away at the beaks; the interior is pearly, but not brilliant. This species inhabits the rapid streams of mountainous districts in various parts of the British Islands.
"The Pearl Mussel, as this mollusk is familiarly called, enjoys a distinguished reputation as one of the few indigenous bivalves which yield the beautiful productions whose name it bears. In ancient times Britain enjoyed some celebrity for its pearls, as they constituted one of its attractions for Julius Cæsar, who, however, does not seem to have reaped a very rich harvest, so far as quality went, though he obtained enough in quantity to cover with them a buckler, which he dedicated to Venus Genetrix, and suspended in her temple. The pearls used for the construction of his present were probably such as Roman ladies would have scorned to wear, although they were ostentatiously offered to the goddess of beauty; for Pliny, who narrates the circumstance, states at the same time that the pearls from Britain were small and lustreless, and not to be compared with those from the East. Tacitus, in his Life of Agricola, describes the pearls of Britain as 'subfusca ac liventia;' and among ancient Christian writers they are mentioned by Origen and Bede. Pennant, and other writers, who have treated of pearls, have all taken it for granted that those mentioned by the ancient authors quoted were derived from the Unio. This, however, is by no means clear, and Cæsar's buckler was more probably covered with pearls from Mytilus edulis, very much inferior in quality and size to those from the fresh-water Pearl Mussel, and agreeing better with the disparaging account of them in Pliny. Those mentioned by Camden, as occurring at the mouth of the Irt, in Cumberland, seem to have been of the same nature. The pearl-fishery at the mouth of the Conway [to which I have already referred] also concerns the Mytilus and not the Unio. Higher up the latter river, however, and in many rivers of all parts of the kingdom, especially in the neighbourhood of mountainous districts, the Unio has been at various times fished to a great extent for pearls, and, in all probability, the fame of British pearls that attracted the Roman conqueror was due to the products of the shell before us. The best account of these fisheries of the freshwater Pearl Mussel is contained in a curious paper in the seventeenth volume of the 'Philosophical Transactions,' (1693,) written by Sir Robert Redding, and communicated by Dr. Martin Lister. This paper has been often referred to by subsequent writers, who, however, seem to have made use of Pennant's short notice of it only, which itself was taken from the abridgement, and not from the original. It is a remarkable paper, on account of the correctness of observation displayed in the personal statements of the author, who seems to have been a person with considerable natural-history powers. His description of both shell and animal is curiously correct as far it goes. He states that they were fished in the rivers of Tyrone, Derry, Donegal, near Dundalk, near Waterford, and in Kerry. The poor people fished them in the warm months before harvest time, when the rivers were low. They took them with their hoes, or wooden tongs, or by thrusting a stick into the shells which they caught sight of among the stones as they lay in part opened, with the white foot protruded, 'like a tongue out of the mouth.' Sir Robert saw them lying on their sides, and his informants described them as 'set up in the sand like eggs in salt, with the sharp edge downwards, and the opening side turned from the torrent.' One in a hundred might contain a pearl, and about one in a hundred of the pearls was tolerably clear. There were no pearls in the young mussels. 'Some gentlemen of the country made good advantage thereof, and I myself whilst there saw one pearl bought for fifty shillings that weighed thirty-six carats, and was valued at forty pounds. Everybody abounds with stories of the good pennyworths of the country, but I will add one more. A miller took out a pearl which he sold for four pounds ten shillings to a man who sold it for ten pounds, who sold it to the late Lady Glenealy for thirty pounds, with whom I saw it in a necklace; she refused eighty pounds for it from the late Duchess of Ormond.' 'The pearl,' Sir Robert observes, 'lies in the toe, or lesser end, at the extremity of the gut, and out of the body of the fish, between the two films or skins that line the shell.' He remarks that they correspond with calculi in other animals.
"The pearls of the Conway had great fame. According to Pennant a notion prevails in Wales, 'that Sir Richard Wynne, of Gwydir, Chamberlain to Catherine, queen of Charles the Second, presented her majesty with a pearl from the Conway, which is to this day honoured with a place in the regal crown.' He says the Pearl Mussels are called by the Welsh Cregin Diluw, or Deluge Shells, as if left there by the flood. Mr. Wilson, of Warrington, in 'Loudon's Magazine of Natural History' for June, 1830, says they are taken in the upper part of the Conway, near Llanrwst, but the search is very precarious. He mentions a Scotch pearl half an inch in diameter. In Scotland, the Tay was the seat of a pearl-fishery, extending from Perth to Loch Tay. 'It is said,' writes Captain Brown, 'that the pearls sent from thence to London, from the year 1761 to 1764, were worth ten thousand pounds sterling; and it is not uncommon at the present time to find pearls in the Teith and Tay worth from one to two pounds each.' The variety Roissyi of this Unio was formerly much sought for in the river near Braddan, in the Isle of Man, on account of its pearls."[4]
Family Cardiadæ.
(Cockles.)
This is a large, important, and, generally speaking, well-defined group of bivalves, the characters of which are thus given by Cuvier. In all the mantle is open anteriorly, but posteriorly it presents two siphons or orifices, one for respiration, the other for the egesta. These form tubes sometimes distinct, sometimes united into a single mass. There is always a transverse adductor muscle at each extremity of the shell, and a foot which generally serves for the purpose of locomotion. It may be laid down as a rule, that the species with elongated siphons live buried in the mud, or sand. This condition of organization may be recognised on the shell, by the more or less developed contour which the impression of attachment of the borders of the mantle describes, before uniting with the impression of the posterior transverse muscle;" or, in other words, by the course of the pallial impression.
These Mollusks are all marine, with one or two doubtful exceptions; they frequently attain large size, and are valued for the delicacy and wholesomeness of their flesh; their abundance often renders them easy to be procured.
Genus Cardium.
The shells of this well-known genus are globose and heart-shaped: the valves are equal, with beaks rolled in towards each other. The hinge is formed by four teeth in each valve, of which the two primary ones called cardinal are oblique, and one 
FOOT-MUSCLES OF COCKLE.lateral remote on each side. The valves are almost always covered with well marked ribs alternating with furrows, which radiate from the beaks to the margins. These ribs are sometimes studded with spinous scales. The ligament is external and conspicuous though short.
