PEARL (from ML. perula, perulus, perla, pearl, probably variants of pirula, diminutive of Lat. pinta', pear). A morbid product formed from the pearly nacre of the interior of the pearl mussel (11eleagrina margaritifcra) and other bivalves. It is often clue to the irritation caused by the presence of a grain of sand or some other foreign body lodged between the mantle of the animal and the shell; an extra amount of pearly matter is thus secreted and forms roughnesses or projections on the inside of the shell, which, if becoming free and regularly spherical, form one or more pearls. Recent authorities state that pearls are also due to the presence of distoma.. It has been noticed that when the pearl oyster (Meleagrina) is large, well formed, and with ample space for individual development, pearls scarcely occur at all, but when the shells are crowded together, and become humped and dis torted, as well as affording cover for all kinds of marine worms and parasitic creatures, then pearls are sure to be found (Cooke). The pearl oyster fishery is carried on in the Aru Islands, the Sulu Archipelago, the Philippine Islands, Burma. the Persian Gulf, the Red Sea, at Cey lon, along the coasts of North Queensland, North eastern Australia, and of West Australia, at New Guinea, and at the Pearl Islands on the Bay of Panama. This species also occurs along the coast of Lower California, and in the South Pacific in the Pa umota, Gambier, and Navigator Islands, and also at Madagascar. The shell is
very large, about eight to ten and even fifteen inches in diameter, while the valves are very thick and heavy, and in young individuals the outer surface is very rough and corrugated. The largest pearl known is said to measure two inches long, four inches round, and to weigh 1800 grains.
The `mother-of-pearl' is the internal nacre or nacreous larnime of this oyster. It is utilized in the manufacture of buttons, studs, knife handles, fans, eard-cases, brooches, boxes, and all kinds of inlaid work.
Pearls may be formed in almost any bivalve, and some of the most valuable are taken from the shells of fresh-water mussels, but these are usual ly small and called 'seed-pearls;' they also occur in tolerable abundance in the common mussel of our coast (3lytilus erlulis). very fair specimens at times hieing found; also in oysters, in Placuna placenta of the Pacific, in many species of Pinna, and in the giant clam (Tridacna). Pearls are sometimes formed in univalve shells; thus pink pearls have been taken from the giant conch shell (Strombus gigas) of the West Indies, as well as from certain species of Turbinella.