After this general view of the organization of the group, we shall proceed to introduce the reader to some of their more characteristic species.

Under the leaves of water-lilies (Nymphea), pond-weed (Potamogeton), or upon floating fragments of submerged wood, are generally to be found certain Bryozoaires, animals described by Trembley under the name of plumed polyps. These are Plumatellæ (Fig. 121). These little diaphanous creatures constitute colonies which under the microscope resemble small branching shrubs; they consist of small slender tubes grafted one to the other, and having from forty to sixty retractile tentacula, which expand like the petals of a flower; they are furnished with vibratile cilia, the movements of which serve the purpose of leading food into the mouth.

Another genus, which is found in ponds in France, and which is also found in fresh water in Britain, is the Cristatella of Cuvier. "Perfect specimens of C. mucedo occur from six lines to twenty-four in length by two or three in breadth," says Sir J. G. Dalyel, "of a flattened figure, fine translucent green colour, and fleshy consistence. Some of the shorter tend to an elliptical form, but those of larger dimensions are linear, with parallel sides, and curved extremities. The middle of the upper and the whole of the under surface are smooth, the former somewhat convex, occasioned by a border of seventy or eighty, even up to three hundred and fifty, individual polypi, dispersed in a triple row, their number depending entirely on the size of the specimen. Each of these numerous polypi, though an integral portion of the common mass, is a distinct animal, endowed with separate action and sensation. The body rising about a line above a tubular fleshy stem, is crowned by a head, which may be circumscribed by a circle as much in diameter, of a horse-shoe shape, and bordered by a hundred tentacula. Towards one side, the mouth, of singular mechanism, seems to have projecting lips and to open as a valve, which folds up within, conveying the particles which are absorbed to the wide orifice of an intestinal organ, which descends, perhaps, in a convolution below; and returns again, terminating in an excretory canal under the site of the tentacula."

The inhabitants of the colony are then united in great numbers under one common envelope; these are longish filaments of the size of a swan's feather, reminding one of the appearance of the silk thread known by embroiderers as chenille. The downy appearance is produced by the collection of tentacula belonging to this curious swarm. The filamentous mass is the translucent row of cells in which these animalcules are lodged, and to which they retreat when disturbed. These cells are sometimes free in part, sometimes completely rooted to the stems of aquatic plants. The tentacles are of a fine transparent glass colour, the body being of a brown colour. Fig. 122 represents Cristatella mucedo, which is common both in this country and in France.

Most naturalists have now agreed to place among the Bryozoa certain species of animalcules which long remained imperfectly known. Amongst these are the Flustra, the Eschara, and other ascidians.

The Flustra are marine Bryozoa, whose skin in hardening forms a thin shell of horny or cellular appearance; their little cells, more or less horny, are grouped symmetrically, somewhat like the cells in a bee-hive. Sometimes they form a crust which covers the algæ and other submarine bodies; sometimes they form ribbon-like stems. In some species the cells are only found on one side; in others they occupy both. Their orifices are extremely small, and defended by spines quite microscopic (Fig. 123).

Their tentacles are covered with cilia, always vibratile, disposed in a straight line, which in their movements produce the effect which a row of animated pearls might be supposed to produce if rolled upwards from the base to the summit of the organ.

The Eschara form leaf-like expansions, the entrance to their cells having also their protecting spines.

The expansions still represent microscopic bee-hives, the inhabitants of which enjoy at once a common and an independent existence. As it is with the corals, so it is here; each eats for the benefit of itself and for the community. Labour and nutrition for the community, labour and food for itself.

Tunicata.

On seeing one of the Tunicata for the first time, a stranger to zoology would scarcely take them for animals at all. Almost always attached to submarine rocks, these beings have the form of a simple sac. Their skin, gelatinous, horny, or rock-like, is at times covered with marine plants and polyps. They have neither arms, nor feet, nor head. But then they have a mouth, placed at the entrance of a digestive tube, and, in connection with the latter, a special opening intended for evacuations. The mouth is preceded by a great cavity, the walls of which are covered with vessels; for this cavity is the seat of respiration, and is covered with vibratile cilia. Thus the same canal serves first for respiration, and then, farther on, for digestion: another instance of the economy of Nature. Another remarkable instance of circulation is found: they have a heart, but no head.

This heart is the centre of a well-developed vascular system, but very unlike what usually obtains. The blood which traverses it takes such a course, that, in the space of a very few minutes, the heart changes its aurical into ventrical and its ventrical into aurical blood. At the same time the arteries are changed into veins and the veins into arteries. The consequence is, that the current which traverses these canals changes its direction with each contraction of the heart.

