A seashell, also known as a sea shell, or simply as a shell, is the common name for a hard, protective outer layer, a shell, or in some cases a "test", that was created by a sea creature, a marine organism. The shell is part of the body of a marine animal. In most cases a shell is an exoskeleton, usually that of an animal without a backbone, an invertebrate. Seashells are most often found washed up empty on beaches or another part of the coastline after the soft parts of the animal have either been eaten by another animal that attacked it (predation), or after the animal has died and the soft parts have been eaten by scavengers or have simply rotted out.
The word seashell is most often used to mean the shells of marine mollusks, i.e. mollusk shells. It can however also be used to mean the shells of a wide variety of other marine animals from various different marine invertebrates.
As well as marine mollusks, many other kinds of sea animals have exoskeletons or even internal shells which sometimes, after death, wash up on the beach and may be picked up by beachcombers. These shells include remains from species in other invertebrate phyla, such as the moulted shells or exuviae of crabs and lobsters, the shells of barnacles, horseshoe crab shells, the tests (endoskeletons) of sea urchins, sand dollars and seastars, brachiopod shells, and the shells of marine annelid worms in the family Serpulidae, which create calcareous tubes cemented onto other surfaces.
Seashells have been admired, studied and used by humans for many different purposes throughout history and pre-history.
Sea shells are not the only kind of shells, in a variety of habitats it is possible to find shells from freshwater animals such as freshwater mussels and freshwater snails, and it is also possible to find shells from land snails.
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