CHAPTER XI.

ACEPHALOUS MOLLUSCA.

"Sigiliatim mortales, cunctum perpetul."

APULEIUS.

The Mollusca proper were divided by Cuvier into five great classes:--

I. Lamellibranchiata, or Acephalous Mollusca, often called Conchifera. II. Brachiopoda. III. Gasteropoda. IV. Pteropoda. V. Cephalopoda.

The name Mollusca indicates the characters which most struck the ancients: they are soft--in Latin, _mollis_: their flesh is cold, humid, and viscous. In consequence of their very softness, they are generally furnished with an apparatus of defence or protection, in the shape of a calcareous cuirass, called a _shell_. According to the species this test is a coat of mail, a buckler, or a tower. The mollusc is thus armed and defended against all attacks from without, nearly after the manner of a knight of the middle ages; only the knight was not quite shut up in his armour, while the mollusc is attached to it by indissoluble organic bonds. "Such a life and such a habitation!" says Michelet. "In no other creature is there the same identity between the inhabitant and the nest. Drawn from its own substance, the edifice is the continuation of its fleshy mantle. It follows its form and tints. The architect has communicated its own substance to the edifice."

The shell of the Mollusca has been variously appreciated by naturalists. "We might regard the shell as the bone of the animal which occupies it," says a celebrated French naturalist; and then gives expression to a very different view. "We may say as a general thesis that testaceous molluscs are animals with whom ossification is thrown out on the external surface in place of the interior, as in the Mammals, birds, reptiles, and fishes. In the case of the superior animals the bones lie in the depths of the body; in the shelled Mollusca the bones are placed on the superficies. It is the same system reversed."

Other zoologists reject as altogether untenable this assimilative theory. "The shell which serves as a dwelling and a shelter cannot," say these authors, "be considered as a skeleton, because it does not assume the external form of the animal; because it does not attach itself to the organs of locomotion; and, finally, because it is the product of secretion, which increases in proportion to the development of the body itself." This last opinion appears to us to be the most acceptable.

However that may be, from the immense variety of form and size, from the beauty and brilliancy of their colours, the shells of the molluscs are among the most attractive objects of natural history. Nor is it from their beauty alone that a fine collection of shells becomes interesting: a living creature has inhabited the shell, a creature which in its organization and its life, above all, by its habits, excites in a high degree our interest, curiosity, and admiration. It has been said that the shell "is like a medal struck by the hand of Nature to commemorate climates." In short, the waters of different regions of the globe, whether fresh or salt, are characterized by the presence of particular shells; moreover, the comparison of living shells with those which lie in a fossilized state buried in the depths of the soil is a most important element of our knowledge touching the origin of the different beds out of which our globe is constituted.

Thus, we must not shut our eyes to these beings, in appearance so miserable and obscure, if we would possess a general knowledge of the animal kingdom. The Creator has endowed them with many wonderful gifts to embellish their lives, and who would dare to disregard them? Who could examine and compare their structure without being charmed with the study? Man, who descends into the depths of the earth in search of the precious metals--who dives into the deep in pursuit of the treasures it conceals--who stoops his head over works of art--would surely not refuse to bend himself for a moment to the sand of the sea, to gather in his hand, to bring nearer to his eyes, these marvellous works of the Divine Creator!

The true molluscs are divisible into two great classes: the Acephalous, or Headless, and those having a head of structure more or less perfect, which are called Cephalous Molluscs.

The Acephalous or Headless Molluscs are so called from the Greek ἀ, privative, and κεϕαλὴ, head. They have no head; the body is surrounded by the folds of the skin; the shell consists of two valves. Such is a summary description of all the Acephalous Molluscs. They are sometimes naked, and sometimes enclosed in a shell, whence they are known as Testaceous Molluscs. They are called _bivalves_, because their shell consists of two halves, or valves united by a hinge. They are sheltered in this double carapace as a book is in its cover.

Although they have no head, they can feed themselves, and they reproduce their kind. They have friendships and enmities, perhaps even passions; probably these are not very lively, for most of them scarcely ever change their place, even to make the least movement. Many of them remain fixed to the rock on which they were hatched, and tumultuous sensations are not quite compatible with immobility.

The bivalves[8] are found in every sea. The shell of the bivalve is ovoid, globulous, trigonal, heart-shaped, elongated like a pea-pod, or flat like the leaves of a tree, having an opening down the ventral side. In some one valve is flat, the other round and swelling in the centre. The shell is thus an outer envelope, consisting of two pieces, more or less corresponding to each other in size and shape (of which the oyster is an example), formed of carbonate of lime deposited in membranous cells in its outer layers, the inner layers being composed of thin coatings of lime deposited in the outer surface of the tissue, called the mantle leaves. The valves are united to the animal by the insertion of certain muscles, and by the horny epidermis of the mantle, which stretches over the edge of the valves. The hinge and ligament which unite the two valves consist of a dense elastic substance, somewhat resembling india-rubber; the hinge is formed of teeth, and cavities into which the teeth fit. The ligament acts in opposition to certain contractile muscles within, which draw them together, and is placed either within or without the hinge, or partly both. On separating the valves, the two leaves of the mantle present themselves. These are thin delicate leaves, furnished at the margin with sensitive tentacles and other organs of sense, and with glands sometimes highly coloured. The use of these organs is thus described by Mr. Rymer Jones:--

"When the animal is engaged in increasing the dimensions of its abode, the margin of the mantle is protruded and firmly adherent all round to the circumference of the valve with which it corresponds. Thus circumstanced, it secretes calcareous matter and deposits it upon the extreme edge of the shell, when the secretion hardens and becomes converted into a layer of solid testaceous substance. At intervals this process is repeated, and every newly-formed layer enlarges the diameter of the valve. The concentric strata thus deposited remain distinguishable externally, and thus the lines of growth marking the progressive increase of size may easily be traced."