Simple as is their organization, the Tunicata have a nervous system. It is an unique ganglion, connected with divers small fillets. The organs of sensation present themselves in a very rudimentary fashion. We find eyes, and, after very minute search, a single ear has been found. They are propagated by budding, and also from eggs. The young are subject to some very curious metamorphoses, some particulars of which will be given farther on.

The Tunicata are divided into Ascidia and Salpa, to which some naturalists add the Brachiopoda.

Ascidians.

The Ascidia, from the Greek word ἀσχιδίον, leather bottle, have, as the name indicates, the shape of a bottle or purse. The analogy becomes more evident when it is considered that these creatures are habitually filled with water, which can be expelled by very slight pressure.

The Ascidians are sometimes free, sometimes united to others in a manner more or less intimate. Hence their division into the three groups of simple, social, and composite Ascidians.

Simple Ascidians attach themselves, each individual singly, to rocks and other submarine bodies, and generally at a fixed depth. Ascidia microcosmus a Mediterranean species, represented in Fig. 124, may be quoted as a type of this division of Ascidians. The name of Microcosmus, or the little world, is probably given from its being inhabited by quite an animated colony of algæ and corals, which dwell upon its surface, and give it a very peculiar, but not very attractive, appearance. The flavour of these molluscoids is very strong, which does not, however, hinder the poorer dwellers on the sea shore from eating them. The genus Phallusia is another type of the group. Phallusia grossularia is of a reddish colour, and about the size of a currant-berry: it usually lodges itself in the oysters of certain localities. At Ostend another species, Phallusia ampulloïdes, is found in prodigious quantities in the oyster parks, and is parasitic on living lobsters.

Social Ascidians comprehend living Tunicata, connected together on a common prolongation by the roots, but free and unconnected in all other respects. Ascidia pedunculata (Fig. 125) may be quoted as an example.

The Composite Ascidians are still more intimately associated together; a great number of these little beings live together in a single mass. Such are the Botryllus and the Pyrosoma.

The Botryllus is a genera the most interesting of all the groups under consideration. Only imagine from ten to twenty individuals, oval in form, more or less flattened, adhering by their dorsal surface to some submarine body, and holding on by their sides, so as to form a sort of wheel. "When we excite one of the branches," says Frédol, "a single mollusc contracts itself; when we touch the centre, they all seem to contract themselves (Cuvier). The buccal orifice is at the outer extremity of the radius; but the intestinal terminations abut on the common cavity, which occupies the centre of the wheel. Here we behold certain animals which eat separately, but which fulfil together as a community very singular functions—a kind of union and communism of which the moral world presents no prototype. With our molluscs, in place of two individuals united, we have a score. We may consider the entire star as one single animal with many mouths. But then, we have with it a luxury of organs for the function of intelligence which seeks and chooses, and parsimony of the organ of stupidity, which neither seeks nor chooses."

While the Botryllus is fixed and adherent, the Pyrosoma, on the contrary, is perfectly free. The animal colony which constitutes it floats and balances itself upon the waters, like the sea-pen or the physalia, of which we have spoken in treating of the zoophytes.

The name Pyrosoma has been given to these animals in consequence of their brilliant phosphorescent properties. According to the observations of Péron and Lesueur, nothing can exceed the brilliant and dazzling light emitted in the bosom of the ocean by these animals. From the manner in which the colonists dispose themselves, they form occasionally long trains of fire; but it is a singular fact that this phosphorescence presents the same curious characteristics that distinguish the cilia of the Beroë; namely, that the colours vary instantaneously, passing with wonderful rapidity from the most intense red to yellow, from golden colour to orange, to green, or to azure blue. Von Humboldt saw a flock of these brilliant living colonies floating by the side of his ship, and projecting circles of light having a radius of not less than twenty inches in diameter. He could see by this light the fishes which followed the ship's track, during many days, at the depth of from two to three fathoms.

Bibra, a Brazilian navigator, having caught six Pyrosoma, employed them to light up his cabin. The light produced by these little creatures was so bright, that he could read to one of his friends the description he had written of these his living torches.

Three species of Pyrosoma are known; namely, P. elegans, two or three inches in length, which inhabits the Mediterranean; P. giganteum, which is found in the same sea. It is a long bluish cylinder, bristling with tubercles, each of which is the abode of an animal, a citizen of this moving republic, and is attached to its colleagues by means of its gelatinous envelope: an alliance imposed by inexorable Nature—a forced species of socialism.

The third species, P. atlanticum, was discovered by Péron and Lesueur in the Equatorial seas.

