structed like any other part of the shell are not true pearls.
The
normal instinctive action of the mollusk is self-protective and
adaptive. By the secretive action of its mantle it gathers from the
water in which it lives, material to build a shell with a rough and
rugged exterior for its enemies, and adapted to resist the chemical
activities by which it is surrounded, and a perfectly smooth lining
suitable as an interposition for its own delicate organism.
Barring
accidents, the building functions of the animal are employed only in
the extension of the shell to meet the needs of its own growth and
protection. But should a particle of secretion intended for the shell,
harden within the folds of the oyster's mantle, or some parasite or
other intruder present itself within the nacre-forming sphere, the
instinctive action which lines the rougher part of the shell is also
directed toward the foreigner, and it is at once covered with a like
deposit. This is the birth of a pearl, and it grows layer by layer as
long as it remains within the scope of the nacre building instinct.
These layers, or skins as they are called, are
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