The animal is shaped much like the shell, being round and swollen, with the mantle open in front, the edges of which are usually plain, but strongly fringed in the neighbourhood of the two siphons; the latter are short, slightly separated and fringed around the edge. The foot is very large, cylindrical, with a conical tip directed forwards. It is bent, with an abrupt elbow near the middle. These animals inhabit sandy or muddy shores, in which they live immersed at a moderate depth, or covered only by the tide at high-water. The powerful and versatile organ known as the foot is admirably adapted for the situations in which they reside. This organ in the present family is developed to a very great size, and is moved by an immense number of muscles of the most elaborate structure and arrangement, as will be seen by the preceding engraving of the dissected foot in a native species, Cardium rusticum.
In order to enable the animal to burrow in the sand, the foot, by means of its conical point, is thrust beneath the surface. When stretched to its utmost, it is bent into a hook, the whole organ is then forcibly contracted, and the animal, shell and all, is thus dragged a little way down into the yielding sand. This process is successively repeated, until the animal is sufficiently buried. But the foot can be made an instrument of locomotion, of quite another kind. The Cockle is a vigorous leaper, having been known to clear the gunwale of a boat when laid on the bottom boards. In order to effect this, the end of the foot bent into a hook is pressed firmly against the plane on which the animal is lying, in the position represented in the following engraving. The foot is then stiffened, when a sudden spring-like action of the elongating muscles jerks the animal into the air, with a force and agility that cannot fail to surprise those who witness it for the first time.
The Cockles are widely distributed, being found in nearly every sea, but their great metropolis is in the Indian Ocean, whence, as from a centre, the number of species diminishes in the ratio of distance. About two hundred species exist in collections, of which about ten are found on the British coasts. The shells of this genus are generally remarkable for elegance of form, and often for brilliancy of colouring. Our British species are among the handsomest of our bivalves. "There is a pleasure," observe the learned authors of the British Mollusca, "in investigating such a group as this, wherein we find not only the greatest variety, but also the greatest distinctness and consequent certainty of specific determination."
COCKLE LEAPING.
A certain Parrot's loquacity had made her a favourite with all the members of a family, except the cook, whose resentment was incurred by Poll's thievish propensities. A jar of pickled cockles was on a shelf in the cupboard, and whenever the door was casually left open, the watchful bird failed not to pay it a visit, though always scolded when found out. One day Cook, coming suddenly into the kitchen, caught Poll in the act of emerging from the cupboard. Unable to restrain her wrath, she cried, "What! you've been at the pickled cockles again, have you?" and at the same instant dashed at the offending bird a tureen full of hot soup which was in her hand.
The poor bird was grievously scalded, and the consequences were the loss of all the plumage of her head, and of all her wonted garrulity. Not a word was uttered for weeks, and it was feared she had become hopelessly silent, when, one day, just as the new feathers of her head were beginning to sprout, a gentleman who was bald happened to call. Struck with the similarity of condition displayed by the stranger's smooth crown to her own, Poll broke forth, vociferating with peculiar emphasis, "What! you've been at the pickled cockles, have you?"
Family Cyprinadæ.
A group of fine shells, many of which are of large size, is thus named after the principal genus, Cyprina of Lamarck; though the appellation of the genus and that of the family are alike objectionable, as they have long been pre-occupied with the slightest possible terminal difference by the family and genus of the Carps among Fishes. The Mollusks of this group are distinguished by thick solid shells, often clothed with an epidermis, but usually smooth on the surface, or if sculptured, only with concentric lines running parallel to the edges of the valves. They vary in colouring, some being brilliant, others sombre. The hinge has strongly developed teeth, and an external ligament. The pallial impression is scarcely sinuated in its course. "This depends on a peculiar feature in the organization of the animal, which has, instead of distinct and produced siphonal tubes, only rudimentary ones in the shape of two scarcely separated orifices." The foot is thick and tongue-like.
Genus Isocardia.
This genus presents many points of affinity with the Cockles, and some with the great Clamps (Chama) of tropical seas. The shells are large, very convex, almost globose, heart-shaped, with the beaks much inrolled, curving forwards and outwards; the hinge is composed of two flattened teeth. The animal will be described presently.
The form of the valves has given the name of Heart-shell to the only British species, Isocardia cor; it ranks among the largest of our Mollusca, being sometimes found nearly three inches in length, and the same in breadth and thickness. It is of an uniform dark brown or liver-colour.
HEART-SHELL.
The following interesting account of the animal and its habits was communicated by the Rev. James Bulwer to the second volume of the "Zoological Journal." "Mantle completely lining the shell, double at the outer edge; exterior fold divided in front, open at each end; at the posterior end forming two short siphons or tubes, ciliated at the upper orifices; colour, yellowish white; margin, orange. Foot very muscular, broad, triangular, compressed, pointed, orange. Branchiæ external, concealed between the mantle and the body. Body soft, completely included within the valves. On being placed in a vessel of sea-water, the valves of the shell gradually opened to the extent represented in the drawing [about one-fourth of an inch]; the feelers or ciliated fringe of the upper orifice of the mantle moved slowly, as if in search of animalculæ. Having remained in this situation about ten minutes, "water was ejected with considerable force from the lower orifice, which till now had remained motionless. The expulsion of the water appeared to be effected by a sudden contraction of the muscles, because this was never done without the valves nearly closing at the same instant. After a few seconds the valves gradually returned to their open position, and remained quiescent as before, till the water was again ejected with a jerk; this alternating process was repeated at unequal intervals during the whole time my specimens were under examination, but at shorter intervals on receiving fresh supplies of sea-water, when I suppose food (its quality I could not ascertain) was more abundant.
"The animal appears to be insensible both to sound and light, as the presence and absence of either did not at all interrupt its movements; but its sense of feeling appeared to be very delicate: minute substances being dropped into the orifice of the mantle instantly excited the animal, and a column of water strongly directed expelled them from the shell. With so much strength was the water in some instances ejected, that it rose above the surface of three inches of superincumbent fluid. Animal small in proportion to its shell, occupying when dead barely a third of the space enclosed in the valves. Its mantle is slightly attached to the shell, and to the epidermis at the margin, and appears to be kept distended, and in contact with the interior of the valves, by the included water. The valves fit so closely that the animal can remain two days or more without permitting a single drop of fluid to escape. Locomotion very confined; it is capable, with the assistance of its foot, which it used in the same manner (but in a much more limited degree) as the Cardiacea, of fixing itself firmly in the sand, generally choosing to have the umbones covered by it, and the orifices of the tubes of the mantle nearly perpendicular.
