The word "seashell" typically refers to the hard outer layers of marine mollusks. Certain mollusks use their shells to protect their backbone-free bodies. When you find a seashell on a beach, the mollusc who lived inside has usually died, leaving the shell empty. According to All the Sea, these shells are made up by an outer mantle of tissue, as well as layers of various minerals. You'll find four common types of seashell on a beach: polyplacophorans, cephalopods, gastropods and bivalves.
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Polyplacophorans are also known as chitons. According to Palaeos.com, these mollusks look almost prehistoric, with long, flat, oval shells divided into eight plates. A chiton's shell is fairly fragile and will break apart into segments and small pieces, rarely ever left in one full piece after the chiton dies. Some of the smaller broken segments of shell you find on the beach may be chiton shells.
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Some of the more common cephalopod seashells belong to a mollusc called a chambered nautilus. According to the National Aquarium Baltimore, nautilus seashells are spiral-shaped, and typically grow up to 8 inches in diamater. The inside of the shell features a series of separate chambers, with a single shell containing up to 30 chambers. Because of their aesthetic beauty, nautilus seashells are highly prized, but this unfortunately poses a threat to certain chambered nautilus populations.
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Gastropod shells often belong to marine snails, but may not always resemble the snail shells found on dry land. According to the Living World of Molluscs, many gastropod seashells are coiled into long, spiral shapes. This spiral leads to a pointed tip, or apex, at one end. The spiral can coil either to the left or to the right, helping to identify different species of snail. In addition, the seashells belonging to gastropods other than snails can take different forms, including circles or cones.
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The Paleontological Laboratory at SUNY Cortland defines a bivalve seashell as one that consists of two individual valves, joined by a hinge. Picturing a clam or oyster shell opening up in the centre gives an idea of how a bivalve seashell looks. Usually, the lower half of a bivalve seashell is slightly larger than the upper half. Bivalve seashells also typically feature rows of ridges on the outermost layer, giving an almost "ruffled" appearance.