world do.




SEASHELLS. . .WHAT ARE THEY?



First, a seashell is one of the 100,000 species of backboneless
animals belonging to the zoological group known as the Mollusca.
Mollusks include not only the familiar clams, scallops and snails,
but also the squids, octopus and Chambered Nautilus. Other "shells"
found in the ocean include those of crabs, lobsters, barnacles and
sea urchins.

True molluscan shells come in two main varieties: BIVALVES and
UNIVALVES. Bivalves have two valves, fitting together along a
toothed hinge on one side, and kept closed by means of ADDUCTOR
MUSCLES. Univalves have only one shell, usually coiled, but
sometimes shaped like a cap or miniature volcano. Some marine
univalves can seal themselves inside with an operculum, which
covers the open end of the shell like a trap door. Although shells
take on many different shapes, they are much alike inside. Each
has a foot, a breathing siphon, a tiny brain and heart, and a
fleshy mantle which secretes lime for shell-building. Most true
mollusks have eyes, but a few are blind. Many have teeth, called
RADULAE.

Like any other animal, the mollusk generally moves about. It