A seashell or sea shell, also known simply as a shell, is a hard, protective outer layer created by an animal that lives in the sea. The shell is part of the body of the animal. Empty seashells are often found washed up on beaches by beachcombers. The shells are empty because the animal has died and the soft parts have been eaten by another animal or have rotted out.
The term seashell usually refers to the exoskeleton of an invertebrate (an animal without a backbone). Most shells that are found on beaches are the shells of marine mollusks (“mollusks” in British English), partly because many of these shells endure better than other seashells.
Apart from mollusk shells, other shells that can be found on beaches are those of barnacles, horseshoe crabs and brachiopods. Marine annelid worms in the family Serpulidae create shells made of calcareous tubes cemented onto other surfaces. The shells of sea urchins are called tests, and the molted shells of crabs and lobsters are called exuviae. While most seashells are external, some cephalopods have internal shells.
Seashells have been admired, studied and used by humans for many different purposes throughout history and pre-history. But seashells 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.