"While the margin of the mantle is thus the sole agent in enlarging the circumference of the shell," the professor continues farther on, "its growth in thickness is accomplished by a secretion of a kind of calcareous varnish derived from the external surface of the mantle generally, which, being deposited layer by layer over the whole interior of the previously existing shell, progressively adds to its weight and solidity. There is, however, a remarkable difference in character between the material secreted by the marginal fringe and that furnished by the general surface of the mantle membrane. The former we have found more or less covered by glands appointed for the purpose, situated in the circumference of the mantle; but as these glands do not exist elsewhere, no colouring matter is ever mixed with the layers that increase the thickness of the shell, so that the latter always remain of a delicate whitish hue, and form the well known iridescent material usually distinguished by the name of nacre or mother-of-pearl." (_General Outline_, p. 385.)

The process by which shells attain their beautiful markings is thus described by Mr. Jones:--"The external surface is exclusively deposited by the margin of the mantle, which contains in its substance certain coloured spots, which are found to be of a glandular character, and to owe their peculiar character to a pigment they secrete, which is mixed with the calcareous matter; coloured lines are therefore found on the exterior of the shell wherever these glandular organs exist. Where the deposition of colour is kept up throughout the process of enlargement, the lines are unbroken and perfect; but where the coloured matter is furnished only at intervals, spots and patches of irregular form and increasing in size with the enlargement of the mantle are the consequence."

Bivalves move about and change from place to place by means of an extensible fleshy organ called, from some of its functions, a foot; in fact, it has less resemblance to a foot than to a large tongue. It is a muscular mass, capable of being pushed out from between the mantle and the valves, and varies much in form; it is in turn a hatchet, a ventilator, a pole, an awl, a finger, and a sort of whip. This foot is simple, forked, or fringed. In some species the tissues are spongy, and capable of receiving considerable quantities of water. When the organ swells, it is elongated and stiff; on the other hand, by suddenly expelling all the water, it gets small and pliable, and can now return to its shell. This organ is represented in Fig. 128 (_Donax trunculus_, Linn.), in which it is singularly developed. This bivalve is found on the sea shore in shallow water; it buries itself almost perpendicularly in the sands. They are so abundant on the French side of the Channel and on the shores of the Mediterranean, that they form a considerable portion of the people's food. These bivalves have the singular power of leaping to a considerable height and then throwing themselves to a distance of ten or twelve inches--a spectacle which may be witnessed any day at low water. When abandoned by the retreating tide, they try to regain the sea. If seized by the hand, in order to drag them out of the sand, aided by their compressed, branched, and angular feet, they give to their shell the sudden and energetic movement under which the bounding action takes place.

The shell of the Donax is slightly triangular and compressed; its length exceeds its height; it is regular, univalve, unequally lateral, and its hinge bears three or four teeth on each valve. The action of these feet is very simple, and is compared by Réaumur to that of a man placed on his belly, who, stretching out one hand, seizes upon some fixed object, and draws himself towards it. There is just this difference, that the movement of the member in the mollusc is altogether contractile.

Authors have described more than 30,000 species of molluscs, so that our space only permits us to describe a few families, or rather types of families.

The arrangement of bivalves now most generally adopted in England is that of Woodward, as developed in the last edition of his manual of the mollusca; it is greatly based on that of Lamarck. We have adopted his arrangement altered from a descending to an ascending scale of organisation.

The Conchifera are divisible into two sections, Siphonida, from the animals having respiratory siphons, and Asiphonida, destitute of them.

The solen may be taken as a type of the first, and the oyster of the second. The division Siphonida is divided into two sub-sections, those without and those with a pallial line sinuated. The first family of this section is the Pholadidæ, which includes Teredo, Xylophaga, and Pholas, animals which possess extraordinary powers of boring; not merely as the Solens do, through sand, but through the hardest rocks.

The _Teredos_ are marine animals having a special and irresistible inclination for submerged wood; for while wood exposed to the air becomes a prey to terrestrial animals, so submerged wood is subject to invasion by aquatic animals, of which the Teredo is by far the most formidable. The Teredos in the bosom of the ocean perforate the hardest timbers, whatever be their essence. The galleries bored by these imperceptible miners riddle the whole interior of a piece of wood, destroying it entirely, without the slightest external indication of its ravages. The galleries sometimes follow the grain of the wood; sometimes they cut it at right angles; the miners, in fact, change their route the moment they meet in their way either the furrows hollowed out by one of their congeners, or some ancient and abandoned gallery. By a strange kind of instinct, however multiplied may be their furrows or tubes in the same piece of wood, they never mingle--there is never any communication between them. The wood is thus attacked at a thousand diverse points, until it is invaded and its entire substance destroyed. It is by secret ravages of this kind that the piles and other submarine constructions upon which bridges are built are often riddled and perforated. They appear to all outward examination as solid and perfect as at the moment they were first driven; but they yield to the least effort, bringing ruin and destruction on the edifices they support. Ships have been thus silently and secretly mined, until the planks crumbled into dust under the feet of the sailors. Others have gone down with their crews, entirely caused by the ravages of these relentless enemies, which are terrible from their unapproachable littleness.

