| 词条 | Bilateria |
| 释义 |
| name = Bilaterians | fossil_range = Ediacaran–Present, {{fossil range|560|0|refs=[1]}} | image = Animal_diversity_October_2007.jpg | image_caption = Diversity of bilaterians. | taxon = Bilateria | regnum = Animalia | subregnum = Eumetazoa | unranked_superphylum = ParaHoxozoa | unranked_subphylum = Bilateria | authority = Hatschek, 1888 | subdivision_ranks = Phyla | subdivision = * Proarticulata †
| synonyms = Triploblasts Lankester, 1873 }} The bilateria {{IPAc-en|ˌ|b|aɪ|l|ə|ˈ|t|ɪər|i|ə}}, bilaterians, or triploblasts, are animals with bilateral symmetry, i.e., they have a head (anterior) and a tail (posterior) as well as a back (dorsal) and a belly (ventral); therefore they also have a left side and a right side.[2] The bilateria are a major group of animals, including the majority of phyla but not sponges, ctenophores, placozoans, and cnidarians. 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 pseudocoeloms) or secondary cavities (that appear de novo, for example the coelom).[3][4] Body planSome of the earliest bilaterians were wormlike, and a bilaterian body can be conceptualized as a cylinder with a gut running between two openings, the mouth and the anus. Around the gut it has an internal body cavity, a coelom or pseudocoelom.{{efn|The earliest Bilateria may have had only a single opening, and no coelom.[5]}} Animals with this bilaterally symmetric body plan have a head (anterior) end and a tail (posterior) end as well as a back (dorsal) and a belly (ventral); therefore they also have a left side and a right side.[6][7] Having a front end means that this part of the body encounters stimuli, such as food, favouring cephalisation, the development of a head with sense organs and a mouth.[5] The body stretches back from the head, and many bilaterians have a combination of circular muscles that constrict the body, making it longer, and an opposing set of longitudinal muscles, that shorten the body;[7] these enable soft-bodied animals with a hydrostatic skeleton to move by peristalsis.[6] They also have a gut that extends through the basically cylindrical body from mouth to anus. Many bilaterian phyla have primary larvae which swim with cilia and have an apical organ containing sensory cells. However, there are exceptions to each of these characteristics; for example, adult echinoderms are radially symmetric (unlike their larvae), and certain parasitic worms have extremely simplified body structures.[7][8] EvolutionThe hypothetical most recent common ancestor of all bilateria is termed the "Urbilaterian".[9][10] 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 Ludwig von Graff, Elie Metchnikoff, Libbie Hyman, or {{ill|Luitfried von Salvini-Plawen|nl}}), 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 {{Interlanguage link|Gösta Jägersten|sv}}, 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 {{Ma|555}}.[11] Earlier fossils are controversial; the fossil Vernanimalcula may be the earliest known bilaterian, but may also represent an infilled bubble.[12][13] Fossil embryos are known from around the time of Vernanimalcula ({{Ma|580}}), but none of these have bilaterian affinities.[14] 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.[15] Phylogeny{{See also|List of bilateral animal orders}}There are two main lineages, superphyla, of Bilateria. The deuterostomes include the echinoderms, hemichordates, chordates, and a few smaller phyla. The protostomes include most of the rest, such as arthropods, annelids, mollusks, flatworms, and so forth. There are a number of differences, most notably in how the embryo develops. In particular, the first opening of the embryo becomes the mouth in protostomes, and the anus in deuterostomes. Many taxonomists now recognize at least two more superphyla among the protostomes, Ecdysozoa[16] (molting animals) and Spiralia.[16][17][18][19] The arrow worms (Chaetognatha) have proven difficult to classify; recent studies place them in the gnathifera.[20][21][22] A modern (2011) consensus phylogenetic tree for Bilateria is shown below, although the positions of certain clades are still controversial (dashed lines) and the tree has changed considerably between 2000 and 2010.[23][22][24][25][26] It is indicated when approximately clades radiated into newer clades in millions of years ago (Mya).[27] {{clade| style=font-size:85%;line-height:85%|label1=Planulozoa |sublabel1=680 mya |1={{clade |1=Cnidaria |label2=Bilateria |2={{clade |1=†Proarticulata |label2=Xenacoelomorpha |2={{clade |1=Xenoturbellida |label2=Acoelomorpha |2={{clade |1=Nemertodermatida |2=Acoela }} }} |label3=Nephrozoa |sublabel3=650 mya |3={{clade |label1=Deuterostomia |1={{clade |label1=Chordata |1={{clade |1=Cephalochordata |label2=Olfactores |2={{clade |1=Urochordata |2=Craniata/Vertebrata }} }} |3={{clade |1=†Saccorhytus coronarius |2={{clade |1=†Vetulocystids |2=†Vetulicolians }} }} |label2=Ambulacraria |2={{clade |1=Echinodermata |2=Hemichordata |state3=dashed |3=†Cambroernida }} }} |label2=Protostomia |sublabel2=610 mya |2={{clade |label1=Ecdysozoa|sublabel1=>529 mya |1={{clade |1={{clade |label1=Nematoida |1={{clade |1=Nematoda |2=Nematomorpha |2={{clade|state1=dashed |1=Loricifera |label2=Panarthropoda |2={{clade |1=Onychophora |label2=Tactopoda |2={{clade |1=Tardigrada |2=Arthropoda }} }} }} }} |label2=Scalidophora |2={{clade |1=Priapulida |2=Kinorhyncha }} }} |label2=Spiralia |2={{Clade |label1=Gnathifera |1={{Clade |1=Rotifera and allies |2=Chaetognatha |label2=Platytrochozoa |sublabel2=580 mya |2={{Clade |1=Platyhelminthes and allies |label2=Lophotrochozoa |sublabel2=550 mya |2={{Clade |1=Mollusca |2=Annelida and allies }} }} }} |3=†Kimberella }} }} }} }} }} Evolutionary originThe original bilaterian is hypothesized to have been a bottom dwelling worm with a single body opening.[28] It may have resembled the planula larvae of some cnidaria, which have some bilateral symmetry.[29] See also
Notes{{notelist}}References1. ^{{cite journal | last1 = Martin | first1 = M. W. | last2 = Grazhdankin | first2 = D. V | last3 = Bowring | first3 = S. A | last4 = Evans | first4 = D. A | last5 = Fedonkin | first5 = M. A | last6 = Kirschvink | first6 = J. 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3 : Bilaterians|Subkingdoms|Ediacaran first appearances |
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