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Title: Bilateria  
Author: World Heritage Encyclopedia
Language: English
Subject: Eumetazoa, Ctenophora, Animal, ParaHoxozoa, Flatworm
Collection: Animal Taxonomy, Animals, Ediacaran First Appearances
Publisher: World Heritage Encyclopedia


Temporal range: Cryogenian - Recent 850–0Ma
A Bowfin, Amia calva
Scientific classification
Domain: Eukaryota
(unranked): Opisthokonta
Kingdom: Animalia
Subkingdom: Eumetazoa
(unranked): Bilateria
Hatschek, 1888

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 1
  • Phylogeny 2
  • See also 3
  • References 4
  • External links 5


Illustration of the different types of symmetry in lifeforms (Field Museum, Chicago). Bilateral forms can have heads. Lifeforms with other types of symmetry have corresponding organs, if not a head.

The hypothetical last common ancestor of all bilateria is termed the "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 .[3] Earlier fossils are controversial; the fossil Vernanimalcula may be the earliest known bilaterian, but may also represent an infilled bubble.[4] Fossil embryos are known from around the time of Vernanimalcula (), but none of these have bilaterian affinities.[5] 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.[6]


There are two main superphyla (main lineages) of Bilateria. The deuterostomes include the echinoderms, hemichordates, chordates, and possibly 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[7] (molting animals) and Lophotrochozoa (also referred to as Spiralia).[7] Within the latter, some researchers also recognize another superphylum, Platyzoa,[8] while others reject the Platyzoa monophyly.[9][10][11] The arrow worms (Chaetognatha) have proven particularly difficult to classify, with some taxonomists placing them among the deuterostomes and others placing them among the protostomes. The two most recent studies to address the question of chaetognath origins support protostome affinities.[12][13]

A modern (2011) consensus phylogeny for bilateria is shown below, although the position of certain clades are still controversial and the tree has changed considerably between 2000 and 2010.[14] Nodes marked with * have received broad consensus. A prominent alternative tree is championed by Nielsen (2001).[15] [16]









Craniata (including Vertebrata)

































See also


  1. ^ 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 [1] — URL retrieved November 15, 2006
  2. ^ Balavoine, Guillaume, & Adoutte, Andre. 2003. The segmented Urbilateria: A testable scenario. Integrative & Comparative Biology 43: 137–147. Found at [2] — URL retrieved November 15, 2006
  3. ^ For refs see Ediacara biota
  4. ^ For refs see Vernanimalcula
  5. ^ For refs see Fossil embryos
  6. ^ Aubet, Natalie R. et al. (June 29, 2012). "Bilaterian burrows and grazing behavior at >585 million years ago". Science (American Association for the Advancement of Science) 336 (6089): 1693+.  
  7. ^ 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.  
  8. ^ Giribet, Gonzalo; at al (September 2000). "Triploblastic relationships with emphasis on the acoelomates and the position of Gnathostomulida, Cycliophora, Plathelminthes, and Chaetognatha: a combined approach of 18S rDNA sequences and morphology.". Systematic Biology 49 (3): 539–62.  
  9. ^ 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.  
  10. ^ Telford, Maximilian J. (2008). "Resolving Animal Phylogeny: A Sledgehammer for a Tough Nut?". Developmental Cell 14 (4): 457–459.  
  11. ^ The Invertebrate Animals
  12. ^ Helfenbein, Kevin G.; Fourcade, H. Matthew; Vanjani, Rohit G.; Boore, Jeffrey L. (2004). "The mitochondrial genome of Paraspadella gotoi is highly reduced and reveals that chaetognaths are a sister group to protostomes". Proceedings of the National Academy of Sciences of the United States of America 101 (29): 10639–10643.  
  13. ^ Papillon, Daniel; Perez, Yvan; Caubit, Xavier; Yannick Le, Parco (2004). "Identification of chaetognaths as protostomes is supported by the analysis of their mitochondrial genome". Molecular Biology and Evolution 21 (11): 2122–2129.  
  14. ^ Edgecombe, Gregory D.; Giribet, Gonzalo; Dunn, Casey W.; Hejnol, Andreas; Kristensen, Reinhardt M.; Neves, Ricardo C.; Rouse, Greg W.; Worsaae, Katrine; Sørensen, Martin V. (2011). "Higher-level metazoan relationships: recent progress and remaining questions". Organisms, Diversity and evolution 11 (2): 151.  
  15. ^ Nielsen, C. 2001. Animal Evolution: Interrelationships of the Living Phyla. Second Edition. Oxford University Press, Oxford.
  16. ^ "Bilateria". Tree of Life Web Project. 2002. Retrieved August 11, 2014. 

External links

  • Tree of Life web project — Bilateria
  • University of California Museum of Paleontology — Systematics of the Metazoa
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