The calcareous endoskeleton is made of many small plates that overlap under the skin, forming an armor and a structurâ¦ Starfish have sensory cells in the epithelium and have simple eyespots and touch-sensitive tentacle-like tube feet at the tips of their arms.  In many species of starfish, the large cardiac stomach can be everted and digest food outside the body. An echinoderm is a member of the phylum Echinodermata which contains a number of marine organisms recognized by their pentamerous radial symmetry, calcareous endoskeleton, and a water vascular system which helps operate their small podia. There are a total of about 7,000 extant species of echinoderm as well as about 13,000 extinct species. Each one of these, even the articulating spine of a sea urchin, is composed mineralogically of a crystal of calcite. Two main subdivisions are traditionally recognised: the more familiar motile Eleutherozoa, which encompasses the Asteroidea (starfish, 1,745 recent species), Ophiuroidea (brittle stars, 2,300 species), Echinoidea (sea urchins and sand dollars, 900 species) and Holothuroidea (sea cucumbers, 1,430 species); and the Pelmatozoa, some of which are sessile while others move around.  At the present time, some trials of breeding sea urchins in order to try to compensate the overexploitation of this resource have been made. Direct transdifferentiation of one type of tissue to another during tissue replacement is also observed. The secondary body cavity, the coelom, forms by the partitioning of three body cavities. , Sea urchins are used in research, particularly as model organisms in developmental biology and ecotoxicology. They are also common as fossils dating back 500 million years. It can also be pretty intimidating. Later, the left side of the body grows at the expense of the right side, which is eventually absorbed. The same everted stomach process is used by other starfish to feed on sponges, sea anemones, corals, detritus and algal films. , The first universally accepted echinoderms appear in the Lower Cambrian period, asterozoans appeared in the Ordovician and the crinoids were a dominant group in the Paleozoic. Department of Biology, The University of Alabama at Birmingham, Birmingham, Alabama 35294-1170, USA ABSTRACT: Antarctic echinoderms appear to be adapted to a benthic environment characterized by long-term low availability of food resources. In most of these species, asexual reproduction is by transverse fission with the disc splitting in two. Echinoderms have varied nutrition All echinoderms have a mouth , stomach , and intestines , but their methods of obtaining food vary.  It is not unusual to find starfish with arms of different sizes in various stages of regrowth. , Echinoderms form part of the diet of many organisms such as bony fish, sharks, eider ducks, gulls, crabs, gastropod molluscs, sea otters, Arctic foxes and humans. Others ingest large quantities of sediment, absorb the organic matter and pass the indigestible mineral particles through their guts. Regrowth of both the lost disc area and the missing arms occur so that an individual may have arms of varying lengths.  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.  Within the Asterozoa, there can be a few exceptions from the rule. One group of Cambrian echinoderms, the cinctans (Homalozoa), which are close to the base of the echinoderm origin, have been found to possess external gills used for filter feeding, similar to those possessed by chordates and hemichordates.. This tissue enables a starfish to change from moving flexibly around the seabed to becoming rigid while prying open a bivalve mollusc or preventing itself from being extracted from a crevice. Many live in cracks, hollows and burrows and hardly move at all. , The calcareous tests or shells of echinoderms are used as a source of lime by farmers in areas where limestone is unavailable and some are used in the manufacture of fish meal. , In 2010, 373,000 tonnes of echinoderms were harvested, mainly for consumption.  Some sea cucumbers use their buccal tentacles to transfer their eggs to their underside or back where they are retained. (aâe) Species from each of the five classes of echinoderms are shown. Some also use their articulated spines to push or lever themselves along or lift their oral surfaces off the substrate. They are nearly all gonochoric, though a few species are hermaphroditic. , Echinoderms possess a simple digestive system which varies according to the animal's diet. Asteroidea, starfish, constitutes an interesting group of animals for several reasons.  During development, echinoderm coelom is divided in metacoel, mesocoel and protocoel (also called somatocoel, hydrocoel and axocoel, respectively). Although most echinoderm spines are blunt, those of the crown-of-thorns starfish are long and sharp and can cause a painful puncture wound as the epithelium covering them contains a toxin. For example, a sea urchin has an 'echinopluteus' larva while a brittle star has an 'ophiopluteus' larva. In starfish, the female may carry the eggs in special pouches, under her arms, under her arched body or even in her cardiac stomach. 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