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UNITED STATES, CANADA AND MEXICO 215
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nacreous
knob or lump of pearl, of greater or less size, results from this
defensive and protective action on the part of the oyster. This walling
out of intruders can hardly be regarded as an indication of instinct or
intelligence in the oyster, analogous to the repairing of a damaged web
by a spider or the retunnel-ing of a filled-in gallery by ants: it is a
pathological rather than an intelligent action, induced by irritation
at the point of intrusion. Secondly, knobs, protuberances, and blister
pearls are the result, indirectly, of some intrusive particle, or, it
may be, of an organism which has in some way worked in between the
delicate tissues of the mantle or sac, or some part thereof, and the
interior surface of the shell. This, as may be easily conceived,
produces an irritation, as a rough particle of dust on the surface of
the human eye, and induces a secretion followed by a flow and deposit
of nacreous lymph at the point irritated, and the cause of the
irritation, whether an organic form or an inorganic particle, is
coated with nacre, and plastered down to or upon the inner surface of
the shell. It is rarely the case—but such instances have been
known—that a small fish, having entered the shellwhen the valves were
partially open, and having worked its way between the mantle and the
smooth surface of the shell up to the region where the adductor muscles
are attached (the muscles by which the valves are opened and closed),
has here had a stop put to further explorations into the anatomy of the
oyster, the latter not only clothing the unfortunate intruder in a
pearly shroud, but also burying him in a nacreous tomb.
The
disturbance of the muscular economy of the oyster at the point named,
it may be assumed, would induce immediate and extreme protective
activity in the nacreous deposition.
The
report of the United States National Museum for 1886, page 339,
contains a paper by Robert E. C. Stearns, " On Certain Parasites,
Commensals, and Domiciliares in the Pearl-oysters, Meleagrins," and the
colored plate (No. 8) (from a painting' made for that paper) which
illustrates this chapter, as well as the notes and comments herein
embodied, have kindly been placed at the author's disposal by Dr.
Stearns.
1 Department of Mollusca, United States National Museum, No. 73,934-
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