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THE PEARL
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The
instances cited of the short capsule shaped pearl and the high button
joined to the shell, which seem to escape the nacreous deposit at the
basis of the domes, favor the lapping or licking method of depositing
the nacreous solution and this action by the mollusk would result in a
constant rolling or turning motion imparted to the object if it were
free within the creature's body. The licking and rolling action of the
mollusk, modified by the conceivable influences of position in the
shell, would account for the spherical form with all the various
modifications in which the pearl is found.
To
account for the variation of quality which undoubtedly exists in the
successive skins of some pearls, and the imperfections in the nacre of
the same skin, the theory has been advanced that the secretions for the
lining, the shell proper, and the epidermis, are exuded by different
parts of the mantle; the pearl traverses during growth these different
bands and its skins are modified by the secretions, as they come within
the various zones of influence. But there are several facts which seem
to oppose the theory.
164
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