M. Quatrefages, who has minutely studied the organization and habits of the Teredos in the Port of Saint Sebastian, reports the following fact, which will give the reader some idea of the rapidity with which these dangerous molluscs pursue their ravages:--

"A boat, which served as a passage-boat between two villages on the coast, went down in consequence of an accident at the commencement of spring. Four months after some fishermen, hoping to turn her materials to advantage, raised the boat. But in that short space of time the Teredos had committed such ravages that the planks and timbers were riddled and worm-eaten so as to be totally useless."

At the beginning of the eighteenth century, half the coast of Holland was threatened with annihilation because the piles which support its dikes and sea-walls were attacked by the Teredo; and it proved no contemptible foe. Many hundreds of thousands of pounds were expended in order to avert the threatened danger. Fortunately, a closer attention to the habits of the mollusc has brought a remedy to a most formidable evil; the mollusc has an inveterate antipathy to rust, and timber impregnated by the oxide of iron is safe from its ravages. This taste of the Teredo being known, it is only necessary, in order to scatter this dangerous host, to sink the timber which is to be submerged in a tank of prepared oxide of iron--clothed, in short, in a thick cuirass of that antipathy of the Teredo, iron rust. Ships' timbers are also served with the same protecting coating; but the copper in which ships' bottoms are usually sheathed serves the same purpose.

The singular Acephalous Mollusc known to naturalists as the _Teredo navalis_, and popularly as the Ship Worm (Fig. 129), has the appearance of a long worm without articulations. Between the valves of a little shell, with which it is provided anteriorly, may be seen a sort of smooth truncature, which surrounds a swelling projecting pad or cushion. This cushion is the only part of the body of the animal which can be regarded as a foot. Starting from this point, all the body of the Teredo is enveloped by the shell and mantle, which form a sort of sheath communicating by two syphons with the exterior.

The mantle adheres to the circumference of the shell. Above, it forms two great folds, which may both be swollen by the afflux of the blood, and acquire considerable size. One of these folds placed in advance, which is called the _cephalic hood_, is worthy of attention. The tissue of the mantle is of a greyish tint, very light, and transparent enough, especially in the young, to permit of the mass of liver, the ovary, the branchiæ, and the heart being distinguished in the interior, even to counting its pulsations. The syphons are extensible, and attached the one to the other for about two-thirds of their length, the upper part being longer and thinner than the lower. It is by these tubes that the aërated water enters which feeds and enables the animal to breathe. It is discharged by the second tube, when deprived of its oxygen, and no longer respirable, carrying with it the useless products of digestion. This movement is continuous; but from time to time the animal shuts at once the orifices of both tubes, and slightly contracts itself.

The shell, seen on the side, presents an irregularly triangular form; it is nearly as broad as it is long; its two valves are solidly attached the one to the other, above and below, by the mantle in such a manner as only to permit of very slight movements. It is coloured in yellow and brown lines; sometimes it is quite plain. On the upper edge of the anterior truncature of the body of the animal is the mouth, a sort of funnel, flat and slightly bell-shaped, furnished with four labial palpi, a stomach without any peculiar feature, and a well-developed intestine.

The heart consists of two auricles and a ventricle, which beat at very irregular intervals, four or five in the minute. The blood is colourless, transparent, and charged with small irregular corpuscles. The act of breathing is accomplished in the branchiæ, or gills, and mantle. Nevertheless, the one half of the blood returns to the heart without passing through these branchiæ.

The nervous system is well developed, and consists of a brain, nervous filaments, and of ganglions, which are distributed in the mantle, the branchiæ, and the syphon tubes.

The adult animal is surrounded by a sort of sheath, consisting of a solid mucus, which has sometimes been described erroneously as forming part of the animal. The Teredo, shut up in this tube, is limited in its movements; when observed in a vase, its motions are slow and deliberate--movements of extension and contraction, by the aid of which it contrives with difficulty to change its place; but nothing indicates a true creeping movement. In a state of nature, according to M. Quatrefages, the body of the animal is stretched out to three times its length without diminishing in any respect its proportional thickness; the afflux of water penetrating under the mantle, and of the blood which accumulates in the interior vessels, sufficiently accounting for a phenomenon which at the first glance is very singular.

The Teredo deposits a spherical greenish-yellow egg. Shortly after fecundation, these eggs are transformed into larvæ. At first naked and motionless, these larvæ are soon covered with vibratile cilia, when they begin to move, at first by a revolving pirouette, afterwards swimming about freely in the water. When one of these larvæ has found a piece of submerged wood, without which it probably could not live, the curious spectacle is observed of a being which fabricates, step by step, and as it requires them, the organs necessary for the performance of its functions. It begins by creeping along the surface of the wood by means of the very long feet with which it is furnished. Then it is observed from time to time to open and shut the valves of the little embryo shell which partly envelopes it. As soon as it has found a part of the wood sufficiently soft and porous for its purpose, it pauses, attacks the ligneous substance, and soon produces a little pore, or cell, which will be the entrance to the future canal.

Once fairly lodged in this little cell, the young Teredo is rapidly developed; it covers itself with a coating of mucous matter, which, condensing by degrees, assumes a brownish tint, forming a solid covering, with two small holes for the passage of the syphon tubes. At the end of three days this covering has become quite solid; it is the commencement of the organized tube, in which the animal is to be developed. When secured beneath this opaque screen, the little miner is no longer exposed to observation; but if his cell is opened at the end of a few days, it is found that it has secreted a new shell, larger and more solid than the original one; it is the shell of the adult animal.

