Editing Bilateria

{{Short description|Animals with embryonic bilateral symmetry}}
{{good article}}
{{Automatic taxobox
| fossil_range = [[Ediacaran]]–[[Holocene|Present]], {{fossil range|567|0|refs=<ref>{{cite journal |last=Grazhdankin |first=Dima |year=2004 |title=Patterns of distribution in the Ediacaran biotas: facies versus biogeography and evolution |url=http://paleobiol.geoscienceworld.org/cgi/reprint/30/2/203.pdf |journal=Paleobiology |volume=30 |issue=2 |pages=203–221 |doi=10.1666/0094-8373(2004)030<0203:PODITE>2.0.CO;2 |bibcode=2004Pbio...30..203G |s2cid=129376371}}</ref>}}
| image = EB1911 Crustacea Fig. 12.—Nauplius of a Prawn.jpg
| image_caption = Many animals have [[bilateral symmetry]], at least at the [[embryo]] stage, providing the name for the clade. [[Nauplius (larva)|Nauplius larva]] illustrated.
| display_parents = 2
| taxon = Bilateria
| authority = [[Berthold Hatschek|Hatschek]], 1888
| subdivision_ranks = Subdivisions
| subdivision_ref = <ref name=G&E>{{cite book |last1=Giribet |first1=Gonzalo |last2=Edgecombe |first2=Gregory |title=The Invertebrate Tree of Life |date=3 March 2020 |publisher=[[Princeton University Press]]}}</ref>
| subdivision = * †[[Proarticulata]]?
* [[Xenacoelomorpha]]
* [[Nephrozoa]]
** †''[[Ikaria wariootia|Ikaria]]''
**[[Deuterostomia]]
*** [[Chordata]]
*** [[Ambulacraria]]
** [[Protostomia]]
*** [[Ecdysozoa]]
*** [[Spiralia]]

See [[#Phylogeny|text]] for alternative relationships.
| synonyms = Triploblasta <small>Lankester, 1873</small>
}}

'''Bilateria''' ({{IPAc-en|ˌ|b|aɪ|l|ə|ˈ|t|ɪər|i|ə}})<ref>{{cite Merriam-Webster|bilateria|accessdate=2024-05-12}}</ref> is a large [[clade]] of [[animal]]s characterised by [[bilateral symmetry]] during [[embryonic development]]. This means their [[body plan]]s are laid around a longitudinal axis with a front (or "head") and a rear (or "tail") end, as well as a left–right–symmetrical belly ([[ventral]]) and back ([[dorsal (anatomy)|dorsal]]) surface.<!--<ref name=Brusca2016/>--> Nearly all bilaterians maintain a bilaterally symmetrical body as adults; the most notable exception is the [[echinoderm]]s, which have [[pentaradial symmetry]] as adults, but  bilateral symmetry as [[embryo]]s. With few exceptions, bilaterian embryos are [[triploblastic]], having three [[germ layer]]s: [[endoderm]], [[mesoderm]] and [[ectoderm]], and have complete [[digestive tract]]s with a separate [[mouth]] and [[anus]]. Some bilaterians lack [[body cavities]], while others have a [[Pseudocoelomata|primary body cavity]] derived from the [[blastocoel]],  or a secondary cavity, the [[Coelomata|coelom]]. [[Cephalization]] is a characteristic feature among most bilaterians, where the [[sense]] organs and [[central nervous system|central nerve]] [[ganglia]] become concentrated at the front end of the animal.<!--Lead is only a summary of cited materials in the article body, please do not add anything "new" up here that isn't there already, thanks-->

