140

FISH AND GAME.

to the irritation caused by an intrusive grain of sand was
proved untenable as a normal mode of pearl inducement,
although, indeed, three instances of this sand origin of
pearls were found among several hundred instances traced
to other causes.
Two chief modes of origination were found, correlated
respectively to two classes of pearls which can usually be
determined and separated at sight by an expert. The one
ig the “Orient” or “fine” pearl of commerce ; the other
constitutes the bulk of those irregularly shaped and of those
known as “ seed pearls.”
“Orient” pearls were discovered to be due to the
irritation caused within the tissues of the pearl oyster by
the presence of the dead bodies of the spherical larvae of
a small tapeworm (Z'etrarhynchus) which frequently infests
the Ceylon pearl oyster in considerable numbers; 40 cysts
have been counted in a single pearl oyster. The living
worm does not induce pearl formation ; this occurs only
when death happens to overtake it when within certain
parts of the oyster’s body. Asa consequence pearls aremore
numerous in oysters which have long been infected, where
the worms are older and so more liable to die. Thus, oysters
under 3% years of age seldom produce pearls, and those
but of inconsiderable size.
The parasitic worm in question has been traced by the
writer from the pearl oyster to the trigger-fishes (Balistes
Spp.), which prey upon shellfish, and thence into certain
large fish-eating rays, where it becomes sexually mature
and produces embryos which enter the pearl oyster and
begin a new cycle of life-phases. By this completion of
our knowledge of the life-history of the pearl-inducing
parasite it now becomes feasible to influence artificially the
production of valuable pearls by the pearl oyster, whenever
the oyster be cultivated in landlocked areas where infected
ray fishes can be kept alive in cage enclosures.
The second or inferior class of pearls is primarily due to
the formation of crystalline bodies, analogous to gall stones,
within certain muscles of the oyster. These bodies become
later, by causing irritation, the nuclei of pearls, coats of
nacre, similar in composition to the brilliantly lustrous
mother-of-pearl lining of the shell itself, being deposited
concentricly around. Thus the pearl increases regularly in
size. “Orient” pearls are similarly formed of successive
coats, and differ solely in the character of the nucleus, 3.e., of
the originating irritant body.
Those who desire further acquaintance with the results of
the "investigation above outlined will find ample details