The young Teredo, which feeds on the raspings of the wood, increases rapidly; it passes first from a spheroid form to an elongated shape, and when its body can no longer be contained in the shell, it projects beyond the edge, and would find itself naked were it not protected by its membranous sheath, which adheres to the walls of the ligneous channel, now the dwelling-place of the animal.

The process by which a creature soft and naked like the Teredo should break into a solid piece of the hardest wood so quickly, and destroy it with so much facility, was long a mystery. Until very recently, the shell was looked on as the implement of perforation. But in that case the shell should preserve certain traces of its action upon surfaces so resistant as oak and fir; but the shell, on the contrary, is perfect, with no signs of friction. On the other hand, the muscular apparatus of the Teredo is not calculated to put the shell into rotatory action, were the process a boring one. It does not seem therefore possible to attribute these perforations to a simple physical action.

Some naturalists have suggested, in explanation of this phenomenon, that the animal is furnished with the means of secreting a liquid capable of dissolving the woody fibre. This has been met by the statement that, in whatever way the wood is attacked, whether the gallery is excavated with or across the fibre of the wood, the groove is as exactly and neatly cut as if it had been perforated by the sharpest tool, and that a corroding dissolvent could not act with this regularity, but would attack the harder and more tender parts unequally. This objection, which M. Quatrefages opposes to the idea of a chemical solvent, appears to us to admit of no reply. But, while opposing unassailable reasons against two theories, the learned author does not leave us without a very reasonable explanation of a very puzzling phenomenon. "Let us not forget," he says, "that the interior of the gallery is constantly saturated with water; consequently all the points of the walls which are not protected by the tube are subjected to constant maceration. In this state a mechanical action, even very inconsiderable, would suffice to clear away the bed of fibre thus softened, and, if this action is in any degree continuous, it suffices to explain the excavation of the galleries, however extensive their ramifications. Again, the upper cutaneous folds, especially the cephalic hood already mentioned, having the power of expanding at will by an afflux of blood, covered with a thick coriaceous epidermis, and moved by four strong muscles, seems to me very capable of performing the operation. It appears very probable that it is this hood which is charged with the removal of the woody fibre, rendering it incapable of resistance by previous maceration, which may also be assisted by some secretion from the animal." That the fleshy parts of the mollusc, acting upon the surface, softened by long maceration in water, is the true boring implement employed by the Teredo, is, probably, the only explanation the case admits of; at all events, in the present state of our knowledge, the explanation of the learned naturalist is the most reasonable which can be given.

The engraving (Fig. 130) represents _P. dactylus_, which has hollowed itself a home out of a block of gneiss. This dwelling is a cell just deep enough to contain the animal and its shell, as represented in Fig. 131. To excavate its cell at the bottom of one of these gloomy retreats seems to be all that the animal lives for. To ascend to the summit or sink to the bottom of their narrow house makes up all the accidents of existence to these strange creatures: the hole they dig is at once their dwelling and their grave; which is attested both by the rocks of the past and the present.

In its structure the shell differs notably from other Acephalous Molluscs, which led Linnæus to place it with the multivalve shells. Between the two ordinary valves, in short, this shell presents certain accessary pieces, smaller than the true valves, and placed near the hinge, as represented in _Pholas dactylus_ (Fig. 131), pieces which would not be there without a purpose.

The shell is equivalve, gaping on each side, swelling below, very thin, transparent, and white. The animal is a thick, white, elongated, fleshy body; its mouth opening anteriorly, throws out a long tube traversed by two canals or syphons, through one of which the water necessary for the respiration of the animal is absorbed, and ejected through the other. Through another opening in the mantle a very thick and short foot is protruded.

There are three ways of accounting for this creature's method of boring--the mechanical, the chemical, and the electric; the first being the one generally held. In this case the animal uses its foot as a boring tool. The second presumes on the Pholas secreting an acid which corrodes the rock; the third that it possesses a galvanic battery with similar powers. It is possible that all these three theories may have a measure of truth. That the foot of the borer is used is clear. The luminosity which is so characteristic of the animal is in favour of an electric current, which is almost always accompanied by chemical decomposition, which would set free the hydrochloric acid of the sea water. The small size of the entrance to the chambers of the Pholas is accounted for by the increase of its size during its residence there. De Blainville thought that a simple movement of the shell incessantly repeated would suffice to pierce the stone, macerated by the sea water which passed through the breathing apparatus.

Mr. Robertson, of Brighton, exhibited the living Pholas in the act of boring through masses of chalk, and thinks the process entirely effected by the simple mechanical action of the "hydraulic apparatus, rasp, and syringe."

"If you examine these living shells," says Gosse, "you will see that the fore part, where the foot protrudes, is set with stony points arranged in transverse and longitudinal rows, the former being the result of elevated ridges, radiating from the hinge, the latter that of the edges of successive growths of the shell. These points have the most accurate resemblance to those set on a steel rasp in a blacksmith's shop. It is interesting to know that the shell is preserved from being itself prematurely worn away by the fact that it is composed of aragonite, a substance much harder than those rocks in which the Pholas burrows. The animal," Gosse adds, "turns in its burrow from side to side when at work, adhering to the interior by the foot, and therefore only partially rotating to and fro. The substance is abraded in the form of a fine powder, which is gradually ejected from the mouth of the hole by contraction of the bronchial syphon."