Bilaterians constitute one of the five main lineages of animals, the other four being [[Porifera]] (sponges), [[Cnidaria]] ([[jellyfish]], [[hydrozoa]]ns, [[sea anemone]]s and [[coral]]s), [[Ctenophora]] (comb jellies) and [[Placozoa]]. They rapidly diversified in the late [[Ediacaran]] and the [[Cambrian]], and are now by far the most successful animal lineage, with over 98% of known animal species.<!--<ref name="Cat of Life"/>--> Bilaterians are traditionally classified as either [[deuterostome]]s or [[protostome]]s, based on whether the [[blastopore]] becomes the anus or mouth.<!--<ref name="Hejnol 2009"/>--> The recently erected phylum [[Xenacoelomorpha]], once thought to be [[flatworms]], has provided an extra challenge to bilaterian taxonomy, as they likely do not belong to either group.<!--Lead is only a summary of cited materials in the article body, please do not add anything "new" up here that isn't there already, thanks-->

== Body plan ==

Animals with a bilaterally symmetric [[body plan]] that mainly move in one direction have a head end (anterior)<!--necessary! not implying there's an actual head--> 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.<ref name="Minelli2009"/><ref name=Brusca2016>{{cite book |chapter-url=http://www.sinauer.com/media/wysiwyg/samples/Brusca3e_Chapter_9.pdf |url-status=dead |archive-url=https://web.archive.org/web/20200605145237/http://www.sinauer.com/media/wysiwyg/samples/Brusca3e_Chapter_9.pdf |archive-date=5 June 2020 |chapter=Introduction to the Bilateria and the Phylum Xenacoelomorpha: Triploblasty and Bilateral Symmetry Provide New Avenues for Animal Radiation |title=Invertebrates |last=Brusca |first=Richard C. |date=2016 |publisher=Sinauer Associates |pages=345–372 |isbn=978-1-60535-375-3 }}</ref> 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 organ]]s and a mouth.<ref name=Finnerty>{{cite journal |last=Finnerty |first=John R. |title=Did internal transport, rather than directed locomotion, favor the evolution of bilateral symmetry in animals? |journal=BioEssays |date=November 2005 |volume=27 |issue=11 |pages=1174–1180 |doi=10.1002/bies.20299 |pmid=16237677 |url=http://faculty.weber.edu/rmeyers/PDFs/Finnerty%20-%20symmetry%20evol.pdf |access-date=2018-03-07 |archive-url=https://web.archive.org/web/20190702004055/http://faculty.weber.edu/rmeyers/PDFs/Finnerty%20-%20symmetry%20evol.pdf |archive-date=2019-07-02 |url-status=dead }}</ref> Most bilaterians ([[nephrozoa]]ns) have a gut that extends through the body from mouth to anus (sometimes called a "through gut"<ref>{{Cite journal |last1=Nielsen |first1=Claus |last2=Brunet |first2=Thibaut |last3=Arendt |first3=Detlev |date=2018-08-22 |title=Evolution of the bilaterian mouth and anus |url=https://www.nature.com/articles/s41559-018-0641-0 |journal=Nature Ecology & Evolution |language=en |volume=2 |issue=9 |pages=1358–1376 |doi=10.1038/s41559-018-0641-0 |pmid=30135501 |bibcode=2018NatEE...2.1358N |issn=2397-334X}}</ref>), and sometimes a wormlike body plan with a [[hydrostatic skeleton]]. [[Xenacoelomorpha|Xenacoelomorphs]], on the other hand, have a bag gut with one opening. Many bilaterian phyla have primary [[larva]]e which swim with [[cilia]] and have an apical organ containing sensory cells.<ref name="Minelli2009">{{cite book |last=Minelli |first=Alessandro |title=Perspectives in Animal Phylogeny and Evolution |url=https://books.google.com/books?id=jIASDAAAQBAJ&pg=PA53 |year=2009 |publisher=Oxford University Press |isbn=978-0-19-856620-5 |page=53}}</ref><ref name=Brusca2016/>

Some bilaterians have only weakly condensed nerve nets (similar to those in [[cnidarians]]), while others have either a [[ventral nerve cord]], a [[dorsal nerve cord]], or both (e.g. in [[Hemichordate]]).<ref>https://pmc.ncbi.nlm.nih.gov/articles/PMC5756474/</ref>

