two
valves, the upper one rounded and the lower one flat. They are composed
of consecutive layers of "nacre" secreted by the oyster, each layer
trending away from the hinge and over the horny lip more than its
fellow below ; these layers are thicker at the hinge than at the lip,
and they are continually in process of formation. This continued growth
is necessary, in the first place to enlarge the young shell so as to
accommodate the increasing dimensions of the owner, and secondly in
order to renew and keep up the necessary thickness ofthe shell, which
through external influences decays away ; the nacre is also secreted in
order to cover any foreign substance, such as mud or weed, that may
drift into the shell, and being unable to escape may cause
inconvenience to the delicately constructed mollusc ; it likewise
serves to arrest the progress of the " borers," which attack the shell
and seek to effect an entrance. Conchologists assert, as stated in the
preceeding chapter, that the nacre is secreted by the mantle, but it is
probably that the adductor muscle itself contributes to the formation
of that portion of the shell which bears the adductor impression.
The
resplendent play of colour which the inner surface of the Pearl shell
exhibits—a display that defies any attempt at artificial imitation — is
not