"Resting in this position on the margin of a sandbank, of which the surrounding soil is mud, at too great a depth to be disturbed by storms, the Isocardia of our Irish sea patiently collects its food from the surrounding element, assisted in its choice by the current it is capable of creating by the alternate opening and closing of its valves."[5]
This fine species is nowhere very common; it is, however, occasionally taken in deep water off the extreme north of Scotland, and the extreme south-west of England, but it appears to be most abundant in the neighbourhood of Dublin. The other species are but few, and are found in the East Indian seas, and on the coasts of Australia. They inhabit mud and sand, at a depth ranging from ten to twenty fathoms.
Family Veneridæ.
(Venus Shells.)
The best character of this extensive family is, according to Cuvier, that the teeth and laminæ of the hinge are approximated beneath the beak in a single group. In general they are more flattened and wider in proportion to their length than the Cockles, from which they differ conspicuously also in having the ribs never radiating towards the margin from the beaks, but always, where present at all, in concentric lines parallel to the margin. The ligament often leaves behind the beaks an oval depression, commonly called the corslet. The muscular impressions are strongly marked, and the pallial, or that which indicates the adhesion of the mantle, is much sinuated in its course.
The animal has two siphonal tubes, capable of being protruded in a greater or less degree, sometimes united to each other; their extremities are fringed, though very slightly in some species. The mantle is widely open in front, for the projection of a large compressed foot, which serves for creeping.
This family is peculiarly rich in the number of its species; the genera Venus and Cytherea alone contain about a hundred each still existing, besides half as many more which are fossil. They are found in almost all seas, generally at a moderate distance from the shore, but extending from tidemarks to great depths. Our native species, Venus striatula and ovata, for example, live indifferently at low-water mark, and at the depth of a hundred fathoms. The tropical regions afford, as usual, the greatest number of species, and those most remarkable for beauty of form, sculpture, and colour, for all which, however, the majority of the family may be considered as preeminent among bivalves; whence the various appellations of the goddess of beauty, and similar names have been selected for the genera.
Genus Venus.
This extensive genus, including some of the most highly prized ornaments of cabinets, is distinguished by a shell usually somewhat solid, rounded in outline, or broader than long, equal-valved, but somewhat unequal-sided. The surface is generally adorned with many concentric ridges, often rising into strong ribs, and sometimes divided by radiating furrows. The margins are crenate, or scalloped; the beaks prominent. The hinge is formed of three diverging cardinal teeth in each valve. The muscular impressions are round, and well marked; the pallial sinus wide and pointed.
The animal is thick and globose, or oval; the edges of the mantle, which is open, are fringed. The siphons have fringed orifices, generally separate. The foot is not grooved.
About half-a-dozen members of this beautiful genus inhabit the British seas, which are considered as nearly the northern limit of its geographical range. Of these the finest is Venus casina, of which the valves are about an inch and a half in diameter. They are marked with strong concentric ribs, white, or occasionally painted with two or three crimson radiating bands.
Though this is a rare British shell, it has been taken at various localities all round the coast. I have obtained specimens from deep water off the Isle of Portland.
Family Mactradæ.
In some respects these shells present close resemblances to those of the last family; but their outline is more triangular, and their surface is smooth, or merely marked with fine concentric lines. They are generally swollen; their substance varies much in texture; they are often invested with a strong epidermis. "The valves are connected together by a hinge, consisting of a forked diverging tooth in one, raised on a ligamental fulcrum lodged in a cavity, which is marginated in the other, a connecting cartilage and small external ligament completing the union." The pallial impression closely resembles that of the Veneridæ.
The animals of this family differ in the degree in which the mantle is open or closed in front; there are two siphonal tubes, which are connected and fringed at their orifices, and sometimes studded with pointed warts.
Genus Mactra.
The shell is more or less triangular, slightly gaping, with a smooth or concentrically striated surface, covered with an epidermis. The hinge is composed of a V-shaped cardinal tooth in one valve, locking into a marginated pit in the other, and of a long lateral tooth on each side of the same valve, which also fits into a deep groove with tooth-like margins. The sinus of the pallial impression is shallow but wide.
The animal has the mantle open as far as the siphons, its edges are fringed; the foot is strong, tongue-shaped and bent, capable of protrusion to a considerable extent. It is used, as in the Cockles, for burrowing in sand, which these animals do with great ease and rapidity. They are inhabitants of sandy shores in most seas, and the majority of them are found in the vicinity of low-water mark. Many of them are of a large size. We have a British species, Mactra helvacea, which attains the size of three inches by two and a half in diameter. This, however, is one of the rarest of our native shells, as M. stultorum is one of the most abundant. The latter is found on every sandy beach; it is about half the size just mentioned, somewhat triangular in outline, and varying much in colour, from plain drab to fawn-colour, marked with pale zones and white divergent rays.
Family Donacidæ.
The shell is more or less triangular and compressed, broader than long, equal-valved, but unequal-sided, the hinder side being the shorter. The texture is strong and compact, the hinge variable, but always provided with conspicuous primary teeth.
The animal is generally richly coloured, with the mantle usually fringed; the siphonal tubes are separate and much developed, having their orifices adorned with deeply-cut tentacles. The foot is large, compressed, sharp-edged, and angular; the gills very unequal.
The species of this family, which are rather numerous, occur in all parts of the world, but most abundantly in the Southern Hemisphere. They live buried in sand or sandy mud, with the short side of the shell uppermost, the siphons being protruded, and range from low-water mark to a depth of ten fathoms.
Genus Donax.
In addition to the family characters already mentioned, the hinge in this the typical genus is composed of two primary teeth in one valve, and one in the other, with accessory lateral teeth. The ligament is external, short, and swollen; the muscular impressions are rounded, and the connecting pallial impression is straight, with a wide and deep sinus, hollowed out toward the hinder side. The surface is covered with an epidermis.