The Pholades are met with on every sea shore, and are plentiful in the Channel; on the French coast they are called _Dails_, and sought for their fine flavour. As examples of the genus, we may quote _Pholas dadylus_ (Fig. 131); _Pholas candida_, found in the Channel and in the Atlantic Ocean, which lives buried in the mud or in decayed wood; _Pholas crispata_ (Fig. 132), also found in the Channel; _Pholas papyracea_ (Fig. 133); and _Pholas melanoura_ (Fig. 134).

The bodies of many genera of Mollusca have the property of shining in the dark, but none emit a light more brilliant than that of the Pholades. Those who eat the Pholades in an uncooked state (which is by no means rare, for the flavour of the mollusc does not require the aid of cooking to render it palatable) would appear in the dark as if they had swallowed phosphorus; and the fisherman who, in a spirit of economy, supped on this mollusc in the dark, would give to his little ones the spectacle of a fire-eater on a small scale.

The perforations produced in stone by the Pholades have become important evidence in a geological sense. In many countries there were evident signs of a considerable sinking of the earth. But in no place is the evidence of this so clear as in the monument of high antiquity on the Pozzuolan coast, known as the Temple of Serapis.

In speaking of the culture of oysters by the Romans, we shall have occasion to mention the disappearance of the Lucrin Lake, and its replacement by an enormous mountain, the Monte Nuovo. Now, Pozzuolo is situated at the foot of Monte Nuovo. We need not add that the whole neighbourhood is volcanic. Pozzuolo touches on the Solfaterra, on the Lake Avernus, and is not far from Vesuvius; and in the bay is the monument of other days, erroneously called the Temple of Serapis. In reality it was most probably a thermal establishment, established for its mineral waters, although the world has agreed to call it a temple.

However that may be, the building has been nearly levelled by the hand of time, aided by the hand of man; and the ruins now consist of three magnificent marble columns of about forty feet high. But the curious and important fact is, that these three columns, at about ten feet above the surface, are riddled with holes, and full of cavities bored deeply into the marble, and these borings occupy the space of three feet on each column. The cause of these perforations is no longer doubtful. In some of the cavities the shell of the operator is still found, and it seems settled among naturalists that it belongs to a species of Pholas, although M. Pouchet, a naturalist of Rouen, denies this. "As far," he says, "as I have been able to judge from the fragment which I extracted from this temple, which is destitute of the hinge, it is infinitely more probable that this mollusc is a species of the genus _Corallisphaga_." In spite, however, of M. Pouchet's scepticism, the mass of evidence is opposed to his theory.

There are two modes of explaining the fact to which we have called attention. To enable the stone-boring molluscs which live only in the sea to excavate this marble, the temple and columns must have been buried several fathoms deep in sea-water. It is only in these conditions that the borers could have made an incision, and laboured at their ease, in the marble column.

But since the same traces of perforation are now visible ten feet above the surface, it follows that, after being long immersed under water, the columns have been elevated to their present position. The temple has been restored to its primitive state, carrying with it, engraved in marble, ineffaceable proofs of its immersion. Sir Charles Lyell has consecrated a long chapter to the successive sinking and elevation of this temple, which proves the fact most conclusively.

* * * * *

Family two, the Gastrochænidæ, is a somewhat heterogeneous one, as it contains Saxicava and Aspergillum. We have only space for a short account of the latter, the animal which has received the strange name of the Watering Pot, and is represented in Fig. 135. It inhabits a calcareous tube, thick, solid, of considerable length, and nearly cylindrical, presenting at one extremity an opening fringed with one or many foliaceous folds in the form of frills, and at the other extremity a convex disk, pierced with holes like a watering-pot: whence its name. The animal is attached by certain muscles to the interior of the tube. Chenu, to whom we are indebted for our information respecting this curious mollusc, tells us "that the animal which inhabits this curious shell was first described by Russell, whose account of it is deficient in the anatomical details, which might explain the utility of the holes in the disk of the central fissure, and of the spiriform tubes found there." We suppose that this arrangement is necessary in order to facilitate respiration; and M. De Blainville thinks the small tubes are intended for the passage of the fillets which are necessary to fix the animal to the body on which it is to live, and in such a manner as to admit of its movements round a fixed point.

The animal which inhabits the _Aspergillum_ is elongated, contractile, and only occupies the upper part of the tube, but it can stretch itself out sufficiently for all its wants. Shells of this genus are very rare, although a great number of species are known. They are found in the Red Sea, and in the seas of Australia and Java. The shells are generally of a white or yellowish tint; some have the tube covered with a glutinated sand, mixed with small fragments of shells of diverse colours. We know nothing of their habits, and their singular forms have left naturalists in doubt as to the place which should be assigned to them in the method of arrangement. It is only after having recognized the existence of two valves, which was detected with great difficulty just under the disk, and forming part of the sheath in which the animal is encased, that it has been decided to range them with the _Tubicola_, and with the shells presenting an arrangement analogous or equally singular. These molluscs are, as M. Chenu says, little known, rare, and hence much sought for by collectors. They are exclusively exotic, the most common species being from Java. It is imported into Europe by the Dutch. Our third family, the Anatinidæ, includes Myochama, Pandora, Lyonsia, Myacites, Pholadomya, Thracia, and Anatina, genera which were more important in the former than in the present seas; some, in fact, being wholly extinct, or represented, as in Pholadomya, by one or two living species. Our fourth family, the Myacidæ, including Gycimeris, is found only in America; Panopaæ, now principally extinct; Thetis, Neæra, Corbula, and Mya, or Gaper.

