Kinds of Seashells

Many people collect seashells, creating large displays of various sizes, shapes and colors. The shells were probably found along an ocean shore and no longer contain the live creatures all shells at one time housed. Shells, or mollusks as the live creatures are scientifically called, have over 125,000 species; however, they are classified by three main types and three less common types. The seashells collected along beaches are the discarded external skeleton of these creatures.

Classifications

Discarded seashells and live mollusks range in size from that of a grain of rice up to almost 4 feet across. The three main classifications are bivalves, gastropods and cephalopods. These three groupings make up the most common and numerous of mollusks. Three less commonly found types are chitons, tusk shells and gastroverms.

Bivalves

Bivalves such as clams, oysters, scallops and mussels make up many of the most often found seashells. However, when found by beachcombers, these shells are often only half of the actual shell, as bivalves always contain two-part shells. Within this grouping, there are pelecypods and brachiopods. Pelecypods contain two identical but mirror-imaged halves, while brachiopods, which are very rare today, contain two very similar but not identical halves. The live creature lives between the two shells, with a muscle attaching the two halves tightly. Some mollusks bury themselves in the sand, and some attach to hard surfaces by burrowing or growing more shell to attach it to the surface. They commonly exist in salt-water oceans and seas as well as in freshwater streams, lakes and rivers.

Gastropods

Gastropods, which consist mostly of snail-like creatures, make up the largest and most varied group of mollusks. Live mollusks exist as a coiled animal inside a coiled shell with an extending foot, actually a stomach, by which they move. Gastropod shells include common types such as conchs, whelks, moon snails, cone snails and nautiluses. All contain a spiral-shaped shell and commonly live in both freshwater and salt water, as well as on land.

Cephalopods

Cephalopods include such creatures as squid, octopus and cuttlefish. This type of mollusk sometimes contains a shell but most often has evolved to a point of not needing a shell. Many fossilized shells of the ancestors of squid, octopus and cuttlefish have been found, demonstrating that this mollusk did once routinely contain a shell. Cephalopods evolved to be predatory, intelligent creatures with keen eyesight and tentacles containing suction cups for defending itself and attacking others.

Less common types

Besides the most common shell types of bivalves, gastropods and cephalopods, three less common types of mollusks exist though are rarely found today. All are extremely small and often live in remote or deep waters. Chitons are tiny, with an armadillo-like shell that allows it to roll up into a ball to protect itself. Another type, tusk shells, resemble tiny elephant tusks open on both ends. These primitive creatures bury themselves in the sand with the smaller end sticking out. Gastroverms make up the last of the three less common shell and mollusk types. These are extremely primitive. Inside the shell lives a jointed, worm-like creature with vital organs repeated in every section of its body.