beds
by a bunch of tough threads which pass out through an aperture in the
shells, near the hinge, and fasten on the rocks and stones;
consequently the oysters do not lie flat, as might be supposed, but
maintain an upright position, hinge down, lip end up, and the shell
slightly open for the passage of the food-laden water, as the
fresh-water mussels do. These threads are called the beard or byssus,
and are composed of material similar to the epidermis of the shell.
The
abalone, which is a univalve, holds on to the rocks by the foot, a flat
muscular appendage used for locomotion and also as an anchor on the
principle of the leather toy known to boys as a sucker.
Although
pearls of value are found only in shells containing mother-of-pearl, a
small proportion only of the mother-of-pearl shells contains pearls,
and many varieties in which pearls are found do not yield enough nacre
to make the shells valuable. The size of the meleagrina in some seas is
remarkable. That at page 127, photographed from a Tuamotu shell,
measures 8-7/8 inches by 6-7/8 inches and weighs twenty-eight ounces
troy.
170
