There is, in the middle of their body, an oval bone, thick in the middle, and thin and sharp at the edges, light, spongy, and of whitish colour. These bones were formerly employed in medicine, and are still kept in the druggists' shops. When dried and pulverized, they are used by silversmiths as moulds, in which they cast spoons, rings, and other small work. When burnt or calcined, they are useful for the cleaning and polishing of silver and other hard substances, and sometimes for correcting the acidity of wines.
The body of the cuttle-fish is furnished with a vessel that contains a considerable quantity of dark-coloured or inky fluid, which the animal emits into the water, to conceal its retreat when alarmed by the approach of its enemies. And it is generally supposed that the article called Indian ink is this black fluid, in an inspissated or hardened state, and perfumed with musk and other substances.
272. The PEARL-BEARING MYA (Mya margaritifera) is a testaceous animal, having an oblong double or bivalve shell of somewhat oval shape, but narrower towards the middle than at the ends, and covered externally with a dark-coloured rough epidermis or skin, except on the protuberant parts near the hinge: one of the shells at the hinge has a single tooth or prominent part, which fits into a forked one in the other.
The general depth of the shells is two inches, and breadth about five inches.
Pearl-bearing myas are found in fresh-water rivers in many parts of Britain, and in those of most other countries within the arctic circle. The river Tay in Scotland, and the Conwy in Wales, are particularly noted for them.
In the river Tay some of these shells are found to contain good pearls; but fine ones are very scarce, and the greater part are of little or no value. They are of various shapes, round, oval, or elongated, and cylindrical, hemispherical, and resembling buttons. Several of the oblong ones have a contraction towards the middle, which gives them the appearance of two pearls joined together.
Pearls are a calculus, or morbid concretion, formed in consequence of some external injury which the shell receives, particularly from the operations of certain minute worms which occasionally bore even quite through to the animal. The pearls are formed in the inside on these places. Hence it is easy to ascertain, by the inspection of the outside only, whether a shell is likely to contain pearls. If it be quite smooth, without cavity, perforation, or callosity, it may with certainty be pronounced to contain none. If, on the contrary, the shell be pierced or indented by worms, there will always be found either pearls or the embryos of pearls. It is possible, by artificial perforations of the shells, to cause the formation of these substances. The process which has been chiefly recommended is to drill a small hole through the shell, and to fill this hole with a piece of brass wire, rivetting it on the outside like the head of a nail; and the part of the wire which pierces the interior shining coat of the shell will, it is said, become covered with a pearl.
As to the value of British pearls, some have been found of size so large as to be sold for 20l. each and upwards; and 80l. was once offered and refused for one of them. It is reported in Wales, that a pearl, from the river Conwy, which was presented to the queen of Charles the Second, was afterwards placed in the regal crown.
273. The ORIENTAL PEARL MUSCLE (Mytilus margaritiferus) to which we are indebted for nearly all the pearls of commerce, has a flattened and somewhat circular shell, about eight inches in diameter; the part near the hinge bent, or transverse, and imbricated (or covered like slates on a house) with several coats which are toothed at the edges.
Some of the shells are externally of sea-green colour, others are chesnut, or reddish with white stripes or marks; and others whitish with green marks.