An echinoderm (/ɪˈkaɪnəˌdɜːrm,ˈɛkə-/ )[2] is any member of the phylum Echinodermata (/ɪˌkaɪnoʊˈdɜːrmətə/; from Ancient Greek ἐχῖνος (ekhînos) 'hedgehog', and δέρμα (dérma) 'skin')[3] of marine animals. The adults are recognizable 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".[4] Adult echinoderms are found on the sea bed at every ocean depth, from the intertidal zone to the abyssal zone. The phylum contains about 7000 living species,[5] making it the second-largest grouping of deuterostomes (a superphylum), after the chordates (which include the vertebrates, such as birds, fishes, mammals, and reptiles). Echinoderms are the largest phylum that has no freshwater or terrestrial members.
Aside from the hard-to-classify Arkarua (a Precambrian animal with echinoderm-like pentamerous radial symmetry), the first definitive members of the phylum appeared near the start of the Cambrian. One group of Cambrian echinoderms, the cinctans (Homalozoa), which are close to the base of the echinoderm origin, has been found to possess external gills used for filter feeding, similar to those possessed by chordates and hemichordates.[6]
The echinoderms are important both ecologically and geologically. Ecologically, there are few other groupings so abundant in the biotic desert of 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 skeletons, 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.
Along with the chordates and hemichordates, echinoderms are deuterostomes, one of the two major divisions of the bilaterians, the other being the protostomes. During the early development of the embryo, in deuterostomes, the blastopore (the first opening to form) becomes the anus whereas in the protostomes, it becomes the mouth. In deuterostomes, the mouth develops at a later stage, at the opposite end of the blastula from the blastopore, and a gut forms connecting the two.[7] The larvae of echinoderms have bilateral symmetry but this is lost during metamorphosis when their bodies are reorganised and develop the characteristic radial symmetry of the echinoderm, typically pentamerism.[8] 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.[9]
Early analyses gave inconsistent results, the main hypotheses being that the Ophiuroidea were sister to the Asteroidea, or that they were sister to the (Holothuroidea + Echinoidea).[10] However, a 2014 analysis of 219 genes from all classes of echinoderms gave the following phylogenetic tree.[11] An independent analysis in 2015 of RNA transcriptomes from 23 species across all classes of echinoderms gave the same tree.[10]