Dating and fish in the sea
Common usage frequently finds these names being also applied to ophiuroids, which are correctly referred to as brittle stars or "basket stars".About 1,500 species of starfish occur on the seabed in all the world's oceans, from the tropics to frigid polar waters.They are opportunistic feeders and are mostly predators on benthic invertebrates.Several species have specialized feeding behaviours including eversion of their stomachs and suspension feeding.The dermis contains an endoskeleton of calcium carbonate components known as ossicles.These are honeycombed structures composed of calcite microcrystals arranged in a lattice.They are sometimes collected as curios, used in design or as logos, and in some cultures, despite possible toxicity, they are eaten.The class Asteroidea belongs to the phylum Echinodermata.
The fossil record for starfish is ancient, dating back to the Ordovician around 450 million years ago, but it is rather poor, as starfish tend to disintegrate after death.It is not unusual in species that typically have five arms for some individuals to possess six or more through abnormal development.The body wall consists of a thin cuticle, an epidermis consisting of a single layer of cells, a thick dermis formed of connective tissue and a thin coelomic myoepithelial layer, which provides the longitudinal and circular musculature.They vary in form, with some bearing external granules, tubercles and spines, but most are tabular plates that fit neatly together in a tessellated manner and form the main covering of the aboral surface.Pedicellariae are compound ossicles with forceps-like jaws.
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The Asteroidea occupy several significant ecological roles.