Silver-shell might properly be so denominated even when it first fell from the briny waters on one of the rocks beneath; for not only was there a minute animal, but a covering for it of two valves, or shelly plates, exquisitely adapted to its comfort and security. All mollusks having shells, whether univalve, like the limpet; bivalve, like the mussel or the oyster; or multivalve, like the balanus, so plentifully scattered over the rocks of our coasts, are similarly provided for; the minute creature having, at its first development, a shelly covering of one or more pieces.

But Silver-shell, as we have seen, grew rapidly, and hence, were there no provision against it, serious evil might arise. Every mother knows how suddenly her schoolboy's jacket sleeve sometimes fails to cover his wrists, while his trowsers stand high above his shoes, from that advance to which robust youth is pronea state which led Hood to remark :

"Then a schoolboy, my tailor was nothing in fault,
For an urchin will grow to a lad by degrees;
But how well I remember that pepper-and-salt
That was up to my elbows and down to my knees;"

and there is, perhaps a mingling of pleasure and vexation in her bosom as she says, "How soon you outgrow your clothes!" This fact reminds us of Queen Elizabeth's paying a visit to one of her favourites, Sir Christopher Hatton, and expressing her surprise on

finding him in what she considered a very humble dwelling, when her host replied, in true courtier phrase, "Your Majesty's bounty has made me too large for my house."

Now no such accident occurred to Silver-shell: his clothes could not become too short; his house could not become too small. And why? Because the shell grew exactly in proportion to the size of its inmate Shakespeare has made the fool ask Lear, "Dost know how an oyster makes its shell?" only to remind the poor king, however, of his houseless state; but were it repeated now among oyster-eaters, it is probable that but few would be prepared for an intelligent reply.

In offering one, it should be observed that an oyster is endowed, like other animals, as well as plants, with a power of secretion: the process of separating certain matters from the nutritious fluids of the body. It is probable that in almost every such act a double purpose is served; the blood being freed from particles which would become superfluous or injurious, and the fluid separated answering some secondary purpose. The process, common as it is, in all its essential features, to vegetables and animals, is everywhere performed by the same agency, the development of single cells, each possessing its own independent vitality. It is a part of their regular actions to secrete and withdraw certain ingredients from the nutritious fluids, and after

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wards to set them free again, either by passing through in perspirable vapour, or the rupture of the cell-wall.

Now the oyster secretes a calcareous, earthy matter, precisely adapted to the enlargement of its dwelling, by means of its mantle, the outer membranous layer which invests its body. Whatever, indeed, be the form of a shell, and shells partake of a marvellous variety, the additions required by growth, according to the law of its kind, both in shape and colour, are ascribable to the wonder-working mantle of the inmate.

Duly provided for its work by the secretion of the necessary matter (7), and by the instinct for its proper use, Silver-shell first began the enlargement of its shell; for this, the margin of the mantle protruded, and firmly

adhered all round the circumference of the valve with which it corresponded. Thus the calcareous matter was gradually deposited in a soft state on the extreme edge

of the shell, and, becoming hardened, it was converted into a shelly layer; the process being repeated at intervals, every newly-formed layer enlarged the diameter of the shell. Thus the shell corresponded in its increase with the growth of its tenant.

Nor was this all: a minute mechanism might have been traced in each layer. On the outer surface of oyster shells there are fine thread-like lines, or streaks, which are called striæ. And on this fact Lord Brougham and Sir C. Bell remark: "We should be inclined to say that the earthy matter crystallises, were it not that the striated or fibrous appearance differs in the direction of the fibres in each successive stratum, each layer having the striæ composing it parallel to one another, but directed obliquely to those of the layer previously formed, and the shell exhibiting a strong texture arranged upon well known mechanical principles."

The oyster does not adorn its shell, as some of the soft-bodied creatures do theirs. Their various and often splendid hues are owing to glands situated on the margin of the mantle, and curiously endowed with the power of depositing colouring matter. In many instances an accordance is observable between the patterns or tracings on the shell and the colours as arranged in the organ that secretes them. Thus, in the Banded Snail there are just as many coloured spots on the edge of

this organ as there are zones on the shell, and when a part of the shell is removed, for the sake of experiment, the piece reproduced is brown exactly opposite the dark portion of the organ, and yellow in the other parts. Silver-shell, however, diffused throughout its exterior only a brown tint. The inner part of an oyster-shell is a great contrast to its exterior. As we examine it, we cannot fail to be struck with its silver-like hue and its exquisite smoothness, so completely adapted to the form and the comfort of the little in-dweller ; and, when held to the light, a slight iridescence is frequently discoverable. The hard, silvery, brilliant internal layer of several kinds of shells reminds us of those described by Landor:

"Of pearly hue

Within, and they that lustre have imbibed

In the sun's palace porch, where, when unyoked,
His chariot wheel stands midway in the wave;

and the internal layer of oyster-shells is often variegated with changing purple and azure.

The cause of this effect has been fully explained by Sir David Brewster, as suggested to him accidentally. He had fixed a piece of mother-of-pearl to an instrument for measuring angles by a cement of resin and beeswax. On removing it from the cement when in a hard state, by insinuating the edge of a knife, and making it spring up, he found that the plate of mother-of-pearl