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Ch. 5: Mother-of-Pearl Shells

Ch. 5: Mother-of-Pearl Shells Page of 341 Ch. 5: Mother-of-Pearl Shells Text size:minus plus Restore normal size   Mail page  Print this page
96                                   Pearls,
internal surface of the valve by a single layer of nacre. In Fig. 3, the obstruction has become buried in the substance of the shell, and the inner surface is again level, because the successive layers of nacre are thinner on the top of the foreign body. At the same time, the decay of the outer surface has been advancing, until in Fig. 3, it has nearly reached the Pearl or other intruder ; and finally in Fig. 4, this foreign body has entirely disappeared, together with its encircling layers of nacre, having been set free by the destruction of the external part of the shell.
Besides the possession of this faculty, it is evi­dent that the oyster is sensible to vibrations of either sound or motion. When it is lying agape in .the boat, any slight concussion will cause it im­mediately to close tightly, like a vice. Many a shell is discovered by bubbles of gas arising from it, in the act of closing, on account of the near ap­proach of a diver, and a consequent tremor of the water. The most remarkable fact, however, is that it is so far conscious of the point of attack of a "borer" (Fig. 1, p. 85), eating its way through the shell, that, in order to guard against the danger of being pierced, the oyster secretes extra-thick layers of nacre at that point, and thus, in course of time, seriously disturbs the natural shape of its pearly
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Ch. 5: Mother-of-Pearl Shells Page of 341 Ch. 5: Mother-of-Pearl Shells
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