POLISHED DONAX.
Among the more beautiful of British shells must be reckoned the Polished Donax (D. politus). It is oblong in form, usually a little more than an inch in breadth, and about half that length. It is always vividly coloured, but varies much; the most common condition is of a pale golden yellow, or faint lilac hue, spotted and clouded with reddish brown; a broad pale ray passes from the beak to the margin. The interior, which, like the exterior, is highly polished, is generally of a rich purple, sometimes varied near the beaks with golden yellow or orange. The animal is of a bright yellow colour, with orange stripes and pink fringes; the foot is large and white. This lovely species, nowhere common, is found in many places on our southern coasts.
Family Tellinadæ.
(Tellens.)
This is a family very numerous in species, which are generally remarkable for the elegance of their form, the delicacy of their texture, and the brilliancy of their colour. The shell is generally much compressed, greatly produced in breadth, regular, and nearly equal-sided. The hinder side, which is often angular, has an irregular bend or fold at its lower border, more or less conspicuous. The beaks are very small, the hinge varies much, the ligament is swollen and lengthened, the pallial impression deeply hollowed. In general the valves are exceedingly thin and fragile, but painted with the most beautiful colours, often arranged in elegant patterns.
TELLINA.
The animal has the mantle moderately open, bordered with fringes; the gills are unequal on both sides. The siphons are very much lengthened, separate, and diverging, returning into a fold of the mantle; their orifices but slightly fringed. Notwithstanding the gaiety of the shell, the animal in this family is always arrayed in white, "simplex munditiis;" differing in this respect from Donax, the inhabitant of which usually rivals its shell in gorgeousness of hues.
About seventy species are enumerated, which live at various depths, chiefly in the warmer seas; their habits closely agree with those of the preceding family. They are much preyed upon by the carnivorous Gasteropoda, as Strombus, Buccinum, &c., their thin shells exposing them with more than ordinary ease to the depredations of those boring Mollusks.
Genus Psammobia.
The shell in this lovely genus is more or less oval, the breadth commonly being double the length; the surface smooth, or delicately striated, and clothed with a thin epidermis. The hinge is composed of two cardinal teeth in one valve, receiving one from the other; the ligament is prominent.
The animal is white, with the mantle open, and slightly fringed; the very long siphons are marked along their length with lines of cilia, and fringed at their orifices. The foot is large and tongue-shaped.
These Mollusks have the habits common to all these allied families. They live, as their name 
PSAMMOBIA VESPERTINA..indicates, in sand covered by the sea, from the water's edge to great depths. They are said to be active in their motions, burrowing with rapidity and ease in the sandy bed of the sea. We have several British species, one of the most elegant of which is sometimes called by collectors the Setting Sun (Psammobia vespertina), from its warm pink hue, and the pale bands which radiate from the beaks to all parts of the margin, like the rays which frequently stretch across the sky from the evening horizon. This species is commonly about two inches in breadth, and is not at all uncommon, burrowing beneath the surface near low-water mark on most of our sandy beaches, where the detached valves washed on shore by the tides are often picked up, and always admired.
Family Solenidæ.
(Razor Shells.)
Every one who has paced along the water's edge on a sandy beach, is familiar with the shells which form the type of this family. Their extreme narrowness and length, (or, to speak strictly, shortness and breadth,) their parallel sides and truncate extremities are so different from the forms of all other shells, that even the unscientific observer regards them with curiosity and interest. The resemblance which the valves bear to the handle (scales) of a razor is obvious, especially in such species as are slightly curved. The valves are thin and brittle, covered with an olive epidermis which readily peels off. Beneath this the surface is marked with striæ recording the progress of growth, which, following courses parallel to all of the margins, impart a singular and peculiar aspect to these shells.
The animals have the mantle united for a portion of its edges, but allowing the protrusion of an enormous foot, which is thick, long, and somewhat club-shaped at the extremity. The siphons are short, united more or less completely, and fringed at the tips.
The species of this genus, which are not numerous, live in sandy beaches near the verge of low-water, or buried in the soil at greater depths. They are most powerful and skilful burrowers, often lying buried in a vertical position two feet deep, though their ordinary habit is to go only so low in the sand or mud, as to allow the extremities of the siphons just to reach the top. "They may be said to have regular burrows. When the animal is undisturbed, and the tide is in, it lies with the tubes at the entrance of its perpendicular hole. If it be disturbed, down it goes. In short, its life is spent in descending to the depths of its burrow, and ascending from it again, by means of the extension and contraction of its great muscular foot, which is situated at that part of the shell which is lowest."
Genus Solen.
SOLEN.
The characters already enumerated belong pre-eminently to this, the principal genus; one or two others which are associated with it presenting them with less distinctness and with more approximation to the neigbouring families. The hinge is usually composed of a well-defined cardinal tooth and a lateral fulcrum in each valve. It varies in position. The hinder muscular impression is long, the anterior much lengthened. The pallial impression is sinuated.
Several species are common upon our sandy shores, and as their flesh is highly esteemed, they are taken in considerable quantities. When properly cooked, as by broiling, which is said to be the most effective method, they are among the most delicious of shell-fish.
The largest native species is Solen siliqua, which is occasionally found eight inches in breadth, and one in length. It may seem preposterous thus to use the terms breadth and length, but the structure of the shell as compared with other genera compels such an application. The valves are nearly straight, smooth, whitish, with purplish concentric bands, covered with a yellowish epidermis. The animal is large, and of a yellowish white hue.
