therein.
After confinement in this cyst for a period of two months or more, the
small mollusk works its way out and falls to the bottom of the river or
pond, where its development continues along lines more conventional to
molluscan life.
In
most of the species of Unios the sexes are separate ; but it has been
determined that in some the individuals are provided with both sets of
sexual organs. It is claimed by some naturalists that certain species
may change from one sex to another ; yet this does not seem to have
been positively established.
Not
the least interesting of the habits of the Unios is the manner in which
they "walk," bushels of them changing their habitation in a few hours.
The shell opens slightly and the muscular tongue-like "foot" is thrust
out, and by pressure of this on the bottom, the mollusk is propelled
in a jerky, jumpy movement with more speed than one would suppose
possible for the apparently inert creature.
The
number of eggs produced by an individual in one season ranges from a
few hundred in some species to many millions in others, as in the Quadrilla héros, for
instance. Most of the fresh-water mol-lusks are of slow growth,
reaching maturity in six or eight years, and it is believed that if
undisturbed they live to be from fifteen to fifty years old; indeed,
some writers credit them with attaining an age of one hundred years.
While
outwardly there is no positive indication of the existence of pearls,
they are relatively scarce in young mollusks, and likewise in those
having a normal, healthy appearance, with smooth exterior free from
blemishes, and they are found generally in the older, irregular, and
deformed shells, which bear excrescences and the marks of having
parasites. However, some of the choicest pearls have come from shells
relatively young and apparently in perfect condition.
It
has been pointed out that with the fresh-water Unios there are three
indications on which the fishermen to some extent rely for determining
the presence of pearls from the outward aspects of the shell. There
are, first, the thread or elevated ridge extending from the vertex to
the edge ; second, the kidney-shape of the shell, and third, the
contortion of both valves toward the middle plane of the mollusk.
A
single mollusk may contain several small pearls,—more than one hundred
have been found,—but in such cases usually none has commercial value.
Ordinarily only one is found in the examination of very many shells. Of
these objects it may be truthfully said that "many are found, but few
are chosen," few that are of first quality or are worthy of a fine
necklace. In many instances, several pounds of cheap pearls would be
gladly exchanged for a choice gem weighing an equal number of grains.
