§ I. Bivalves, or those having two shells.
THE PEARL OYSTER. (Avicula Margaritifera.)
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]
[C] For a very interesting article on this subject, see Beckmann’s “History of Inventions,” vol. i. p. 259. (Bohn’s Standard Library.)