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Echinoderm, members of the phylum Echinodermata, are a group of entirely marine animals with no known freshwater or terrestrial species. The phylum contains about 7000 living species, making it the second-largest grouping of deuterostomes (a superphylum), after the chordates (vertebrates such as birds, fishes, mammals, and reptiles). Most are important to the marine ecosystems they inhabit, keeping things such as algae in check and stirring up sediment. Most are able to reproduce asexually and regenerate tissue, organs, and limbs; in some cases, they can undergo complete regeneration from a single limb.

Echinoderms evolved from animals with bilateral symmetry, and adults are recognizable by their (usually pentaradial, or five-sided) symmetry. Echinoderm larvae, however, are often ciliated, free-swimming organisms. The left sides of their bodies eventual grow at the expense of the right side, which is eventually absorbed, and then these left sides grow in a (often pentaradially) symmetric fashion, in which the body is arranged around a central axis. This gives them their generally symmetric forms, which are iconic in most echinoderms, of which the most well known is the starfish.

Species

• crinoid
• sea_cucumber
• sea_pig
• sea_urchin
• sand_dollar
• starfish
• sunflower_sea_star

See also

• marine
• ambient_sealife