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
pushes along on the ocean floor on its foot, or it might swim a
little. It lays millions of eggs and hatches countless baby
mollusks. It lives its life in its shell, lugging it around,
snuggling into it when alarmed, burrowing into mud, fastening
itself to a rock and creating ingenious camouflage. It builds its
calcareous house with a great instinctive talent for color and
sculpture. . .and the closer it lives to the tropical zones, the
more beautifully spectacular is its art.
The two parts of a bivalve shell are like thin saucers, concave
inside, convex outside. The inside is smooth, polished. The outside
is rougher, sometimes with graceful ribs or concentric ridges or
combinations of both. Univalves are conical and spiraling, with
a series of whorls coming down like widening steps from the tiny
nucleus on top. Univalves may have spines on their shoulders.
The opening, called the aperture, has a delicate right-hand rim
called the lip and a heavy, left-hand edge called the columella.
[figure captions]
BIVALVE'S anatomy: a) foot, b) adductor muscles, c) gills, d) hinge,
e) adductor muscles, f) siphon, g) stomach, h) mantle. Oysters,
clams, mussels all have them.
UNIVALVE'S anatomy: As before, a) foot, b) siphon, c) mantle, but
also d) operculum. Univalves include whelks, winkles, conchs.
Chambered nautilus is brother to the octopus, but he wears his
castle permanently--and on the outside.