" This shell is common on most of our sandy shores, found buried to the depth of a foot or more, near low-water mark; it frequently elevates one end a little above the surface, and protrudes its body in search of food: upon being disturbed, it suddenly recedes. This place is known by a small depression on the surface. In many places it is sought after for food by the common people."[6]
" The mode in which a dish full of these esculents is rapidly collected by children, might successfully be imitated by conchologists, for other than culinary purposes; a long narrow wire, bent and sharpened at one end, is suddenly thrust into the hollows of the sands, indicative of the presence of these animals, and passing between the valves, the barbed portion fixes itself on retraction in the animal, and forces it to the surface." [7]
The habits of a species closely allied to this (S. marginatus) were made the subject of investigation by the celebrated Reaumur, who published an account of them, illustrated by figures, in the "Mémoires de l'Académie des Sciences," for 1712. It burrows in sand near low-water mark, spring-tides, to the depth of from a foot and a half to two feet. The Solens lie in their holes nearly vertical, and their places are marked by perforations shaped like keyholes, corresponding to the form of the extremities of their united siphons. They are nearly vertical, and do not remain quiet, but rise up and down, now and then shifting themselves partly above the sand, as if to learn what is going on in the world above. When the tide goes out they sink deeper. The fishermen then endeavour to tempt them out as little boys would catch birds if they could—by putting salt on their tails. The salt penetrating the perforation, reaches and irritates the extremities of the siphons, and the Solen, annoyed and pained, rises suddenly to clear itself of the nuisance. His vigilant human enemy watches the moment, and seizes the opportunity—and the Solen, if he can catch it; but unless very quick in his motions, those of the Solen may be quicker, and once aware of the danger impending, the sensible shell-fish will not rise again, but submits patiently to the indignity of being salted alive, rather than run the risk of being caught and roasted, or else cut up for bait. But if it be not touched, a second dose of salt will cause it again to rise, which shows that knowledge and recollection of the danger is the impediment to its reappearance in the former case. Fishermen in England have a queerly absurd fancy that when the razor-fish feels the salt, it thinks the tide is coming in, and therefore rises in its hole.[8]
"If the Solen be taken out of its hole and placed upon the sand, it immediately prepares to rebury itself. It stretches out its foot to full length, and then bends it so as to use the extremity as a sort of auger. When the end has sunk into the sand, it draws up its shell, which, first oblique, and afterwards perpendicular, soon becomes immersed and rapidly disappears. M. Deshayes, during his Algerian researches, observed a remarkable instinct of S. marginatus to swim when desirous of changing its locality. When it finds itself on ground too hard to be penetrated by its foot, it fills the cavity of its mantle with water, and then contracting and closing exactly at the same time its siphonal orifices, elongates its foot; then recontracting that organ, it ejects the water with force from the tubes, and thus propels itself after the manner of a cuttle-fish for a foot or two forwards. Then if it finds the surface favourable, it bores and buries itself, but if not, it makes another leap to try its chance anew."[9]
Family Myadæ.
(Gapers.)
A large assemblage of genera of small importance, presenting considerable variation in external form and appearance, but having much more in common, may, in a work such as the present, be conveniently associated under the above title, the more especially as they are united in one family by M. de Blainville. He has named the group Pyloridea, including in it the Solenidæ already described, as well as the genera of which Messrs. Forbes & Hanley constitute the three families Myadæ, Corbulidæ, and Pandoridæ; the British representatives of the last-named two are few, and for the most part small, though remarkable for the curious form of the shell, which projects at one side as if distorted.
The shell in the Myadæ is nearly regular and equal-valved, oblong, and somewhat coarse in appearance, gaping at the two extremities. The hinge is incomplete, the teeth being gradually effaced, but generally composed of one or two oblique diverging folds. The ligament is sometimes internal, sometimes external. There are two distinct muscular impressions connected by a pallial impression, widely sinuated behind. The animal is compressed, becoming in the different genera more and more cylindrical, as well as more completely closed, and prolonged backwards into two long siphons, which are ordinarily united, with fringed extremities. In front, the mantle allows the protrusion of a very small foot. The gills are narrow, free, and prolonged into one of the siphons.
All the animals of this family live plunged in the mud or sand, which is ordinarily covered by the sea. They rarely change their place or their position, invariably being vertically immersed, with the siphons above and the foot beneath. Many of the species grow to a large size, and as they are both wholesome and palatable, they are on some coasts much prized for the table.
Genus Mya.
The hinge in this genus is composed of one or two oblique folds, diverging backwards from a horizontal spoon-shaped process, belonging to the left valve, and corresponding to a horizontal groove or socket in the right. The ligament is internal, inserted between the socket and the spoon-shaped process. The shell is generally white, exhibiting rude concentric lines of growth, and covered with a wrinkled epidermis, which is continued over the mantle and tubes of the animal.
The mollusk, thus encased in a thick and leathery case, presents a close resemblance to one of those Tunicated Ascidiæ, which I shall presently notice, with the addition of a bivalve shell; "and no better mode could be devised of impressing on the tyro in malacology the close affinity of two great sections, so unlike in most of their proper members, than the placing before him, side by side, examples of the genera Mya and Cynthia.
The species of this genus are eaten not only by man; Otho Fabricius informs us, that on the inhospitable shores of Greenland, where they are numerous, they are greedily devoured by the walrus, the arctic fox, and by many predaceous birds, as well as by large fishes.
We have two native species, both of considerable size, of which the larger is the Sand Gaper (Mya arenaria), which attains a breadth of four inches, and a length of nearly two and a half. It is a coarse shell, of an earthy reddish or yellowish hue, stained with dirty black, and marked by irregular lines of growth. The animal, when its epidermis is stripped off, is yellowish-white, the orifices tinged with red; the investing coat is brown, rough, and wrinkled.
This Gaper is common on many parts of our seashore, in gravelly, clayey, or sandy ground, at low water, particularly near the mouths of rivers; and, where it occurs, it is usually abundant. At Southampton they are dug up and sold for food, under the name of "Old Maids." Their burrow, which descends to the depth of a foot or more, is indicated by a slight depression on the wet surface, from which, on the approach of a footfall, a jet of water from the siphon al tube is spouted. A narrow spade or fork is instantly plunged down in an oblique direction, with force sufficient to reach the animal, which is then raised with a jerk, an evicted tenant.
SAND GAPER.
One of the rarest of British bivalves (Panopæa Norvagica), allied to Mya, is an inhabitant of deep water, and is interesting for its unusual square form, and for its large size.