Our fifth family, Solenidæ, contains the Solens, which under the name of "razor-fish" are so abundant on the sandy shores of all parts of the globe. These molluscs live buried vertically in the sand, a short distance from the shore; the hole which they have hollowed, and which they never quit, sometimes attains as much as two yards in depth; by means of their foot, which is large, conical, swollen in the middle, and pointed at its extremity, they raise themselves with great agility to the entrance of their hole. They bury themselves rapidly, and disappear on the slightest approach of danger.

When the sea retires, the presence of the Solen is indicated by a small orifice in the sand, whence escape at intervals bubbles of air. In order to attract them to the surface, the fishermen throw into the hole a pinch of salt; immediately the sand becomes stirred, and the animal presents itself just above the point of its shell. It must be seized at once, for it disappears again very quickly, and no renewed efforts will bring it to the surface a second time. Its retreat is commonly cut short by a knife being passed below it; for it burrows into the ground with such velocity that it is difficult to capture it with the hands alone.

This shell has by some been compared to a knife-handle; by others to a razor, which has become its popular name. It is a thin, transparent, long, and slender equivalved bivalve, with parallel edges, gaping and truncated at both extremities. The tints are rose-coloured, bluish-grey, and violet; the valves slightly covered with an epidermis of a greenish brown.

The animal which lives in this elegant dwelling has the form of an elongated cylinder. Its mantle is closed in its whole length, and only open at the ends at one side for the passage of the food, and at the other for the passage of a tube formed of two syphons united together. This curious shell, various species of which are represented in PL. XIX., are known as razor-fish, sabre-fish, and other names, which in some respects indicate the peculiar form of the shell, as well as its sharpness.

The Tellinidæ, the sixth family in our table, is very important, as including a vast number of genera and species, of which, as types, we will particularise Tellina and Donax; but Galatea, Mesodesma, Semele, Sanguinolaria, Psammobia, and Capsula, are important genera.

Along the shores of the Channel and in the Mediterranean there are few bivalves more abundant than the several species of the genus _Donax_. They live near the shore in shallow water, burying themselves perpendicularly in the sand. They have the very singular habit, considering their apparent helplessness, of being able to leap to a certain height and then project themselves ten or twelve inches. This may often be witnessed in the case of individuals left by the retreating tide. If seized by the hand, and attempts are made to disengage them from the sand, they continue to impress on their shell a sudden and energetic movement, aided by the elasticity of their foot, which is at once decisive and angular.

The shell of the _Donax_ is nearly triangular in shape, compressed, longer than it is high, regular, equivalve, not equilateral; the hinge with three or four teeth on each valve.

The animal is slightly compressed, and more or less triangular. Its mantle, which forms two symmetrical lobes enveloping the body, is open pretty nearly in all its extent, but it is united posteriorly, and terminates in two syphons or nearly equal tubes, as in Fig. 130, p. 326. One of these tubes serves the purpose of respiration: it is the _bronchial syphon_. The other, serving the purpose of ejecting the products of digestion, is termed the _anal_ tube. The tentacles of the bronchial tube seem to be possessed of exquisite sensibility. When touched, the animal draws in its syphon, and only puts it forth anew when the danger has passed. The species of _Donax_ are very numerous, especially in the Asiatic and American seas. Among the European species we may mention _Donax rugosus_ (Fig. 136) and _Donax denticulatus_ (Fig. 137).

Next to _Donax_ naturalists rank the genus _Tellina_, which includes many species of very minute shells, all remarkable for their beauty of form, and for their brilliant and varied colours. One of these, called the Rising Sun (_Tellina radiata_), is represented in Fig. 138. The Tellinas are found in every sea; the French coast furnishes many species: examples, _Tellina virgata_ (Fig. 139) and _Tellina sulphurea_ (Lamarck) (Fig. 140). In Fig. 141 _Tellina donacina_ is represented with its two vital tubes, or syphons.

The seventh family, or Mactridæ, include Lutraria and Mactra, or the otter and kneading-trough shells. They are widely distributed; there are several British species of both.

The eighth family, Veneridæ, includes Venus, Cytherea, Meroe, and Artemis; beautiful genera, and as such called by Linnæus and his followers after heroines of Greek mythology. Petricola, Venerupis, Tapes, Lucinopsis, and Trigona, also belong to the family. These acephalæ of size so small, like their congeners, inhabit every sea; they are found in every region of the globe, more than a hundred and fifty species being known. The shell is elliptic in form, the valves smooth, striated, spiny, and lamellous, like those of Cardium and Donax. Like these, they bury themselves in the sand.

Among the vast number of species, many of them are extremely rare, and much sought after by collectors in consequence of their great beauty. In the principal ports of France, _Venus verrucosa_ (Fig. 142), and another species known in the south of France under the name of Clovisse, are eaten there like oysters. Prepared with fine herbs, the Clovisse, we have M. Figuier's authority for saying, is not to be despised. "We may be believed also," he says, "if we add that nothing is more delicious than to eat the living Clovisse torn from the rock of the Phara of Lake Thau, when the Mediterranean sun of a day in winter is shining down upon us, the heart rejoicing in manhood's strength." In PL. XVIII. some of the principal species are represented, along with some of the more remarkable species of _Cytherea_. In Fig. 143 we have the elegantly pencilled shell of _Cytherea geographica_, together with the animal in its natural connection.