== Evolution ==

=== Common ancestor ===

{{main|Urbilaterian}}

The hypothetical [[most recent common ancestor]] of all Bilateria is termed the '[[urbilaterian]]'. The nature of this 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}}). This means that the urbilaterian had a solid body, and all body cavities therefore secondarily arose later in different groups. The other side poses that the urbilaterian had a coelom, meaning that the main acoelomate phyla ([[flatworm]]s and [[gastrotrich]]s) have secondarily lost their body cavities.<ref>{{cite journal |last1=Knoll |first1=Andrew H. |last2=Carroll |first2=Sean B. |author2-link=Sean B. Carroll |s2cid=8908451 |date=25 June 1999 |title=Early Animal Evolution: Emerging Views from Comparative Biology and Geology |journal=Science |volume=284 |issue=5423 |pages=2129–2137 |doi=10.1126/science.284.5423.2129 |pmid=10381872}}</ref><ref>{{cite journal |last1=Balavoine |first1=G. |last2=Adoutte |first2=Andre |title=The segmented Urbilateria: A testable scenario |journal=Integrative and Comparative Biology |date=2003 |volume=43 |issue=1 |pages=137–147 |doi=10.1093/icb/43.1.137 |pmid=21680418 |citeseerx=10.1.1.560.8727 |s2cid=80975506 }}</ref> 
This is the Archicoelomata hypothesis first proposed by A. T. Masterman in 1899.<ref name="Masterman 1899">{{cite journal |last=Masterman |first=A. T. |title=On the Theory of Archimeric Segmentation and its bearing upon the Phyletic Classification of the Cœlomata |journal=Proceedings of the Royal Society of Edinburgh |volume=22 |date=1899 |doi=10.1017/S0370164600051245 |pages=270–310}}</ref> Variations of the Archicoelomata hypothesis are the [[Gastraea]] by [[Ernst Haeckel]] in 1872<ref name="Levit 2022">{{cite journal |last1=Levit |first1=Georgy S. |last2=Hoßfeld |first2=Uwe |last3=Naumann |first3=Benjamin |last4=Lukas |first4=Paul |last5=Olsson |first5=Lennart |title=The biogenetic law and the Gastraea theory: From Ernst Haeckel's discoveries to contemporary views |journal=Journal of Experimental Zoology Part B: Molecular and Developmental Evolution |volume=338 |issue=1–2 |date=2022 |doi=10.1002/jez.b.23039 |doi-access=free |pages=13–27|pmid=33724681 |bibcode=2022JEZB..338...13L }}</ref> or [[Adam Sedgwick (zoologist)|Adam Sedgwick]], and more recently the Bilaterogastrea by {{Interlanguage link|Gösta Jägersten|sv}},<ref name="Olsson 2007">{{cite journal |last=Olsson |first=Lennart |title=A clash of traditions: the history of comparative and experimental embryology in Sweden as exemplified by the research of Gösta Jägersten and Sven Hörstadius |journal=Theory in Biosciences |volume=126 |issue=4 |date=2007 |doi=10.1007/s12064-007-0008-6 |pages=117–129|pmid=18008099 }}</ref> and the Trochaea by Claus Nielsen.<ref name="Nielsen 1985">{{cite journal |last=Nielsen |first=Claus |title=Animal phylogeny in the light of the trochaea theory |journal=Biological Journal of the Linnean Society |volume=25 |issue=3 |date=1985 |doi=10.1111/j.1095-8312.1985.tb00396.x |pages=243–299}}</ref>

[[File:Xenoturbella japonica.jpg|thumb|One view is that the original bilaterian was a marine worm somewhat like ''[[Xenoturbella]]''.]]

