CHAPTER IV.

THE OYSTER AT HOME.

Its Nature, Colour, and Structure; Natural Food; Perception of the changes of Light; Uses of the Cilia; Fecundity and Means of Propagation; Age; Fossil Oysters in Berkshire and in the Pacific; Power of Locomotion.

The Oyster belongs to those Mollusks which are headless, having their gills in the form of membranous plates, and are named _Lamellibranchiata_, from the Latin word _Lamella_, a plate; or _Conchæ_, the Latin name for the whole family of oyster, scollop, cockle, mussel, and other well-known bivalves. Properly speaking, only six kinds are fit to take part in the gastromal treat, to say nothing of the sanitary advantages the family are good enough to provide for the world at large. These six peculiar and most agreeable aristocrats all belong to the family of the common oyster, _Ostrea edulis_, by far the most important tribe, and in fact, that in behalf of whose meritorious qualities I have more particularly taken up my pen.

The oyster bears different names in accordance to the localities in which it is found, whether on rocky ground, mud, or sand, and has different colours in different places. In Spain, oysters are found of a red and russet colour; in Illyria they are brown, but the fish is black, and in the Red Sea, of the colours of the rainbow. The green oyster, the Parisian delicacy, is brought from Brittany; but the same flavour and colour can be produced by putting oysters into pits where the water is about three feet deep in the salt marshes, and where the sun has great power. In these they become green in three or four days; for these colours are derived from the elementary substance on which they feed; not, however, that it produces any peculiar difference as to flavour. I may, however, as well decide at once that the green oyster is, to my taste, the oyster _par excellence_, in which decision I shall doubtless be borne out by most _gourmets_ whose knowledge extends to a choice of the good things of this life.

I know, in this, some of my friends north of the Tweed may differ, and, if still living, amongst them I should have had to include Professor Wilson, so long the very life and soul of oyster-suppers and whisky-toddy. But nobody can judge of the true flavour of an oyster without well _masticating_ his delicious food; and, by his own showing, both he and the "Shepherd" bolted their "Pandores." These same "Pandores," by the way, are large fat oysters, much relished in modern Athens, which are said to owe their superior excellence to the brackish contents of the pans of the adjacent salt-works of Prestonpans flowing out upon the beds. Taken away young and transferred to the Ostend beds, these Pandores furnish the very best oysters to be met with on the Continent, surpassing even the far-famed ones of Flensburg, in Holstein. Had "Christopher North" tickled the fish first to death with his incisors before he swallowed it, I might have submitted my judgment to his; but how can a man who bolted his food be quoted as an authority in matters of taste? At best, his must have been but an after-taste, a mere bilious reminder of what the repast had been, in which the whisky played as prominent a part as the "breedy creatures" themselves.

But let us return. The lower shell (*) of the oyster is concave, the upper flat. These shells are opened and closed by the medium of a strong muscle acting upon a hinge (+), far more complete in its structure than ever locksmith could produce, even at the forthcoming Exhibition of all Nations.

On the outside of the shell, when placed in a dark place, we may often observe a shining matter of blueish light, like a flame of brimstone, which sticks to the fingers when touched, and continues shining and giving light for a considerable time, though without any sensible heat. This light is produced by three varieties of minute animalcules, most interesting when examined under the microscope.

The oyster possesses an organ of respiration similar to that of a fish—branchiæ or gills, in fact (_br_), which are fringed by a mantle or beard divided into two lobes (_m_), filled up by small membranous fibres which terminate in the mouth (_b_), in the form of rays, serving the animal also with power to catch and eat. Unlike other shelled mussels the oyster has no feet; thus it is unable to make any other voluntary movement, save that of opening and closing its shell, as already named, in order to receive its food, which consists principally of small microscopical spores and young shoots of marine plants, made soft and thin by the action of the waves; whence arise the green beards or mantles. With some difficulty I have been enabled to separate a small portion of this vegetation from the mantle of an oyster, and having placed it under a strong microscope, discovered sea weed, of precisely the same species as that in which oysters are packed. They also feed on an infusion of sea worms called oyster animalcules. These are very accurately described in the "Journal des Savans," by M. Auzout. Some are irridescent, but others are not, and good specimens of all may be secured immediately the oyster has been taken from the sea.

By means of the beard or mantle described (_m_), the oyster secures his food, bringing it gradually, by means of little hooks bent inwards, to its mouth (_b_), wherein it is crushed and slowly consumed.

The stomach (_i_) is situated near the mouth, and all the organs are very simple. The mantle (_m_ and _m'_) above-named replaces the lungs. The liver (_f_) is small; the gall, comparatively speaking, large; the larger blood vessels little rarefied. The heart (_h_) consists of two cameras at a tolerable distance from one another, resembling small round bladders. The pulse beats rather slowly (caused by, perhaps, the want of food and sea water). From the stomach the rectum (_a_) leads directly to the anus. How digestion is effected in this short and simple way, I can scarce venture to assert. But it is a fact well known, that, after the spawning season, the oyster becomes thin, but a very short time enables it to recover its fat and succulence.

On examining the oyster the mantle (_m_), divided into two lobes (_m_ and _m'_), the edges of which are fringed, will be perceived filling the greater part of the shell; also four membranous leaves crossed with stripes, which at their hinder extremities have as many capillary tubes. These leaves, or veins, unequally divided around the edges of the body perform the functions of the lungs, and separate from the water the necessary air for the maintenance of the animal.

The mouth (_b_) is a kind of trunk, or long aperture surrounded by four lips nearly resembling those of a gill, but far shorter.