The sub-section we shall now treat of is without the pallial line sinuated. The Cyprinidæ form the ninth family of our arrangement of the Conchifera, and contain, Cardia, Cypricardia, Isocardia, Crassatella, Astarte, Circe, and Cyprina, which amount together to some hundred species.

The Cycladidæ are our tenth family, and include Cyrenoides, Cyrena, Pisidium, and Cyclas.

The Lucinidæ is the eleventh family, containing Galeomma, Lepton, Montacuta, Kelia, Diplodonta, Corbis, and Lucina.

In the small family of which we have made the Tridacna the representative, as well as in some preceding families, the mantle of the animal is more or less largely open, but never with such a prolongation as to form tubes. In the _Cardiums_, now under consideration, as well as _Donax_, _Tellina_, and _Venus_, the respiratory organs are somewhat modified, so as to adapt them to the habits of the animal. All these molluscs live buried in the mud or sand, and two great tubes issuing from the interior of their bodies bring the atmospheric air into communication with their respiratory organ--namely, the _branchial leaves_.

The twelfth family, Cardiadæ, contains the familiar cockles--Cardium--which is derived from καρδὶα, _a heart_, which they are supposed to resemble in form, are amongst the most widely-distributed of shells. The shell is convex, as we see in _C. hians_ (Fig. 144), somewhat heart-shaped, equivalved, the edges dentate or corrugated, the hinge furnished with four teeth upon each valve. The accessary ornaments vary with the species, some being smooth, as in _Cardium Greenlandicum_, Chemnitz (Fig. 145); others, and by far the greater number, are furnished with regular sides, generally obtuse, sometimes in ridges diverging from the point and armed with straight or curved spines, arranged in the oddest manner, as in _Cardium aculeatum_ (Fig. 146).

In _C. hians_ (Fig. 144), the mantle has a large opening in front, fringed anteriorly with papillæ in the form of tentacula; the inhabitant of the shell has a very large foot, with a bend or knee near the middle; its mouth is transverse and funnel-shaped, and furnished with a triangular appendage. One of the peculiarities in the organization of these molluscs is its direct connection with their mode of life. In short, these molluscs, which most commonly live on the sea shore, and bury themselves in the sand to the depth of four or five inches, are enabled to breathe, to draw water for their nourishment, and also to throw off the products of digestion, by having the mantle prolonged into two tubes, the orifices of which reach to the surface of the soil. By means of these feet and an extremely curious organ of locomotion, the Cardiums can at will issue from their holes and re-enter them. The fishermen of the shore easily recognize the presence of these animals by the little jets of water which they throw up through the sands.

These molluscs are found in every sea on the globe, and under all latitudes. Many of them belong to our own and the French coasts, where they are eagerly sought for by collectors, as well as for food. The flesh of the animal, however, is coriaceous, and little esteemed. The species most common on the littoral of the Atlantic is _Cardium edulis_ (Fig. 147), its white or fawn-coloured shell being hollowed out into six and twenty furrows, forming so many corrugated ripples on its side.

_Cardium costatum_ (Fig. 148) is an exotic species which inhabits the coast of Guinea and the Senegal, the shell of which, white and fragile, is much sought after by collectors.

The thirteenth family of our table, Tridacnidæ, consists of only eight or ten species, but it contains the largest of all, the giant Tridacna. The historian of the wars of Alexander the Great speaks of oysters inhabiting the Indian Ocean which were more than a foot long; these were probably _Tridacna_, the shells of which were most likely to be seen by the Macedonian conquerors. The valves of _Tridacna gigas_ are sometimes found a yard and a half in length, and weighing five hundred pounds. Magnificent examples may be seen in the church of Saint Sulpice, Paris, where they hold the holy water. These beautiful shells were the gift of the Venetian Republic to Francis I. Under Louis XIV., the curé Languet had them presented to the church of Saint Sulpice, where they are used as fonts for holy water. Another pair are exhibited in the church of Saint Eulala, at Montpelier, but much smaller in size. The shells of _Tridacna_ are formed, as represented in PL. XVII., of three acute angles, festooned on their edges by broad sides bristling with white scales. The hinges have two teeth; the ligament is elongated and external.

The animal of _Tridacna_ is remarkable for its fine colours. _Tridacna safrana_ is of a beautiful blue round the edges, rayed through a shade of very pale blue. More in the interior is a row of small moons of a yellowish green; the centre is a bright violet, with brownish longitudinal punctured lines. "We have at this moment before our eyes," say the travellers Quoy and Gaimard, "one of the most charming spectacles that can be seen, when at a little depth beneath the surface a number of these animals display the brilliant velvety colours and varying shades of their submarine parterres. As we can only perceive the gaping opening of the valves, we may imagine to ourselves what is its first aspect." The mantle of the animal is closed and ample; its edges are swollen, and reunited in nearly its whole circumference in such a manner as to leave only three very small openings--two in the upper part; the one serves the purpose of discharging the products of digestion, the other gives entrance and exit to the water necessary for respiratory purposes. The third opening is in the lower part of the body, and free; it leaves an opening for the passage of the foot, which is enormous, and is surrounded with an ample tuft of byssoidal fibres.

