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.