There are about 7,000 extant species of echinoderm as well as about 13,000 extinct species.
The context of the echinoderms within the Bilateria is: Bilateria An independent analysis in 2015 of RNA transcriptomes from 23 species across all classes of echinoderms gave the same tree. However, a 2014 analysis of 219 genes from all classes of echinoderms revised the phylogenetic tree.
Historically, taxonomists believed that the Ophiuroidea were sister to the Asteroidea, or that they were sister to the (Holothuroidea + Echinoidea). The characteristics of adult echinoderms are the possession of a water vascular system with external tube feet and a calcareous endoskeleton consisting of ossicles connected by a mesh of collagen fibres. Among the bilaterians, they belong to the deuterostome division, meaning that the blastopore, the first opening to form during embryo development, becomes the anus instead of the mouth. Echinoderms are bilaterians, meaning that their ancestors were mirror-symmetric. The name echinoderm is from Ancient Greek ἐχῖνος ( ekhînos) 'hedgehog', and δέρμα ( dérma) 'skin'. Further, some scientists hold that the radiation of echinoderms was responsible for the Mesozoic Marine Revolution. They were the most used species in regenerative research in the 19th and 20th centuries. Geologically, the value of echinoderms is in their ossified skeletons, which are major contributors to many limestone formations, and can provide valuable clues as to the geological environment. 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.
Ecologically, there are few other groupings so abundant in the biotic desert of the deep sea, as well as shallower oceans. The echinoderms are important both ecologically and geologically. The first definitive echinoderms appeared near the start of the Cambrian. Echinoderms are the largest entirely marine phylum. The phylum contains about 7,000 living species, making it the second-largest grouping of deuterostomes, after the chordates. Adult echinoderms are found on the sea bed at every ocean depth, from the intertidal zone to the abyssal zone. The adults are recognisable by their (usually five-point) radial symmetry, and include starfish, brittle stars, sea urchins, sand dollars, and sea cucumbers, as well as the sea lilies or "stone lilies". Homalozoa † Gill & Caster, 1960 Cincta † Soluta † Stylophora † Ctenocystoidea † Robison & Sprinkle, 1969Ĭrinozoa Crinoidea Edrioasteroidea † Cystoidea † Rhombifera †Įchinozoa Echinoidea Holothuroidea Ophiocistioidea † Helicoplacoidea †īlastoidea † Cystoidea † von Buch, 1846 Eocrinoidea † Jaekel, 1899 Paracrinoidea † Regnéll, 1945Īn echinoderm ( / ɪ ˈ k aɪ n ə ˌ d ɜːr m, ˈ ɛ k ə-/) is any member of the phylum Echinodermata ( / ɪ ˌ k aɪ n oʊ ˈ d ɜːr m ə t ə/).
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