bed. This is seen in Figs. 5, 6 and 7. {See next page). The
borer is represented in Fig. 5, attacking the exterior of the shell,
and the oyster has secreted an unusually thick layer of nacre opposite
the point of attack. In Fig. 6, the borer has penetrated into the
substance of the shell, but successive deposits of nacre have been
formed to resist the intrusion. In Fig. 7, the decay of the external
part of the shell has removed the borer, while the internal surface has
resumed its natural form. The three figures are placed at different
levels in order to shew that as external decay proceeds, the oyster
adds to the internal surface. All these figures are taken from
photographs of the Pearl shell.
The oyster's sole means of protection against
crabs and other enemies, is that of closing its shell ;
if, however, this shell is pierced completely through
by a " borer," (Fig. 1, p. 85), a breach of its walls is
effected, and the defenceless citadel is open to attack.
Thus dead shells generally exhibit the cause of their
late owner's disappearance. A large species of
Murex, a Univalve, is also an inveterate enemy of
the Mother-of-Pearl oyster, attaching itself to the
shell, and boring through it. When, therefore, a
shell is found with any unnatural protuberance on
its interior surface, the pearler scrapes away a little
of the decaying shell at the back of this spot, and if
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