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
