A seashell or sea shell, sometimes known simply as a shell, is a hard, protective outer covering that is generally formed by a sea animal. The animal’s shell is a component of its body. Beachcombers frequently come upon empty seashells washed up on shore. The soft portions of the animal have been devoured by another animal or have decayed, thus the shells are empty.

A seashell is often made of calcium carbonate or chitin and is the exoskeleton of an invertebrate (an animal without a backbone). The majority of beach shells are the shells of marine mollusks, partially because these shells are formed of calcium carbonate, which lasts longer than chitin shells.

Aside from mollusk shells, barnacles, horseshoe crabs, and brachiopods have shells that may be found on beaches. The Serpulidae family of marine annelid worms produce shells, which are calcium carbonate tubes bonded to other surfaces. The moulted shells of crabs and lobsters are termed exuviae, and the shells of sea urchins are called “tests.” Some cephalopods have inside shells, although most seashells are exterior.

Humans have utilized seashells for a variety of reasons throughout history and prehistory. Seashells aren’t the only sort of shells; freshwater creatures’ shells, such as freshwater mussels and freshwater snails, and land snail shells may be found in a variety of environments.

The term “seashell” is frequently used to refer only to the shell of a marine mollusc. The shells of marine species of bivalves (or clams), gastropods (or snails), scaphopods (or tusk shells), polyplacophorans (or chitons), and cephalopods are the shells most likely to be termed “seashells” by beachcombers (such as nautilus and spirula). These shells are frequently found in the wild and for sale as ornamental items.

Gastropods and bivalves found in the sea are more abundant than those found on land or in freshwater, and their shells are frequently bigger and more robust. Marine species’ shells also frequently have greater sculpting and color, however this is not always the case.

There are considerably more species of colorful, big, shallow water shelled marine mollusks in tropical and sub-tropical sections of the world than there are in temperate zones and places closer to the poles.

Although there are a few big species of shelled mollusks, there are also a huge number of very tiny species (see micromollusks).

Not all mollusks are found in the sea. There are many different types of land and freshwater mollusks, such as snails and freshwater bivalves. Furthermore, not all mollusks have an exterior shell: some, such as cephalopods (squid and octopuses), have an interior shell, while many, such as slugs and nudibranchs, have none.

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