the irridescent pearl
within the rudely-formed shell of the oyster. Poets have feigned that
pearls are

Rain from the sky,
Which turns into pearls as it falls in the sea;

we need scarcely add that science has exploded this imaginative
fertility.

Pearl is, in fact, a calcareous secretion by the fish of bivalve
shells; and principally by such as inhabit shells of foliated
structure, as sea and fresh water muscles, oysters, &c. A pearl
consists of carbonate of lime, in the form of nacre, and animal matter
arranged in concentric layers around a nucleus; the solution
indicating no trace of any phosphate of lime. To this lamellar
structure the irridescence is to be ascribed. Each layer is _presumed_
to be annual; so that a pearl must be of slow growth, and those of
large size can only be found in full-grown oysters. The finest and
largest are produced from the Meleagrina margaratifera, (_Lamarck_,) a
native of the sea, and of various coasts. A considerable number are
likewise taken from the Unio margaratifera, which inhabits the rivers
of Europe; and, it is singular, as remarked by Humboldt, that though
several species of this genus abound in the rivers of South America,
no pearls are ever found in them. The pearls are situated in the body
of the oyster, or they lie loose between it and the shell; or, lastly,
they are fixed to the latter by a kind of neck; and it is said they do
not appear until the animal has reached its fourth year.

Naturalists have much disputed the formation of pearls. Mr. Gray
justly observes they are merely the internal nacred coat of the shell,
which has been forced, by some extraneous cause, to assume a spherical
form. Lister, on the other hand, states "a distemper in the creature
produces them," and compares them with calculi in the kidneys of man.
But, as observed by a more recent inquirer,[12] "though they are
accidental formations, and, of course, not always to be found in the
shellfish which are known usually to contain them, still they are the
products of a regular secretion, applied, however, in an unusual way,
either to avert harm or allay irritation. That, in many instances they
are formed by the oyster, to protect itself against aggression, is
evident; for, with a plug of this nacred and solid material it shuts
out worms and other intruders which have perforated the softer shell,
and are intent on making prey of the hapless inmate: and it was
apparently the knowledge of this