One proposal, by Johanna Taylor Cannon and colleagues, is that the original bilaterian was a bottom dwelling worm with a single body opening, similar to ''[[Xenoturbella]]''.<ref name=Cannon2016>{{Cite journal |last1=Cannon |first1=Johanna Taylor |last2=Vellutini |first2=Bruno Cossermelli |last3=Smith |first3=Julian |last4=Ronquist |first4=Fredrik |last5=Jondelius |first5=Ulf |last6=Hejnol |first6=Andreas |title=Xenacoelomorpha is the sister group to Nephrozoa |journal=Nature |volume=530 |issue=7588 |pages=89–93 |doi=10.1038/nature16520 |pmid=26842059 |year=2016 |bibcode=2016Natur.530...89C |s2cid=205247296 |url=http://urn.kb.se/resolve?urn=urn:nbn:se:nrm:diva-1844 }}</ref> An alternative proposal, by Jaume Baguñà and colleagues, is that it may have resembled the [[planula]] larvae of some [[cnidaria]]ns, which unlike the radially-symmetric adults have some bilateral symmetry.<ref>{{Cite journal |last1=Baguñà |first1=Jaume |last2=Martinez |first2=Pere |last3=Paps |first3=Jordi |last4=Riutort |first4=Marta |date=April 2008 |title=Back in time: a new systematic proposal for the Bilateria |journal=Philosophical Transactions of the Royal Society B: Biological Sciences |volume=363 |issue=1496 |pages=1481–1491 |doi=10.1098/rstb.2007.2238 |pmc=2615819 |pmid=18192186}}</ref> However, [[Lewis I. Held]] presents evidence that it was segmented, as the mechanism for creating segments is shared between vertebrates ([[deuterostome]]s) and arthropods ([[protostome]]s).<ref name="Held 2014">{{cite book |last=Held |first=Lewis I. |author-link=Lewis I. Held |title=How the Snake Lost its Legs. Curious Tales from the Frontier of Evo-Devo |title-link=How the Snake Lost its Legs |date=2014 |publisher=[[Cambridge University Press]] |isbn=978-1-107-62139-8 |page=11 }}</ref>

Bilaterians, presumably including the urbilaterian, share many more [[Hox gene]]s [[Evolutionary developmental biology|controlling the development]] of their more complex bodies, including [[Cephalization|of their heads]], than do the Cnidaria and the Acoelomorpha.<ref name="Hombria 2021">{{cite journal |last1=Hombría |first1=James C.-G. |last2=García-Ferrés |first2=Mar |last3=Sánchez-Higueras |first3=Carlos |title=Anterior Hox Genes and the Process of Cephalization |journal=Frontiers in Cell and Developmental Biology |volume=9 |date=5 August 2021 |pmid=34422836 |pmc=8374599 |doi=10.3389/fcell.2021.718175 |doi-access=free}}<!--paper is CC-by-SA 4.0, could use image(s)--></ref>

=== Fossil record ===

[[File:Ikaria wariootia.jpg|thumb|upright|''[[Ikaria wariootia]]'', living 571–539 million years ago, is one of the oldest bilaterians identified.<ref>{{cite journal |last1=Evans |first1=Scott D. |last2=Hughes |first2=Ian V. |last3=Gehling |first3=James G. |last4=Droser |first4=Mary L. |title=Discovery of the oldest bilaterian from the Ediacaran of South Australia |journal=Proceedings of the National Academy of Sciences |date=7 April 2020 |volume=117 |issue=14 |pages=7845–7850 |doi=10.1073/pnas.2001045117 |pmid=32205432 |pmc=7149385 |bibcode=2020PNAS..117.7845E |doi-access=free }}</ref>]]

