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| image = Amia calva1.jpg | image_width = 225px | image_caption = A Bowfin, ''Amia calva'' | taxon = Bilateria | authority = Hatschek, 1888 | subdivision_ranks = Phyla | subdivision = * Orthonectida * Rhombozoa * Acoelomorpha (disputed) * Chaetognatha * Tullimonstrum † * Proarticulata † * Superphylum Deuterostomia * * Chordata * * Hemichordata * * Echinodermata * * Xenoturbellida * * Vetulicolia † * ''Protostomia (unranked)'' * * Superphylum Ecdysozoa * * * Scalidophora (unranked) * * * * Kinorhyncha * * * * Loricifera * * * * Priapulida * * * Nematoda * * * Nematomorpha * * * Panarthropoda (unranked) * * * * Lobopodia * * * * * Dinocarida † * * * * * "Xenusia" † * * * * * Onychophora * * * * * Tardigrada * * * * Arthropoda * * Superphylum Platyzoa * * * Platyhelminthes * * * Gastrotricha * * * Rotifera * * * Acanthocephala * * * Gnathostomulida * * * Micrognathozoa * * * Cycliophora * * Superphylum Lophotrochozoa * * * Sipuncula * * * Hyolitha † * * * Nemertea * * * Phoronida * * * Bryozoa * * * Entoprocta * * * Brachiopoda * * * Mollusca * * * Annelida * * * Echiura }} The bilateria are the animals with bilateral symmetry, i.e., they have a front and a back end, as well as an upside and downside, and therefore a left and a right. In contrast, radially symmetrical animals like jellyfish have a topside and downside, but no front and back. The bilateria are a major group of animals, including the majority of phyla but not sponges, cnidarians, placozoans and ctenophores. For the most part, bilateral embryos are triploblastic, having three germ layers: endoderm, mesoderm, and ectoderm. Nearly all are bilaterally symmetrical, or approximately so; the most notable exception is the echinoderms, which achieve near-radial symmetry as adults, but are bilaterally symmetrical as larvae. Except for a few phyla (i.e. flatworms and gnathostomulids), bilaterians have complete digestive tracts with a separate mouth and anus. Some bilaterians lack body cavities (acoelomates, i.e. Platyhelminthes, Gastrotricha and Gnathostomulida), while others display primary body cavities (deriving from the blastocoel, as pseudocoel) and/or secondary cavities (that appear ''de novo'', for example the coelom). ==Evolution== The hypothetical most recent common ancestor of all bilateria is termed the "Urbilaterian".〔Knoll, Andrew H. and Sean B. Carroll. (1999) Early Animal Evolution: Emerging Views from Comparative Biology and Geology. ''Science''. 25 June 1999: Vol. 284. no. 5423, pp. 2129–2137. Found at () — URL retrieved November 15, 2006〕〔Balavoine, Guillaume, & Adoutte, Andre. 2003. The segmented Urbilateria: A testable scenario. ''Integrative & Comparative Biology'' 43: 137–147. Found at () — URL retrieved November 15, 2006〕 The nature of the first bilaterian is a matter of debate. One side suggests that acoelomates gave rise to the other groups (planuloid-aceloid hypothesis by Graff, Metchnikoff, Hyman, or Salvini Plawen), while the other poses that the first bilaterian was a coelomate organism and the main acoelomate phyla (flatworms and gastrotrichs) have lost body cavities secondarily (the Archicoelomata hypothesis and its variations such as the Gastrea by Haeckel or Sedgwick, the Bilaterosgastrea by Jägersten, or the Trochaea by Nielsen). The first evidence of bilateria in the fossil record comes from trace fossils in Ediacaran sediments, and the first ''bona fide'' bilaterian fossil is ''Kimberella'', dating to .〔For refs see ''Ediacara biota''〕 Earlier fossils are controversial; the fossil ''Vernanimalcula'' may be the earliest known bilaterian, but may also represent an infilled bubble.〔For refs see ''Vernanimalcula''〕 Fossil embryos are known from around the time of ''Vernanimalcula'' (), but none of these have bilaterian affinities.〔For refs see Fossil embryos〕 Burrows believed to have been created by bilaterian life forms have been found in the Tacuarí Formation of Uruguay, and are believed to be at least 585 million years old. 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「bilateria」の詳細全文を読む スポンサード リンク
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