page_title: Seashell
section_title: Mollusks
hierarchical_section_title: Seashell / Molluscan seashells / Molluscan seashells used by other animals / Mollusks
caption_attribution_description: ja:イイダコ（飯蛸 英名：en: ''Amphioctopus fangsiao'' (d'Orbigny, 1839) (synonym: Ocellated octopus 学名：Octopus ocellatus）
context_page_description: 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 decomposed.
  A seashell is usually the exoskeleton of an invertebrate, and is typically composed of calcium carbonate or chitin. Most shells that are found on beaches are the shells of marine mollusks, partly because these shells are usually made of calcium carbonate, and endure better than shells made of chitin.
  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 which are tubes made of calcium carbonate cemented onto other surfaces. The shells of sea urchins are called "tests", and the moulted shells of crabs and lobsters are exuviae. While most seashells are external, some cephalopods have internal shells.
context_section_description: Carrier shells in the family Xenophoridae are marine shelled gastropods, fairly large sea snails. Most species of xenophorids cement a series of objects to the rim of their shells as they grow. These objects are sometimes small pebbles or other hard detritus. Very often shells of bivalves or smaller gastropods are used, depending on what is available on the particular substrate where the snail itself lives. It is not clear whether these shell attachments serve as camouflage, or whether they are intended to help prevent the shell sinking into a soft substrate.
  Small octopuses sometimes use an empty shell as a sort of cave to hide in, or hold seashells around themselves as a form of protection like a temporary fortress.
caption: An ocellated (spotted) octopus using a clamshell as a shelter
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