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== Echinoderm ==

'''Echinoderm''' (pronunciation: /ˈɛkɪnoʊˌdɜːrm/; from the Greek ''echinos'', meaning "hedgehog", and ''derma'', meaning "skin") is a phylum of marine animals found at every ocean depth. The phylum contains about 7000 living species, making it the second-largest grouping of deuterostomes, after the chordates (which include the vertebrates, such as birds, fishes, mammals, and reptiles).

Echinoderms are characterized by radial symmetry, several arms (five or more, often in multiples of five) radiating from a central body, and a body wall stiffened by calcium carbonate plates and spines. They are also known for their remarkable capability for regeneration. Most species reproduce sexually, and the group includes some of the few animals that can regenerate a complete body from a small fragment.

== Etymology ==

The term "echinoderm" is derived from the Greek ''echinos'' (hedgehog) and ''derma'' (skin), a reference to the spiny skin that is a common characteristic of animals in this phylum.

== Classification ==

Echinoderms are exclusively marine and include the well-known starfish, sea urchins, sand dollars, and sea cucumbers, as well as the less familiar crinoids and brittle stars. They are classified under the [[Phylum]] Echinodermata, which is split into five extant [[Class (biology)|classes]]:

* [[Asteroidea]] (starfish or sea stars)
* [[Ophiuroidea]] (brittle stars)
* [[Echinoidea]] (sea urchins and sand dollars)
* [[Crinoidea]] (sea lilies and feather stars)
* [[Holothuroidea]] (sea cucumbers)

== Related Terms ==

* [[Radial symmetry]]
* [[Deuterostomes]]
* [[Chordates]]
* [[Regeneration (biology)|Regeneration]]
* [[Calcium carbonate]]
* [[Marine biology]]

== See Also ==

* [[List of echinoderm orders]]
* [[Starfish]]
* [[Sea urchin]]
* [[Sand dollar]]
* [[Sea cucumber]]

[[Category:Marine biology]]
[[Category:Animal classes]]
[[Category:Echinoderms]]

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