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}}.<ref name=Fedonkin1997>{{Cite journal |last1=Fedonkin |first1=M. A. |last2=Waggoner |first2=B. M. |date=November 1997 |title=The Late Precambrian fossil Kimberella is a mollusc-like bilaterian organism |journal=Nature |volume=388 |issue=6645 |pages=868–871 |doi=10.1038/42242 |bibcode=1997Natur.388..868F|s2cid=4395089 |doi-access=free }}</ref> Earlier fossils are controversial; the fossil ''[[Vernanimalcula]]'' may be the earliest known bilaterian, but may also represent an infilled bubble.<ref name=Bengtson2004>{{cite journal |last1=Bengtson |first1=S. |last2=Budd |first2=G. |date=19 November 2004 |title=Comment on 'small bilaterian fossils from 40 to 55 million years before the Cambrian'. |journal=Science |volume=306 |pages=1291a |doi=10.1126/science.1101338 |pmid=15550644 |issue=5700 |doi-access=free }}</ref><ref name=VernAcritarch>{{cite journal |last1=Bengtson |first1=S. |last2=Donoghue |first2=P. C. J. |last3=Cunningham |first3=J. A. |author4=Yin, C. |year=2012 |title=A merciful death for the 'earliest bilaterian,' Vernanimalcula |journal=Evolution & Development |volume=14 |issue=5 |pages=421–427 |doi=10.1111/j.1525-142X.2012.00562.x |pmid=22947315 |s2cid=205675058 |url=http://urn.kb.se/resolve?urn=urn:nbn:se:nrm:diva-597 }}</ref> [[Fossil embryos]] are known from around the time of ''Vernanimalcula'' ({{Ma|580}}), but none of these have bilaterian affinities.<ref name=Hagadorn2006>{{Cite journal |last1=Hagadorn |first1=J. W. |last2=Xiao |first2=S. |last3=Donoghue |first3=P. C. J. |last4=Bengtson |first4=S. |last5=Gostling |first5=N. J. |last6=Pawlowska |first6=M. |last7=Raff |first7=E. C. |last8=Raff |first8=R. A. |last9=Turner |first9=F. R. |last10=Chongyu |doi=10.1126/science.1133129 |first10=Y. |last11=Zhou |first11=C. |last12=Yuan |first12=X. |last13=McFeely |first13=M. B. |last14=Stampanoni |first14=M. |last15=Nealson |first15=K. H. |s2cid=25112751 |title=Cellular and Subcellular Structure of Neoproterozoic Animal Embryos |journal=Science |volume=314 |issue=5797 |pages=291–294 |date=13 October 2006 |pmid= 17038620|bibcode=2006Sci...314..291H }}</ref> Burrows believed to have been created by bilaterian life forms have been found in the [[Tacuarí Formation]] of Uruguay, and were believed to be at least 585 million years old.<ref name="sci">{{cite journal |title=Bilaterian burrows and grazing behavior at >585 million years ago |last1=Pecoits |first1=E. |last2=Konhauser |first2=K. O. |last3=Aubet |first3=N. R. |last4=Heaman |first4=L. M. |last5=Veroslavsky |first5=G. |last6=Stern |first6=R. A. |last7=Gingras |first7=M. K. |s2cid=27970523 |journal=Science |volume=336 |issue=6089 |pages=1693–1696 |doi=10.1126/science.1216295 |pmid=22745427 |date=June 29, 2012 |bibcode=2012Sci...336.1693P}}</ref> However, more recent evidence shows these fossils are actually late Paleozoic, not Ediacaran.<ref>{{cite journal |last1=Verde |first1=Mariano |title=Revisiting the supposed oldest bilaterian trace fossils from Uruguay: Late Paleozoic, not Ediacaran |journal=Palaeogeography, Palaeoclimatology, Palaeoecology |date=15 September 2022 |volume=602 |doi=10.1016/j.palaeo.2022.111158 |bibcode=2022PPP...60211158V |url=https://www.sciencedirect.com/science/article/abs/pii/S0031018222003285}}</ref>

