Book V — Molluscous Animals
§ I Bivalves, or Those Having Two Shells
Who that sees the beauty and delicacy of pearls would imagine that they were the production of disease? Such, however, is the case, as they are either formed in the body of the oyster which inhabits the shell; or they rise from cracks in the shell itself, the delicate, silvery, half-transparent lining of which forms the substance generally called Mother-of-Pearl, or Nacre. Their formation is generally caused by the introduction of some foreign body between the mantle or skin of the animal and its shell; the irritation thus produced causes successive coats of pearly matter to be deposited on the intruding object, and thus the pearl is formed. The best pearls are those which are fairly imbedded in the substance of the mantle. These shells are found in the Persian Gulf and at Ceylon, where they form an important article of commerce.
The Chinese form pearls by casting into the shell of a certain kind of muscle artificial beads, which at the end of a year become covered with a pearly crust, in such a manner that they cannot be distinguished from the natural pearl.C
Has long been in favour with man for its delicacy as an article of food; the Lucrine lake used to be as much in renown among the Romans for the choicest kind of Oysters, as Cancalle Bay with the French, and the Colchester beds with us. The two shells of the Oyster are generally unequal in size; the hinge is without teeth, but furnished with a somewhat oval cavity, and generally with lateral transverse grooves. Oysters sometimes grow to a very large size; in the East Indies they are said sometimes to measure nearly two feet in diameter.
The principal breeding season of oysters is in the months of April and May, when they cast their young, which are enveloped in slime, and in this state called spats by the fishermen, upon rocks, stones, shells, or any other hard substance that happens to be near the place where they lie; and to these the spats immediately adhere. Till they obtain their film or crust, they are somewhat like the end of a candle, but of a greenish hue. The substances to which they adhere, of whatever nature, are called cultch. From the spawning time till about the end of July, Oysters are said to be sick; but by the end of August they become perfectly recovered; from May till August they are out of season and unwholesome. The Oyster-fishery of our principal coasts is regulated by a court of admiralty. In the month of May the fishermen are allowed to take the Oysters, in order to separate the spawn from the cultch, the latter of which is thrown in again, for the purpose of preserving the bed for the future. After this month it is felony to carry away the cultch, and otherwise punishable to take any Oyster, between whose shells, when closed, a shilling will rattle. The reason of the heavy penalty on destroying the cultch is, that when this is taken away, muscles and cockles will breed on the bed; and, by gradually occupying all the places on which the spawn should be cast, will destroy the Oysters.
The Oyster has been represented, by many authors, as an animal destitute not only of motion, but of every species of sensation. It is able, however, to perform movements which are perfectly consonant to its wants, to the dangers it apprehends, and to the enemies by which it is attacked. The gills, through which the Oyster breathes, are what is commonly called the beard, and are very indigestible. The scallop is nearly allied to the Oyster.
The Common Cockle. (Cardium Edule.)
Few of our shell-fish are more common, in inlets and bays near the mouths of rivers, than these. In such situations they are usually found immersed at the depth of two or three inches in the sand, the place of each being marked by a small, circular, depressed spot. When they open their shells, the entrance into them is protected by a soft membrane, which entirely closes up the front, except in two places, at each of which there is a small, yellow, and fringed tube; by means of which they receive and eject the water which conveys to their body the nutriment necessary for their support.
Cockles are in great request as food among the labouring classes, and are caught chiefly in the winter months. Their size varies from five or six inches to half an inch in diameter. The shell is generally white; it has twenty-six longitudinal ridges, is transversely wrinkled, and has somewhat imbricated striæ. The foot of these animals is largely developed, and is to them a most important organ, as they use it not merely for progression, but in the excavation of hollows in the sand or mud in which they dwell.
The Chama, which is akin to the cockle, was used by the ancients to engrave various figures upon, from which circumstance those small bas-reliefs, so valued now, have obtained among the Italians and collectors the name of Cameos. The shells of some of these are decorated with red or yellow stripes, diverging from the hinge, and spreading to the edges. The Giant Chama has been found to weigh more than five hundred pounds, and the oyster-like animal within was large enough to furnish a meal for twenty men. The animals which inhabit these shells are sometimes called Clams. The shells are often used in Catholic countries for containing holy water.
This is a shell of a rather elongated form, gaping at both ends, and terminated in front by a point; it is white and chalky in its appearance, and the anterior end is roughened by numerous sharp spines and tubercles. The animal which inhabits this shell bores deeply into the rocks of the sea-shore, forming cylindrical holes, in which it lives; and the water which it requires for its food and respiration is conveyed to and from the interior of the shell by a pair of tubes which reach to the outer orifice of its dwelling-place. It is supposed that the Pholas is enabled to bore into the hard rock by means of its large and strong foot, but this is still a matter of dispute.
There are many other boring shells, most of which are related to the Pholas. Some of them burrow in rocks, others in wood, and some indifferently in either material. Of the wood-borers, the most remarkable is the Ship Worm (Teredo navalis), which penetrates deeply into floating or submerged timber, and lines the cavity of its burrow with a coating of shell. In this way the Teredo has often done much injury to piles and other woodwork exposed to the sea, and in 1731 and 1732 it excited so much alarm in Holland by attacking the piles of the great dikes, that even statesmen condescended to study its natural history. We must remember, however, that in the grand economy of nature even this destructive creature has its use; by penetrating in every direction through any floating mass of timber it promotes the breaking up of the latter, and prevents the surface of the sea from being encumbered with quantities of wreck.
Like the oyster, the Mussel inhabits a bivalve shell, to which it adheres by a strong cartilaginous tie. The shells of several of the species are beautiful. The Mussel possesses the property of locomotion, which it performs with the member called its tongue, by which it gets hold of the rock, and is enabled to draw itself along; it has also the property of emitting a kind of thread, called the byssus, which, fixing the sides of the shell upon the ground, answers the purpose of a cable, to keep the body of the fish steady.
Notes
C For a very interesting article on this subject, see Beckmann’s “History of Inventions,” vol. i. p. 259. (Bohn’s Standard Library.)