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The bilateria [ˌbɪlaˈtɛrɪ.a], 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]

Temporal range: EdiacaranPresent, 560–0 Ma[1]
Animal diversity October 2007.jpg
Diversity of bilaterians.
Scientific classification e
Kingdom: Animalia
Subkingdom: Eumetazoa
Clade: ParaHoxozoa
Clade: Bilateria
Hatschek, 1888

Triploblasts Lankester, 1873

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 planEdit

Idealised wormlike bilaterian body plan. With a cylindrical body and a direction of movement the animal has head and tail ends. Sense organs and mouth form the basis of the head. Opposed circular and longitudinal muscles enable peristaltic motion.

Some 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.[a] 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.[8] 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.[9] 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.[6][7]


The hypothetical most recent common ancestor of all bilateria is termed the "Urbilaterian".[10][11] 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 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 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 555 million years ago.[12] Earlier fossils are controversial; the fossil Vernanimalcula may be the earliest known bilaterian, but may also represent an infilled bubble.[13][14] Fossil embryos are known from around the time of Vernanimalcula (580 million years ago), but none of these have bilaterian affinities.[15] 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.[16]


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[17] (molting animals) and Spiralia.[17][18][19][20] The arrow worms (Chaetognatha) have proven difficult to classify; recent studies place them in the gnathifera.[21][22][23]

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.[24][23][25][26][27] In 2019 it was proposed that the Ambulacraria are sister to Xenacoelomorpha together forming the Xenambulacraria. Nephrozoa is shown here sans Ambulacraria. Also Deuterostomes was proposed to be obsoleted altogether.[28][29][30] It is indicated when approximately clades radiated into newer clades in millions of years ago (Mya).[31]
































>529 mya

Rotifera and allies  



Platyhelminthes and allies  



Annelida and allies  

550 mya
580 mya


610 mya

Saccorhytus coronarius



650 mya
680 mya

Evolutionary originEdit

The original bilaterian is hypothesized to have been a bottom dwelling worm with a single body opening.[5] It may have resembled the planula larvae of some cnidaria, which have some bilateral symmetry.[32]

See alsoEdit


  1. ^ The earliest Bilateria may have had only a single opening, and no coelom.[5]


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  7. ^ a b c Brusca, Richard C. (2016). Introduction to the Bilateria and the Phylum Xenacoelomorpha | Triploblasty and Bilateral Symmetry Provide New Avenues for Animal Radiation (PDF). Invertebrates. Sinauer Associates. pp. 345–372. ISBN 978-1605353753.
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  15. ^ Hagadorn, J. W.; Xiao, S.; Donoghue, P. C. J.; Bengtson, S.; Gostling, N. J.; Pawlowska, M.; Raff, E. C.; Raff, R. A.; Turner, F. R.; Chongyu, Y.; Zhou, C.; Yuan, X.; McFeely, M. B.; Stampanoni, M.; Nealson, K. H. (13 October 2006). "Cellular and Subcellular Structure of Neoproterozoic Animal Embryos". Science. 314 (5797): 291–294. Bibcode:2006Sci...314..291H. doi:10.1126/science.1133129. PMID 17038620.
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  17. ^ a b Halanych, K.; Bacheller, J.; Aguinaldo, A.; Liva, S.; Hillis, D.; Lake, J. (17 March 1995). "Evidence from 18S ribosomal DNA that the lophophorates are protostome animals". Science. 267 (5204): 1641–1643. Bibcode:1995Sci...267.1641H. doi:10.1126/science.7886451. PMID 7886451.
  18. ^ Paps, J.; Baguna, J.; Riutort, M. (14 July 2009). "Bilaterian phylogeny: a broad sampling of 13 nuclear genes provides a new Lophotrochozoa phylogeny and supports a paraphyletic basal Acoelomorpha". Molecular Biology and Evolution. 26 (10): 2397–2406. doi:10.1093/molbev/msp150. PMID 19602542.
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External linksEdit