Pearls have been lately studied by zoologists, and their true history made known. They are a disease, caused, like so many other diseases, by an infecting parasite. It is common knowledge that they are found much as we see them in jewellery, as little lustrous spheres embedded in the soft bodies of various shellfish, such as mussels, oysters, and even some kinds of whelks. They are not found in the shellfish like crabs and lobsters, called Crustacea, but only in those like snails, clams and oysters, called Mollusca. Pink pearls are found in some kinds of pink-shelled whelks. A pearl-mussel or pearl-oyster has a pearly lining to its shell, which is always being laid down layer by layer by the surface of the mussel’s or oyster’s body, where it rests in contact with the shell, which consequently increases in thickness. If a grain of sand or a little fish gets in between the shell and the soft body of its maker, it rapidly is coated over with a layer of pearl, and so a pearly boss or lump is produced, projecting on the inner face of the shell, and forming part of it. These are called “blister-pearls,” and are very beautiful, though of little value, since they are not complete all round, but merely knobs of the general “mother-of-pearl” surface. These blister-pearls can be produced artificially by introducing a hard body between the shell and the living oyster or mussel.

It used to be thought that the true spherical pearls were caused by a hard granule of some kind pressing its way into the soft substance of the shell-fish, pushing a layer of the pearl-producing surface like a pocket in front of it. But it is now known that this “pushing in” is the work, not of an inanimate granule, but of a minute parasitic worm, which becomes thus enclosed by a pocket of the outer skin. The pocket closes up at its neck, and lays down layer after layer of pearl substance around the intrusive parasite, the dead remains of which can be detected with the microscope in sections of the pearl forming there a central kernel or nucleus. These parasitic worms were first detected in the small pearls formed by the common edible sea-mussel.

Though they are very small, sea-mussel pearls are collected for the market at Conway, in North Wales, and also on the coast of France. The parasitic worm is the young of a worm which, when adult, lives in the intestine of carnivorous fishes. It appears that it has to pass from and with the mussel into shellfish-eating sea fishes, where, although the mussel is digested, the parasite is not, but grows in size and alters its shape considerably. Then after a time the worm is swallowed, with the fish in which it has fixed itself, by sharks, dogfish, and such fish-eating fishes. In these at last it becomes adult and of some size, an inch or so long, varying according to the particular kind, and produces many thousands of eggs, which hatch out as minute creatures swimming in the sea-water, and fortunate if they fall upon a bed of mussels. They enter the mussel’s shell and make their way into its soft substance. A certain number (very few) get encased in the skin and covered up by pearl-layers, which is the mussel’s way of killing them and putting them out of mischief. The others which have entered other regions of the mussel’s body thrive, and have a chance of being swallowed by a mussel-eating fish, and then a further chance of that fish being eaten by a shark. If this happens the lucky worm—like the Italian who gets a winning number in three successive drawings of a lottery—gains the big prize. He becomes adult and produces innumerable young, who in their turn enter upon the chanceful career of a mussel parasite.

Thus we see that a pearl is not only a disease or abnormal growth caused by a parasite, but is actually an elaborately formed tomb or sarcophagus, in which the parasite is enclosed layer upon layer. This mode of disposing of parasites and other intrusive bodies is not unusual in animals. The terrible little flesh-worm—the Trichina—which causes the death of rats, pigs, and men who eat raw meat, is sometimes conquered in this way. It is found in the muscles (flesh) of man and animals enclosed in little pearl-like sacs, half the size of a hempseed, and it dies there, unless the invaded animal should die, and its flesh be eaten (as raw ham for instance) by another animal. The burying of inconvenient corpses in plaster of paris, corresponding to pearls as we now know them, has been a method of concealment occasionally adopted by criminals. On the whole, pearls have not very pleasant associations.

The history of the special parasitic worm which invades the beautiful little pearl-oyster of Ceylon has recently been followed out by skilful naturalists. There, too, a smaller oyster-eating fish of a peculiar kind, and a larger fish which eats the first fish, are necessary for the reproduction and multiplication of the pearl-producing parasites. The new Ceylon Pearl-Fishing Company has, therefore, to see to it that both these kinds of fish are encouraged to live in the sea near where the pearl oysters are found, and it is their object to increase the parasitic disease by which pearls are formed, and ensure an abundance of parasites.

An interesting new method has been recently applied to the examination of pearl oysters for pearls. The Rontgen rays are used to produce a skiagraph (such as surgeons use in searching for a bullet) of the pearl oysters when brought into harbour. They are thus rapidly examined one by one, without injury, and the shadow-picture shows the pearl or pearls inside those oysters which are infected. The pearlless oysters are returned to the depths of the sea, whence they came—those with small pearls only are kept in special reserves or sea-lakes, in order that the pearl may grow in size, whilst only those with good-sized pearls are opened at once, in order that the pearl may be extracted and sent to market.

There were great findings of pearls in the fresh-water pearl mussels of the Scotch rivers in former days. In the last forty years of the eighteenth century these pearls were exported from Scotland to France to the value of £100,000.

In the eighteenth century not only did they get their pearls from European rivers instead of from the East; but, instead of being excited about the artificial production of diamonds, they were driven wild with astonishment by the demonstration of the volatilisation of these stones—the disappearance of diamonds into invisible vapour when sufficiently heated. That the hardest stone in nature could be thus dissipated into thin air seemed incredible. On Aug. 10, 1771, a chemist named Rouelle invited to his laboratory to witness this wonder a company comprising the Margrave of Baden and the Princess his wife, the Dukes of Chaulne and of Nivernois, the Marchionesses of Nesle and of Pons, the Countess of Polignac, and some members of the Academy of Sciences, including the great chemist Lavoisier. Four diamonds—the largest belonging to the Count Lauraguais—were submitted before the eyes of all to the heat of a furnace, and in three hours had completely evaporated. There was, no doubt, room here for a mystification and for the abstraction of the diamonds with a view to dishonest appropriation. But no such purpose existed. The experiment was a genuine one, and Rouelle and his brother were honest investigators. They established the fact, now demonstrated as a lecture experiment, that the diamond is volatilised at very high temperatures. A more celebrated “evaporation” of diamonds—that which is known as “the affair of the Queen’s necklace”—took place a few years later in Paris, when no scientific investigation was connected with the embarrassing disappearance of the Royal trinket.