An echinoderm () is any animal of the phylum Echinodermata (), which includes starfish, brittle stars, sea urchins, sand dollars and sea cucumbers, as well as the sessile sea lilies or "stone lilies". While bilaterally symmetrical as larvae, as adults echinoderms are recognisable by their usually five-pointed radial symmetry (pentamerous symmetry), and are found on the sea bed at every ocean depth from the intertidal zone to the abyssal zone. The phylum contains about 7,600 living species, making it the second-largest group of deuterostomes after the chordates, as well as the largest marine-only phylum. The first definitive echinoderms appeared near the start of the Cambrian. Echinoderms are important both ecologically and geologically. Ecologically, there are few other groupings so abundant in the deep sea, as well as shallower oceans. Most echinoderms are able to reproduce asexually and regenerate tissue, organs and limbs; in some cases, they can undergo complete regeneration from a single limb. Geologically, the value of echinoderms is in their ossified dermal endoskeletons, which are major contributors to many limestone formations and can provide valuable clues as to the geological environment. They were the most used species in regenerative research in the 19th and 20th centuries. Further, some scientists hold that the radiation of echinoderms was responsible for the Mesozoic Marine Revolution.
Etymology The name echinoderm is from Ancient Greek ἐχῖνος (ekhînos) 'hedgehog' and δέρμα (dérma) 'skin'. The name Echinodermata was originated by Jacob Theodor Klein in 1734, but only in reference to echinoids. It was expanded to the phylum level by Jean Guillaume Bruguière, first informally in 1789 and then in formal Latin in 1791. In 1955, Libbie Hyman attributed the name to "Bruguière, 1791 [ex Klein, 1734]." This attribution has become common and is listed by the Integrated Taxonomic Information System (ITIS), although some workers believe that the ITIS rules should result in attributing "Klein, 1778" due to a 2nd edition of his work published by Leske in that year. While Echinodermata has been in common use since the mid-1800s, several other names had been proposed. Notably, Francis Arthur Bather called the phylum "Echinoderma" (apparently after Latreille, 1825) in his 1900 treatise on the phylum, but this name now refers to a fungus.
Diversity There are about 10,500 extant species of echinoderm as well as about 13,000 known extinct species. All echinoderms are marine, but they are found in habitats ranging from shallow intertidal areas to abyssal depths. Five extant classes of echinoderms are generally recognized: the Asteroidea (starfish, with over 1900 species), Ophiuroidea (brittle stars, with around 2,300 species), Echinoidea (sea urchins and sand dollars, with some 900 species), Holothuroidea (sea cucumbers, with about 1,430 species) and Crinoidea (feather stars and sea lilies, with around 580 species).
Anatomy and physiology Echinoderms evolved from animals with bilateral symmetry. Although adult echinoderms possess pentaradial symmetry, their larvae are ciliated, free-swimming organisms with bilateral symmetry. Later, during metamorphosis, the left side of the body grows at the expense of the right side, which is eventually absorbed. The left side then grows in a pentaradially symmetric fashion, in which the body is arranged in five parts around a central axis. Within the Asterozoa, there are a few exceptions from the rule. Most starfish in the genus Leptasterias have six arms, although five-armed individuals can occur. The Brisingida also contain some six-armed species. Amongst the brittle stars, six-armed species such as Ophiothela danae, Ophiactis savignyi and Ophionotus hexactis exist, and Ophiacantha vivipara often has more than six. Echinoderms have secondary radial symmetry in portions of their body at some stage of life, most likely an adaptation to a sessile or slow-moving existence. Many crinoids and some seastars are symmetrical in multiples of the basic five; starfish such as Labidiaster annulatus possess up to fifty arms, while the sea-lily Comaster schlegelii has two hundred. Genetic studies have shown that genes directing anterior-most development are expressed along ambulacra in the center of starfish rays, with the next-most-anterior genes expressed in the surrounding fringe of tube feet. Genes related to the beginning of the trunk are expressed at the ray margins, but trunk genes are only expressed in interior tissue rather than on the body surface. This means that a starfish body can more-or-less be considered to consist only of a head.
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