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At Billiers, France, pearls are frequently found in river mussels
and in this locality ducks play an important part in the life history of the
pearls. Often the livers of these ducks harbor small, leaf-like parasites
called flukes, or Trematodes. (1)

The fluke, though a very small animal, is exceedingly prolific and
may produce as many of 50,000 eggs which go down the bile duct into
the intestines, eventually reaching the water. From each egg comes a
larva which is called a miracidium. This larva swims about in great haste
to find a host because, unless one is found within eight hours, it will die.
(4: p. 118)

The miracidum, once it has found a home, usually in a mollusk
which goes by the name, Tapes decussatus, (1) loses its cilia for it now
has no further use for these swimming organs. Having accomplished this,
the larva begins to grow and becomes a hollow sac known as a sporocyst
which is endowed with strange reproductive powers. Five to 12 other
larvae, called rediae, develop inside the first and these, in turn, eventually
produce third larvae having tails and known as cercariae. (4: p. 118)
Being equipped for traveling, the cercariae strike out for a new
home and this frequently is found in the soft tissues of a river mussel. The
scoter ducks, at Billiers, are fond of mussels. When a scoter eats one
containing a cercaria the unfortunate bivalve is doomed but the cercaria,
not at all bothered by the ducks digestive