The following graphic account of Mr. Bean's discovery of this extremely scarce species is extracted from the "Magazine of Natural History," wherein the shell was first actually published as a native of our seas: "We have obtained at Scarborough three specimens of this, in every sense of the word, gigantic prize. To some of the fishermen of our coast it appeared to be well known by the name of the bacca-box, from a fancied resemblance to one of their most useful household goods. They were all caught by the hook, and rescued from destruction in a singular manner. The first was destined for a tobacco-box; the second had the honour of holding the grease belonging to the boat-establishment; and the third, after amusing them (the members of a philosophical society) by squirting water to the ceiling, was at last seen by a learned friend, purchased for a trifle, and generously placed in our cabinet. The animal we have not seen, but its colour is black." Three additional individuals have since then been obtained by Mr. Bean from the same locality, and two single valves have been dredged by Mr. M'Andrew in ninety fathoms water, twenty-five miles east of Zetland. It is likewise captured, though very rarely, in deep water off the Northumberland and Durham coasts (King and Alder).[10]
Family Gastrochænidæ.
(Stone Borers.)
It is characteristic of all the families of Mollusca lately noticed, that they live habitually concealed in burrows of their own making. These burrows have, however, been commonly perforations in a yielding medium, such as sand or mud, easily and rapidly made by the animal, and obliterated as soon as relinquished.
The species of the family last described occasionally penetrate more resisting substances, such as clay, and even chalk. But I have now to describe genera which have the power of perforating the hardest and most solid rocks. Some species, however, both of this and of the succeeding family, though decided stone-borers, occasionally content themselves with burrows in the softer materials.
In the family before us the shell is equal-valved, but very unequal-sided, the valves usually gaping, and connected by a hinge, which varies exceedingly, sometimes being merely rudimentary, at others having cardinal teeth. These latter, however, when present, are in some species lost under certain conditions of growth.
The animal is oblong or club-shaped, with siphons capable of great elongation, and united through their whole length. The mantle is closed except to give existence in front to a minute foot.
In the genus Gastrochæna, represented by one native species, the gallery perforated in the stone is lined with a deposit of calcareous substance, which forms a shelly tube frequently projecting from the stone, and serving for the protection of the siphons when they are extended. Such an one is now before me, which has been in my possession
GASTROCHÆNA.
in a living state for several months, during which time it has slightly increased the length of its projecting tube by the addition of shelly matter. Internally the tube gives indications of a division into two, but the partition does not extend to the middle from either side. The crimson tips of the siphons may often be seen when the animal is undisturbed, just reaching to the tip of the tube, or projecting in the smallest degree beyond it. The investing tube is found, on carefully breaking the stone, to enclose the animal with its valves in a sort of flask or bottle. This species (G. modiolina) is reckoned among our rarer shells.
Genus Saxicava.
No accessory tube is formed by the animals of this genus, which live in the hardest rocks, especially those of calcareous formation. The shell is oblong and gaping, with prominent beaks; the hinge furnished at one period of life with cardinal teeth. The ligament is external and projecting. The animal is oblong, with the mantle closed, except in front, where exit is afforded for a minute finger-like foot, furnished with a byssal groove. The siphons are united, and their orifices, which are large, are margined with a fringe of simple filaments.
SAXICAVA.
"The Saxicavæ," observe the authors of the British Mollusca, "are borers, although the habit of boring does not seem necessary to their existence, since we find them very commonly free. If there be a crevice, however, in rock, shell, coral, or sea- weed, into which they can thrust themselves, they do so; and if near a limestone rock, perforate it, and form crypts, in which they live. Mr. Osier states that, when young, they are very active animals, and soon commence to perforate. Both that gentleman and Mr. Garner have noticed that their excavations are not round, nor the sides smoothed off, like those of the holes made by Pholas. As for us, we only know of their boring into calcareous rocks, but Mr. Clarke has noticed an instance of their perforating triassic sandstone at Exmouth. Wherever we have a sea-coast of mountain limestone, the surface of the rocks is almost invariably found riddled by Saxicava. The whole front of the Plymouth breakwater has been attacked by it, and much alarm for its safety excited. Mr. Couch observes that the Saxicava never bores deeper than six inches, and that, consequently, unless a new surface be exposed by the destruction of the perforated part, there is not much danger. Owing, however, to the thinness of the partitions, which often are the only separation between the crypts of these mollusks, there is a great probability of the action of the sea rapidly forming new surfaces in such cases. How they bore has been as much discussed as the question how Pholas bores. The general opinion has been, that Saxicava bores by means of an acid secretion; an opinion held by many who will not admit the probability of such an agent being used by the Pholadidæ. Mr. Osier, though inclined to such a view, could detect no acid, nor, for reasons previously stated, is it likely. Mr. Hancock, as we have seen when treating of Pholas, expressly asserts that the Saxicavæ. bore by rasping, effected by means of siliceous particles contained in the anterior part of the mantle. Mr. Couch entertains a similar view. We have not been able to satisfy ourselves of the presence of such particles, though inclined to regard such a view with favour, as in this case the surface of the shell does not seem devised for rasping, as is that of the shells of the Pholadidæ''[11]
Family Pholadidæ.
(Borers.)
The members of this family have a very close affinity with those of the last, both in structure and habit, and have indeed been often united with them by zoologists. They possess peculiarities, however, which make their separation more natural. The genera differ among themselves more in outward appearance than in organization; they have the body usually lengthened in various degrees, and produced behind into very long siphons, which are united through their length. The mantle is closed except in front, where the foot protrudes; this organ is club-shaped, and has the extremity obliquely flattened, so as to form a broad disk; it is sometimes largely developed, at others minute. The valves are equal, but very one-sided, always gaping at both extremities; the hinge is without teeth, and there is no proper ligament; each valve is furnished with a curved calcareous process beneath its summit, and at the back of the shell there are generally two accessory plates resembling valves.
The mollusks of this group are as indomitable borers as the Gastrochænadæ, but they less exclusively confine themselves to hard substances. Indurated mud, clay, and wood are perforated by these animals as well as stone, and their boring habits render them objects of painful anxiety to those who are interested in submarine works. The ship-worm (Teredo), whose terrible ravages have been already alluded to (see ante, p.' 41), is a member of this family, though the great elongation of its body, the minuteness of its valves, and the shelly tube with which it lines its burrow, long caused it to be associated with the Worms (Annelida), rather than with the Mollusca.
