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 fact that suggested to Linnaeus his
method of producing pearls at pleasure, by puncturing the shell with a
pointed wire.
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