Behind the muscles is to be seen a large fleshy white and cylindrical substance moving on a central muscle, and containing the stomach and intestines (_i_). This part resembles the trunk of other conchæ, but it has no power of opening or contracting. The canal of the intestines is situated on the top of the muscle (_a_).

The oyster has circular vessels, on the bottom of which are to be seen deep muscular cavities, occupying the place of the heart (_h_), and sending their moisture to the small skin through which they come in contact with the water or the air.

In his "Outline of the Animal Kingdom," Professor Rymer Jones most happily describes all these peculiarities. "Wonderful indeed is the elaborate mechanism," are his words, "employed to effect the double purpose of renewing the respired fluid and feeding the helpless inhabitants of these shells! Every filament of the branchial fringe, examined under a powerful microscope, is found to be covered with countless cilia in constant vibration, causing, by their united efforts, powerful and rapid currents, which, sweeping over the surface of the gills, hurry towards the mouth whatever floating animalcules, or nutritious particles, may be brought within the limits of their action, and thus bring streams of nutritive molecules to the very aperture through which they are conveyed to the stomach, the lips and labial fringes acting as sentinels to admit or refuse entrance, as the matter may be of a wholesome or pernicious character."

Nature, too, has given the oyster a sensitive perception of the changes of light as the means of its protection from the many enemies it has to contend with; for if the shadow of an approaching boat is thrown forward so as to cover it, it closes the valves of its shell before any undulation of the water can have reached it. This sensitiveness is easily studied in the marine vivary, where the oyster, with its beautiful cilia, more beautiful by far than the richest lace of a bride's wedding dress, is always an object of great interest.

The oyster is an hermaphrodite animal, and hence its propagation is effected by self-produced eggs, which it bears within in the form of a greenish milky juice which it casts as spat in May, and which, as has already been stated, in this country is protected by wise and prudent acts of the Legislature. "The liquor in the lower shell of the oyster," says a writer in No. 587 of the "Family Herald," "if viewed through a microscope, will be found to contain multitudes of small oysters, covered with shells and swimming nimbly about—120 of which extend about an inch! Besides these young oysters, the liquor contains a variety of animalcules." Indeed, with the aid of a microscope one million of young have been discovered in a single oyster. Guarded by their two tender shells, these swim freely in the sea when ejected by the parent oyster, until, by means of a glutinous substance, they fix themselves so fast to some object that they can be separated only by force. These young are very soon able to produce others, many say at four months after their birth. When the oyster attains the size of a crown the shell is still very tender and thin; it is only after the second, third, or fourth year that it becomes fit for human food.

If we cannot answer the Fool's question in Lear, and "tell how an oyster makes his shell," we can, nevertheless, tell by his shell what is his age.

"A London oysterman," says a correspondent of No. 623 of the "Family Herald," "can tell the ages of his flock to a nicety. The age of an oyster is not to be found out by looking into its mouth. It bears its years upon its back. Everybody who has handled an oyster-shell must have observed that it seemed as if composed of successive layers or plates overlapping each other. These are technically termed 'shoots' and each of them marks a year's growth; so that, by counting them, we can determine at a glance the year when the creature came into the world. Up to the time of its maturity, the shoots are regular and successive; but after that time they become irregular, and are piled one over the other, so that the shell becomes more and more thickened and bulky. Judging from the great thickness to which some oyster-shells have attained, this mollusk is capable, if left to its natural changes unmolested, of attaining a great age." Indeed, fossil oysters have been seen, of which each shell was nine inches thick, whence they may be concluded to have been more than 100 years old.

For the most part the offspring remains near the mother, which accounts for the large oyster banks or beds which are found in almost all the seas of the temperate and torrid zones, and which in some places have been known to attain such magnitude as to cause ships to be wrecked upon them. The lower stratum is necessarily lifeless, being pressed upon by the upper one, so that the oysters beneath are unable to open themselves, and are consequently deprived of food.

The immense propagation of the oyster may be understood from the fossil oyster bed near Reading, in Berkshire. These fossils have the entire shape, figure, and are of the same substance as our recent oyster-shells, and yet must have lain there from time immemorial. This bed occupies about six acres, forming a stratum of about two feet in thickness. But the largest fossil oyster banks are those raised by earth-quakes along the western shores of South America, which measure from sixty to eighty feet in depth, are often forty miles in length, and in many places stretch above two miles into the interior.

The Abbé Dicquemare, fond of trying experiments in the spread of gastronomy, even to the stewing a mess of _Gemmaceæ_, the _Gems_ of our water-vivaries, till they had something of the flavour of oysters, asserts that, when in a state of liberty, oysters can move from one place to another by suddenly admitting sea water into the shell, which they are able to open and shut with extraordinary power and rapidity, whereby they produce a strange sound; and this observation has been confirmed by other naturalists, and is recorded as an ascertained fact in several books of natural science. In like manner they defend themselves against smaller animals, especially against the spider crab, which constantly tries to penetrate into their half open shells. Much natural instinct or foresight is also attributed to the oyster; in proof of which I may name that, when in a position which is exposed to the variations of the tide, oysters seem to be aware that they remain for some hours without water, and consequently provide it within their shells.

This makes such oysters far more fit to be conveyed to a distance, than those taken nearer to the shore, which evacuate the water, thus exposing themselves to the heat of the sun, the cold, or an attack from their enemies; and this, too, is the reason why Colchester or Pyfleet oysters, packed at the beds, are in such request.