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Brachiopods, phylum Brachiopoda, are marine animals that have hard "valves" (shells) on the upper and lower surfaces, unlike the left and right arrangement in bivalve molluscs. Brachiopod valves are hinged at the rear end, while the front can be opened for feeding or closed for protection. Two major groups are recognized, articulate and inarticulate. Articulate brachiopods have toothed hinges and simple opening and closing muscles, while inarticulate brachiopods have untoothed hinges and a more complex system of muscles used to keep the two halves aligned. In a typical brachiopod a stalk-like pedicle projects from an opening in one of the valves, known as the pedicle valve, keeping the animal anchored to the seabed but clear of silt that would obstruct the opening. The word "brachiopod" is formed from the Ancient Greek words βραχίων ("arm") and πούς ("foot"). They are often known as "lamp shells", since the curved shells of the class Terebratulida look rather like pottery oil-lamps. Lifespans range from three to over thirty years. Ripe gametes (ova or sperm) float from the gonads into the main coelom and then exit into the mantle cavity. The larvae of inarticulate brachiopods are miniature adults, with lophophores that enable the larvae to feed and swim for months until the animals become heavy enough to settle to the seabed. The planktonic larvae of articulate species do not resemble the adults, but rather look like blobs with yolk sacs, and remain among the plankton for only a few days before leaving the water column upon metamorphosing. In addition to the traditional classification of brachiopods into inarticulate and articulate, two approaches appeared in the 1990s: one approach groups the inarticulate Craniida with articulate brachiopods, since both use the same material in the mineral layers of their shell; the other approach makes the Craniida a third group, as their outer organic layer is different from that in either of the other two. However, some taxonomists believe it is premature to suggest higher levels of classification such as order and recommend a bottom-up approach that identifies genera and then groups these into intermediate groups. Traditionally, brachiopods have been regarded as members of, or as a sister group to, the deuterostomes, a superphylum that includes chordates and echinoderms. One type of analysis of the evolutionary relationships of brachiopods has always placed brachiopods as protostomes while another type has split between placing brachiopods among the protostomes or the deuterostomes. It was suggested in 2003 that brachiopods had evolved from an ancestor similar to ''Halkieria'', a slug-like Cambrian animal with "chain mail" on its back and a shell at the front and rear end; it was thought that the ancestral brachiopod converted its shells into a pair of valves by folding the rear part of its body under its front. However, new fossils found in 2007 and 2008 showed that the "chain mail" of tommotiids formed the tube of a sessile animal; one tommotiid resembled phoronids, which are close relatives or a subgroup of brachiopods, while the other tommotiid bore two symmetrical plates that might be an early form of brachiopod valves. Lineages of brachiopods that have both fossil and extant taxa appeared in the early Cambrian, Ordovician, and Carboniferous periods, respectively. Other lineages have arisen and then become extinct, sometimes during severe mass extinctions. At their peak in the Paleozoic era, the brachiopods were among the most abundant filter-feeders and reef-builders, and occupied other ecological niches, including swimming in the jet-propulsion style of scallops. Brachiopod fossils have been useful indicators of climate changes during the Paleozoic. However, after the Permian–Triassic extinction event, brachiopods recovered only a third of their former diversity. A study in 2007 concluded the brachiopods were especially vulnerable to the Permian–Triassic extinction, as they built calcareous hard parts (made of calcium carbonate) and had low metabolic rates and weak respiratory systems. It was often thought that brachiopods went into decline after the Permian–Triassic extinction, and were out-competed by bivalves, but a study in 1980 found both brachiopod and bivalve species increased from the Paleozoic to modern times, with bivalves increasing faster; after the Permian–Triassic extinction, brachiopods became for the first time less diverse than bivalves. Brachiopods live only in the sea, and most species avoid locations with strong currents or waves. The larvae of articulate species settle in quickly and form dense populations in well-defined areas while the larvae of inarticulate species swim for up to a month and have wide ranges. Brachiopods now live mainly in cold water and low light. Fish and crustaceans seem to find brachiopod flesh distasteful and seldom attack them. Among brachiopods, only the lingulids (''Lingula sp.''〔http://www.tandfonline.com/doi/pdf/10.1080/10042857.2013.800376〕) have been fished commercially, on a very small scale. One brachiopod species (''Coptothyrus adamsi'') may be a measure of environmental conditions around an oil terminal being built in Russia on the shore of the Sea of Japan. ==Description== 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「brachiopod」の詳細全文を読む スポンサード リンク
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