nacre
sufficient to cover them, or to produce one large pearl, might
reasonably be expected to result in a considerable distortion of the
shell. It may also be that the displacement of the mantle, caused by
the wrapping of itself about the growing pearl, interferes with the
even deposit of shell material about the edges of the shell and so
distorts it.
Because
deformed shells are more fruitful of pearls some have advocated the
practice of throwing perfectly-formed shells back into the sea
unopened, but, inasmuch as the mother-of-pearl of the shells often
exceeds in value the pearls found in them, this is not likely to
happen. Few fisheries could be made to pay if they were fished for the
pearls alone. In many of them the shells yield 90 per cent, of the
total value and are in fact the sole incentive for the investment of
the necessary capital.
Luckily
for the world's supply of pearls, however, the disturbers of the
mollusk which cause these gems by their intrusions appear to be more
abundant in waters where the shell is valueless, the banks about Ceylon
especially being infested with the cestodes which are
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