The fullest and most carefully prepared account of the natural history of this animal was communicated by Mr. W. Thompson to the Edinburgh New Philosophical Journal, from which paper the following interesting details of one of our British species (of which we have unfortunately no fewer than six) are extracted: —
"The greatest diameter of the testaceous tube or case, at the larger end, is seven-eighths of an inch; at the smaller, it varies from one and a half to two lines. All of the specimens have from one and a half to two inches and upwards of the smaller end of the tube greatly contracted within by laminæ, also the partition producing the double aperture extending but a few lines from the very extremity. The greatest thickness of the shell is at the smaller end, where, at the commencement of the laminæ, its consistence is from one-twentieth to one-fortieth part of an inch: from this it becomes gradually thinner towards the greater end, which in the very largest specimens is found to be closed up, but in several others there is no deposition whatever of testaceous matter for some distance from the termination of the cell. In one perforation, about twenty inches long, the body of the animal has had no testaceous covering for the last three and a half inches; in two other cells, of about two feet, no deposition appears for four and a half, and four inches and three quarters from their termination. All the timber at Portpatrick in which the Teredo had formed its habitation, is pine, and perhaps to this circumstance the superior size of the animal may chiefly be attributed. Though it is well known that the Teredo bores in the direction of the grain, it may be observed that it does so whether the position of the wood be perpendicular or otherwise. Captan Fayrer remarked, that it has a decided disposition to work horizontally. It is, however, often obliged to deviate from a straightforward course, to avoid such obstructions as nails, timber-knots, and tubes of its fellows, and make a winding or angular habitation according as such impediments occur; but these circumstances seem not eventually to impede the progress of the animal, as some of the very largest specimens I have examined are the most tortuous. During the nine or ten years that the Teredo has been established at Portpatrick, it has not degenerated; as specimens just received, which were alive in their native element a few days ago, are of equal size to those sent from the same place live years since, showing that it has not been affected by the cold of the winter season, as we might reasonably expect were the animals truly exotic. If this animal had originally been introduced, and has been preserved only by occasional importations, should we not rather look for it in those parts of the United Kingdom where vessels from every quarter of the globe are congregated, than in the obscure harbour of Portpatrick, which has never been visited by a foreign craft? "[12]
Genus Pholas.
The shells of this genus are of delicate and fragile texture, and of a pure white hue, when undefiled by extraneous substances. The valves are much developed behind, gaping at both ends, but especially in front. Their exterior surface is rough, with transverse scaly ridges and furrows. A spoon-shaped process springs from beneath the beaks in each valve, directed forwards. The beaks are covered by a callosity, and there are accessory plates or valves at the back of the shell.
The animal is thick and club-shaped, with long siphonal tubes united externally into one. The foot generally is large, thick, oblique, and flat at its extremity. The burrows in this genus are never lined with a shelly coat.
Frequent mention has been made in the preceding pages of the siphonal tubes, with which many genera of Conchifera are furnished; and it has been explained that, whether apparently united in one or obviously distinct, these tubes are the external organs accessory to respiration. Even when apparently united, as in the present genus, a glance at the very tip shows that there are two openings, one of which is a little smaller than the other, and commonly this subordinate orifice diverges at a slight angle from the principal one.
The latter is the entrance, the former the exit for the water, a perpetual change of which is absolutely indispensable to the life of the animal. The interior of these tubes is said to be lined with innumerable delicate cilia; by the action of which the surrounding water is drawn towards the entering orifice, and conveyed in a strong current through the tube over the surface of the gills. Then, having been deprived of its oxygen, it is poured through the other tube and expelled in a jet at its extremity, by a similar machinery.
This apparatus of double siphonal tubes is principally developed in those species which burrow, whether in sand, mud, wood or stone. As the burrowing bivalve usually, if not always, dwells in the interior of the passage it has excavated, it is needful that there should be a communication with the external water, and hence a hole is always found extending to the surface of the material bored. The entering and departing currents keep this passage clear, a process which in mud or sand might seem at first not very easy of accomplishment. It is facilitated, however, by the faculty which the boring bivalves have of lengthening the siphonal tubes at will; and the degree to which this may be accomplished depends on the depth of the cavity which the species is accustomed to make.
If we take one of the stone-boring Mollusca, a Pholas or a Saxicava for example, from its excavation, without injuring the animal, and place it in a glass vessel of sea-water, it will not be difficult to detect the currents in question, even with the naked eye; though a lens of moderate power will render them more distinctly appreciable. The vessel should be so placed as that the light may be nearly, but not exactly, opposite to the eye. By this arrangement the minute atoms of floating matter are illuminated while the back-ground is dark, and these by their motion clearly reveal the currents of the fluid in which they are suspended. A few moments of practice will enable even an unaccustomed eye to perceive the atoms converging from all points around, with an even but increasing velocity, towards the principal tube, down which they disappear like the streams of passengers and traffic in the neighbourhood of a great city, converging towards it as to a common centre of attraction by a hundred different routes. The current of the expelling tube is even still more marked in its character; a forcible jet of water is periodically ejected from this orifice, which draws the surrounding particles into its vortex, and shoots them forward to a distance of many inches. It is by the expulsive force of this anal current, chiefly, that the passage is kept free from the deposit of mud and other substances, which would otherwise soon choke it up.
A fresh, supply of water for respiration, and its dismissal when no longer fit for use, are efficiently provided for by this contrivance. But since many particles of matter float in the water, which from their form or other qualities might be hurtful to the delicate tissues of the viscera to be traversed, how is the entrance of these to be guarded against in an indiscriminating current? A beautiful contrivance is provided for this necessity. The margin of the entering siphon, and sometimes, though more rarely, of the ejecting one, is set round with a number of short tentacular processes, varying indeed in their length, but the longest scarcely more than equalling half the diameter of the mouth of the tube. In Saxicava rugosa, which bores through and through, with small holes, the hardest limestone of our coast, these tentacular appendages are found fringing both the tubes. The tentacles in this species are simple, and appear as if cut off transversely; and some are not more than half as long as the others, with which they irregularly alternate. The object of this diversity in length will be manifested presently. In Pholas parva, the processes are few and short, and are confined to the receiving tube, from the interior margin of which they project, towards the centre. But it is in Pholas dactylus, a noble species of large size, that excavates the softer rocks on our shores, that this apparatus is developed with peculiar beauty, and its use is made most clearly manifest. The tentacular filaments are in this case also confined to the oral tube. They are numerous, each forming a little tree, with pinnate branches, bearing no small resemblance to the flower of feathery branchiæ, that expands around the mouth of a Holothuria. These branched tentacula are ordinarily bent down across the mouth of the tube, the longest of them just meeting in the centre; alternating with these are placed others of similar structure, but inferior size: and the interspaces are occupied by others smaller still, and simply pinnate; so that when the whole occupy their ordinary transverse position, the small ones fill up the angles of the larger, and the branches of all form a net-work of exquisite tracery, spread across the orifice, through the interstices or meshes of which the current of entering water freely percolates, while they exclude all except the most minute floating atoms of extraneous matter.
