Editing Seashell (section)

{{short description|Hard, protective outer layers created by an animal that lives in the sea}}
{{distinguish|Seychelles}}
{{for-multi|the coaster ship|MV Seashell|the color|Seashell (color)}}
{{More citations needed|date=January 2009}}
{{Use dmy dates|date=September 2020}}
{{sea shell topics}}

[[File:Conchiglie Seashells 01.jpg|right|thumb|Seashells washed up on the beach in [[Valencia, Spain]]; nearly all are single valves of [[bivalve]] [[mollusk]]s, mostly of ''[[Mactra corallina]]'']]
[[File:Selection of seashells.jpg|right|thumb|Hand-picked molluscan seashells ([[bivalve]]s and [[gastropod]]s) from the beach at [[Clacton on Sea]] in England]]
[[File:Seashells.JPG|thumb|A group of seashells, mostly bivalves in the family Pholadidae]]
[[File:Seashell unknown 3.jpg|thumb|226x226px|Mixed shells on a beach in Venezuela]]
[[File:Seashells living in Persian Gulf.ogv|thumb|[[Hermit crab]]s inhabiting marine gastropod shells that lived in the [[Persian Gulf]]]]
[[File:Different types of shells.jpg|thumb|A group of beachworn sea snail shells that vary in size, form and pattern combination.]]

A '''seashell''' or '''sea shell''', also known simply as a '''shell''', is a hard, protective outer layer usually created by an [[animal]] or organism that lives in the sea. Most seashells are made by [[Mollusca|mollusks]], such as [[snail]]s, [[clam]]s, and [[oyster]]s to protect their soft insides.<ref name=":0">{{Cite web |title=How are seashells made? |url=https://www.whoi.edu/know-your-ocean/did-you-know/how-are-seashells-made/ |access-date=14 May 2024 |website=Woods Hole Oceanographic Institution |language=en-US |archive-date=22 March 2022 |archive-url=https://web.archive.org/web/20220322101724/https://www.whoi.edu/know-your-ocean/did-you-know/how-are-seashells-made/ |url-status=live }}</ref> Empty seashells are often found washed up on [[beach]]es by [[beachcombing|beachcombers]]. The shells are empty because the animal has died and the soft parts have [[decomposition|decomposed]] or been eaten by another organism.

A seashell is usually the [[exoskeleton]] of an [[invertebrate]] (an animal without a backbone), and is typically composed of [[calcium carbonate]]<ref name=":0" /> or chitin. Most shells that are found on beaches are the shells of [[marine (ocean)|marine]] mollusks, partly because these shells are usually made of calcium carbonate, and endure better than shells made of chitin.

Apart from [[mollusk shell]]s, other shells that can be found on beaches are those of [[barnacle]]s, [[horseshoe crab]]s and [[brachiopod]]s. Marine [[annelid]] worms in the family [[Serpulidae]] create shells which are tubes made of calcium carbonate cemented onto other surfaces. The shells of [[sea urchin]]s are called "[[test (zoology)|test]]s", and the moulted shells of [[crab]]s and [[lobster]]s are [[exuviae]]. While most seashells are external, some [[cephalopods]] have internal shells.

Seashells have been used by humans for many different purposes throughout history and prehistory. However, seashells are not the only kind of shells; in various habitats, there are shells from freshwater animals such as [[freshwater mussel]]s and [[freshwater snail]]s, and shells of [[land snail]]s.