These curious Ascidians are so created in rings as to constitute a long fine cylindrical tube, closed at one end and open at the other. By the contraction and dilatation of the mass of beings, this great cylinder swims slowly through the open sea, lighting up the ocean with its phosphorescent light, shining through the water like a glowing fire. Mr. Bennet thus describes one of these pelagic appearances: "On the 8th of June, being then in lat. 30° S. and 27° 5' W. long., having fine weather and a fresh south-easterly trade-wind, and the thermometer ranging from 78° to 84°, late at night the mate of the watch called me to witness a very unusual appearance in the water. This was a broad and extensive sheet of phosphorescence extending from east to west as far as the eye could reach. I immediately cast the towing-net over the stern of the ship, which soon cleaved through the brilliant mass, the disturbance causing strong flashes of light to be emitted, and the shoal, judging from the time the vessel took in passing through the mass, may have been a mile in breadth. On taking in the towing-net, it was found half filled with Pyrosoma atlanticum, which shone with a beautiful pale greenish light. After the mass had been passed through by the ship, the light was still seen astern, until it became invisible in the distance, and the ocean became hidden in the darkness as before this took place.

"The second occasion of my meeting these creatures was in a high latitude, and during the winter season. It was on the 19th of August, the weather dark and gloomy, with light breezes from north-northeast, in lat. 40° 30' S., and 138° 3' E. long., at the western entrance to Bass's Straits, and about 8 o'clock p.m., when the ship's wake was perceived to be luminous, while scintillations of the same light were abundant all round. To ascertain the cause, I threw the towing-net overboard, and in twenty minutes succeeded in capturing several Pyrosoma, which gave out their usual pale green light; and it was, no doubt, detached groups of these animals which were the occasion of the light in question. The beautiful light given out by these molluscans soon ceased to be seen; but by moving them about it could be reproduced for some length of time after. The luminosity of the water gradually decreased during the night, and toward morning was no longer seen."

The genus Salpa forms another interesting group of Tunicata.

The Biphora or Salpa (Fig. 126) are long transparent threads of the more delicate tissues, composed of rows of individuals placed side by side, and grafted, as it were, transversely: ribbons, in which each animal is grafted end on end to its sister: double parallel chains of social creatures, sometimes alternate, sometimes opposite; living chaplets, of which each pearl is an individual. Each individual presents an oblong diaphanous or prismatic body, more or less symmetrical, and often furnished in front, rarely behind, with tentaculiform appendages. So great is the transparency, that the various organs may be observed through the skin as they perform their several functions.

Momus, an ancient philosopher, thought it a subject of regret that Nature had not thought of piercing the body with an opening sufficiently large for each one to see what was passing in the interior. The creature which now occupies our attention would surely have satisfied the demands of our critic: its body is, metaphorically speaking, a house of glass.

In order to move itself, the Salpa has recourse to a singular artifice. It introduces water into its body through a posterior opening, furnished with a valve, which it expels by an anterior outlet situated near the mouth. It is thus pushed backwards, and swims, as it were, by recoil. Moreover, it swims with its belly upwards. All the elements of a chain of Salpas act in concert; they contract and dilate simultaneously; they advance as a single individual. One of them floats on the surface with the undulations of a serpent, so that among sailors they have gained the appellation of sea-serpents. These long, living trains abound in the Mediterranean, principally towards the African coast, and in the Equatorial seas. They are inhabitants of the open sea, and live immerged at considerable depths; but when the nights are calm they show themselves on the surface. As they spread themselves abroad with a strong phosphorescent light, they resemble long ribbons of fire, unrolling their long waving lines in spite of the waves, as in Fig. 127. What wonders they see who go down into the great deep! What sights are reserved for the navigator who traverses the Tropical seas during the silence of night!

When a chain of Salpas is drawn from the water, the rings separate, and they can no longer be made to adhere. The social bond has been dissolved by a superior force.

Salpas are sometimes met with, isolated and solitary, whose exterior conformation differs much from that which is proper to the connected Salpa; so different, indeed, that it might belong to another type. Chamisso, Krohn, and Milne Edwards have ascertained that the Biphora is viviparous, and that each species is propogated by alternate generation, the young creature being unlike its immediate parent. One of these generations is represented by the solitary individuals, the other by an aggregation of individuals. Each solitary Biphora engenders a new group—a chain; each constituted member of the chain engenders a solitary Salpa.

Thus a Salpa is not organized like its mother or daughter, but rather like its sister, its grandmother, or granddaughter—another example of alternate generation, which has already been discussed in treating of zoophytes.

Those marine creatures which pass their lives in a forced community—animals which eat, sleep, or rest always in company—who abandon themselves together to the soft caresses of the waves,—these colonies, or, rather, republics of animals, leading constantly the same monotonous existence,—reveal to us very strange things: an identical community of sentiments in a crowd of beings riveted by the same chain—a chain at once physical, intellectual, and moral!

CHAPTER XI.

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.