The mode in which the boring Mollusca execute their perforations is yet a matter of uncertainty. The principal hypotheses that have been put forth on the subject have been already briefly enumerated, but it may not be amiss to add an expression of opinion on their value, and a careful summing up of the evidence, by zoologists eminently worthy of deference, the able authors of the "British Mollusca."
PHOLAS."Of all these theories," observe Messrs. Forbes and Hanley, "the chemical one, so far as a secreted solvent is concerned, bears least examination in the case of the Pholadidæ. The substances perforated are wood, limestones, hard and soft, argillaceous shales, clays, sandstone, and (in the case of a Pholas, in the magnificent collection of Mr. Cuming) wax. The notion of a secreted solvent, that would act indifferently on all these substances is, at present at least, purely hypothetical; and, since all attempted tests have failed to detect an acid, gratuitously so; for we can hardly suppose that any of those who have taken this view of the cause would maintain, that the animals have the power of secreting different acids at will, according to the substance they have to attack. Yet this notion has been most favoured by naturalists, who, sceptical as to the perforating power of such fragile instruments as are the shells of many of these creatures, endowed the animals with supernatural chemical qualifications. Even good experimental observers—Mr. Osler for one—whilst they proved that the Pholas could bore mechanically by the rotation of its valves, could not free their minds from the prejudice in favour of a solvent. The important statement put forward by Mr. Albany Hancock, respecting the instruments by which Mollusca bore, and which, so far as Gasteropoda are concerned, appear to furnish us with a true explanation,—namely, that it was effected by means of silicious particles, variously arranged in certain portions of the animal's body,—led us to hope that a better cause than any yet alleged had been discovered. But we cannot bear it out with respect to the Pholadidæ. We can find no such particles in the mantle of the Teredo, nor have any been noticed by Home or Deshayes, or by the most recent observers, Fray and Leuckart, who paid especial attention to the structure of the tissues in this genus. Nor could we, although aided by the anatomical and microscopical skill of Mr. Busk, detect any silicious particles in either the mantle, foot, or siphon tube of Pholas candida. If present in any species, therefore, they are exceptional, so far as the genus Pholas and its allies are concerned. The shells of several British species of Pholas, and that of Pholadidea have been chemically examined by our friend, Mr. Trenham Reeks, with a negative result as regards the presence of particles of silex in their substance, where, after the statement of Mr. Hancock respecting the structure of the mantle, we thought they might possibly be found. On the other hand, taking into consideration its mineralogical nature, as stated by M. Necker, there is no reason for supposing that the shell of the Pholadidæ is so weak a perforating instrument as some have fancied. With its peculiar form, and the saw-like asperities of its surface, especially of its anteal extremity, it is well adapted for an auger, when wielded fresh and elastic by its well-muscled animal inhabitant, whose foot, in all the members of this tribe, even in Teredo, where it is least developed, seems specially organized to serve as a fulcrum. We have no evidence that they perforate any substances essentially harder than their shells, or so hard. The sandstones in which they occasionally occur are either friable or marly, when fresh, though cabinet specimens seem so solid. The explanation of Necker accounts for their perforations in the hardest limestones. Wood, wax, and other substances in which they occur, offer no difficulty. The statements put forward respecting their boring in lava and granite, have long ago been shown to be mistakes. That they exhibit a rotatory motion, during the action of boring, has been proved by competent observers; and the cavities they excavate, if examined when fresh, invariably show transverse groovings, which could have been caused only by such motions. Currents of water, set in motion by cilia, doubtless aid materially the animal's operations, and possibly may be the means by which the larvæ effect their first lodgment; but, considering the arrangement of the parts of the body in the adult animal, it seems to us that Mr. Garner's view of their being the primary cause of the perforation, whilst the rasping of the valves is secondary, should be reversed. Such currents must be most effective in clearing away loosened and loosening particles. If there be any chemical action aiding, it must be due to the carbonic acid set free during the respiratory process. Evidence of a secreted solvent there is none."
To the same authority I am indebted for another interesting fact in the history of this genus.
"A remarkable property of the animals of this genus, and one which has long attracted notice, is their phosphorescence when placed in the dark. This phenomenon is exhibited by some other acephalous mollusks, and by the compound tunicated genus Pyrosoma. The light is of a bluish-white hue, and is regarded by Mayer to proceed from a luminous mucus, like that given off by the Medusæ. This mucus is thrown off into the surrounding water, so that the currents proceeding from the animal are luminous. Dr. Coldstream states, that the light is given out most strongly by the internal surfaces of the respiratory tubes, and that it is strongest in summer. Professor John Müller has observed, that when Pholades are placed in a vacuum, the light disappears, but reappears on the admission of air; also that, when dried, they recover their luminous property on being rubbed or moistened."[13]
- ↑ Forbes and Hanley.
- ↑ Brit. Mollusca, ii. 174, et seq.
- ↑ Ann. Nat. Hist. vi.
- ↑ Forbes and Hanley; Brit. Moll. ii. 149; et seq.
- ↑ Zool. Journ. ii. 359.
- ↑ Montagu, Test. Brit. 47.
- ↑ Forbes and Hanley, i. 248.
- ↑ Forbes and Hanley, i. 244.
- ↑ Ibid. i. 245.
- ↑ Cited in Br. Moll. i. 177.
- ↑ Brit. Moll. i. 138.
- ↑ Edin. New Phil. Journ. 1835.
- ↑ Brit. Moll. i. 104, et seq.