Aided by this silky tuft, the animal attaches itself to the rocks, and suspends its weighty shell from them. If it is intended to remove those attached to the sides of the rock, it is necessary to cut the cords of the tendonous byssus, by which it is held suspended, with a hatchet.

All the species are inhabitants of the Tropical seas. The _Tridacna_ _gigas_ is a native of the Indian Ocean. The flesh, though coriaceous, and by no means of an agreeable flavour, is a great resource to the poor Indians. The accompanying representations of _Tridacna squamosa_ (Figs. 149 and 150) will convey a general idea of the genus.

Our fourteenth family, Hippuritidæ, is entirely fossil; but the fifteenth, Chamidæ, of which the best example is the rugose genus Charina, is widely distributed in tropical seas.

The very numerous division of shells called Asiphonidæ, possesses animals without respiratory siphons. The shells we shall now describe belong to the sixteenth family, Unionidæ, which contain Iridina, Anodon, and Unio.

The pond mussels, _Anodon_, are found in lakes, rivers, and seas of almost every region of the globe. Their shells are rounded or oval, generally very thin, regular, and equivalve, not gaping, the hinges without teeth, whence their name, from the Greek, ὀδότοϛ, _without teeth_. These shells are nacred inside, and generally smooth.

The _Anodon cygnea_ (Fig. III., PL. XVI.) is broad, deep, and light, and is sometimes employed for skimming the cream off milk. The genus is divided into many groups, the principal forms of which are represented in PL. XVI.

The river mussels, _Unio_, are, like the Anodon, found in the muddy bottoms of all countries. The animal resembles the Anodon, but the shell presents a toothed hinge. The lower face of the valve is nacrous, but shaded with purplish violet, copreous, and iridescent; the anterior face is of a green colour, which varies from tender to blackish green.

Among the species found in European seas may be noted the Rhine mussel, a large species, the nacre of which is employed for ornamental purposes. _Unio littoralis_ (Cuvier), represented in Fig. 151, and the painter's mussel, _Unio_ _pictorum_ (Fig. 152), employed in the arts to contain certain colours. Those known as the river mussels are leathery, of an insipid taste, and scarcely eatable: the finest species are found in the great American rivers.

Mussels, as we have seen, produce pearls of moderate value. Linnæus, who was aware of the origin of the Pintadine pearls, and of pearls in general, was also aware of the possibility of producing them artificially from various molluscs. He suggested bringing together a number of mussels, piercing holes in their shells with an augur in order to produce a wound, and afterwards leave them for five or six years, to give the pearl time to form. The Swedish Government consented to try the experiment, and long did so in secret; pearls were produced, but they were of no value, and the enterprise was abandoned as unsuccessful.

Scottish pearls were much celebrated in the middle ages, and between the years 1761 and 1784 pearls to the value of £10,000 were sent to London from the rivers Tay and Isla; "and the trade carried on in the corresponding years in the present century," says Mr. Bertram, "is far more than double that amount." The pearl, according to Mr. Bertram, is found in a variety of the mussel, which is characterised by the valves being united by a broad hinge, and having a strong fibrous byssus, with which it attaches itself to other shells, to rocks, and other solid substances. "The pearl fisheries of Scotland," he adds, "may become a source of wealth to the people living on the large rivers, if prudently conducted." Mr. Unger, a dealer in gems in Edinburgh, having discerned the capabilities of the Scotch pearl as a gem of value, has established a scale of prices which he gives for them, according to their size and quality; and it is now a fact that the beautiful pearls of our Scottish streams are admired beyond the orient pearl. Empresses and queens, and royal and noble ladies, have made large purchases of these gems; and Mr. Unger estimates the sum paid to pearl-finders in the summer of 1864 at £10,000. The localities successfully fished have been the classic Doon, the Forth, the Tay, the Don, the Spey, the Isla, and most of the Highland rivers of note. Scottish pearls are much whiter in colour than oriental. What tint they have is bluish, while those of the East are yellowish. Pink pearls are produced by several exotic species of Unio.

Our seventeenth family are the Trigoniadæ, affording Trigonia, of which so many occurred in the Jurassic period of Geologic History in the strata of Europe, but of which two or three are alone left alive in the seas of Australia.

The eighteenth family, the Arcadæ, affords between 200 and 300 species of the families of Leda, Nucula, Pectunculus, and Arca.

Of the eighteenth family, Arcadæ, we shall only at present instance Pectunculus.

The genus _Pectunculus_ are abundant on the shores of the Mediterranean and along the Atlantic coast. If we take up at hazard a handful of shells on any part of the French coast, one-third will consist of _Pectunculus_. They are found mixed with Cardium, Venus, Razor-fish, and Pectens. Their round and robust frame attracts much attention. They form the first of those charming infantile collections which are gathered at the mother's feet.

The animal which inhabits this pretty shell is moulded on its curvature; like the shell, it is round and squat; it is furnished with a mouth, large, and thick for its size, and with double branchiæ. When the animal is taken alive, it sometimes exudes a thick mucous liquid over the shell, which has disgusted many a young collector with his capture.

Among numerous species of _Pectunculus_ we note as worthy of representation: _P. aureflua_, Reeve (Fig. 153); _P. delessertii_, Reeve (Fig. 154); _P. pectiniformis_, Lamarck (Fig. 155); and _P. scriptus_, Born (Fig. 156).

FOOTNOTES:

[Footnote 8: The term bivalve as constituting a class must be taken in a limited sense, for several genera, as pholas for example, have six or more valves.]