There is glory in the grass, and splendour in the flower;
and the imagery might have been extended to 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 fact that suggested to Linnaeus his
method of producing pearls at pleasure, by puncturing the shell with a
pointed wire. But this explanation accounts only for the origin of
such pearls as are attached to the shell; while the best and greatest
number, and, indeed, the only ones which can be strung, have no such
attachment, and are formed in the body of the animal itself. 'The
small and middling pearls,' says Sir Alexander Johnston, 'are formed
in the thickest part of the flesh of the oyster, near the union of the
two shells; the large pearls almost loose in that part called the
beard.' Now, these may be the effect merely of an excess in the supply
of calcareous matter, of which the oyster wishes to get rid; or, they
may be formed by an effusion of pearl, to cover some irritating and
extraneous body." The reality of the latter theory is strengthened, if
not proved by the Chinese forcing the swan muscle to make pearls by
throwing into its shell, when open, five or six minute mother-of-pearl
beads, which, being left for a year, are found covered with a crust
perfectly resembling the real pearl. Such is one method of getting
artificial pearls. The extraneous body which naturally serves for the
nucleus, appears to be very often, or, as Sir E. Home says, always, a
blighted ovum or egg. This theory which, however, is here but partly
explained, has been fully adopted by Sir E. Home:--"if," says the
enthusiastic baronet, "I shall prove that this, the richest jewel in a
monarch's crown, which cannot be imitated by any art of man, either in
the beauty of its form or the brilliancy and lustre produced by a
central illuminated cell, is the abortive egg of an oyster enveloped
in its own nacre, of which it receives annually a layer of increase
during the life of the animal, who will not be struck with wonder and
astonishment?" And, we must add, that the proofs are very much in
favour of this conclusion.
[12] The writer of An Introduction to the Natural History of
Molluscous Animals, in a Series of Letters: one of the
most delightful contributions to the _Magazine of Natural
History_, since the establishment of that valuable
journal.
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