=== Phylogeny ===

{{further|Protostome|Deuterostome}}

{{See also|List of bilateral animal orders}}

The Bilateria are now by far the most successful animal lineage, with over 98% of known animal species.<ref name="Cat of Life">{{cite web |title=Animalia |url=https://www.catalogueoflife.org/ |website=The Catalogue of Life |access-date=18 February 2025}}</ref> The group has traditionally been divided into two main lineages or [[superphyla]].<ref name=Nielsen-2008>{{cite journal |last=Nielsen |first=Claus |title=Six major steps in animal evolution: are we derived sponge larvae? |journal=Evolution and Development |year=2008 |volume=10 |issue=2 |pages=241–257 |doi=10.1111/j.1525-142X.2008.00231.x |pmid=18315817 |s2cid=8531859 }}</ref> The deuterostomes traditionally include the [[echinoderm]]s, [[hemichordate]]s, [[chordate]]s, and the extinct [[Vetulicolia]]. The protostomes include most of the rest, such as [[arthropod]]s, [[annelid]]s, [[mollusc]]s, and flatworms. There are several 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 [[Taxonomy (biology)|taxonomists]] now recognise at least two more superphyla among the protostomes, [[Ecdysozoa]]<ref name="Halanych 1641–1643">{{cite journal |last1=Halanych |first1=K. |author2=Bacheller, J. |author3=Aguinaldo, A. |author4=Liva, S. |author5=Hillis, D. |author6=Lake, J. |date=17 March 1995 |title=Evidence from 18S ribosomal DNA that the lophophorates are protostome animals |journal=Science |volume=267 |issue=5204 |pages=1641–1643 |doi=10.1126/science.7886451 |pmid=7886451 |bibcode=1995Sci...267.1641H |s2cid=12196991 }}</ref> and [[Spiralia]].<ref name="Halanych 1641–1643"/><ref>{{cite journal |last1=Paps |first1=J. |author2=Baguna, J. |author3=Riutort, M. |title=Bilaterian phylogeny: a broad sampling of 13 nuclear genes provides a new Lophotrochozoa phylogeny and supports a paraphyletic basal Acoelomorpha |journal=Molecular Biology and Evolution |date=14 July 2009 |volume=26 |issue=10 |pages=2397–2406 |doi=10.1093/molbev/msp150 |doi-access=free |pmid=19602542}}</ref><ref>{{cite journal |last=Telford |first=Maximilian J. |date=15 April 2008 |title=Resolving animal phylogeny: A sledgehammer for a tough nut? |journal=Developmental Cell |volume=14 |issue=4 |pages=457–459 |doi=10.1016/j.devcel.2008.03.016 |pmid=18410719 |doi-access=free }}</ref> The arrow worms ([[Chaetognatha]]) have proven difficult to classify; recent studies place them in the [[gnathifera (clade)|Gnathifera]].<ref>{{cite journal |last1=Helfenbein |first1=Kevin G. |last2=Fourcade |first2=H. Matthew |last3=Vanjani |first3=Rohit G. |last4=Boore |first4=Jeffrey L. |date=20 July 2004 |title=The mitochondrial genome of ''Paraspadella gotoi'' is highly reduced and reveals that chaetognaths are a sister group to protostomes |journal=[[Proceedings of the National Academy of Sciences of the United States of America]] |volume=101 |issue=29 |pages=10639–10643 |doi=10.1073/pnas.0400941101|pmid=15249679 |pmc=489987 |bibcode=2004PNAS..10110639H |doi-access=free }}</ref><ref>{{cite journal |last1=Papillon |first1=Daniel |last2=Perez |first2=Yvan |last3=Caubit |first3=Xavier |last4=Yannick Le |first4=Parco |date=November 2004 |title=Identification of chaetognaths as protostomes is supported by the analysis of their mitochondrial genome |journal=Molecular Biology and Evolution |volume=21 |issue=11 |pages=2122–2129 |doi=10.1093/molbev/msh229|pmid=15306659 |doi-access=free }}</ref><ref name="Fröbius">{{Cite journal |last1=Fröbius |first1=Andreas C. |last2=Funch |first2=Peter |date=2017-04-04 |title=Rotiferan Hox genes give new insights into the evolution of metazoan bodyplans |journal=Nature Communications |volume=8 |issue=1 |pages=9 |doi=10.1038/s41467-017-00020-w |pmid=28377584 |pmc=5431905 |bibcode=2017NatCo...8....9F }}</ref>

The traditional division of Bilateria into Deuterostomia and Protostomia was challenged when new morphological and molecular evidence supported a sister relationship between the acoelomate taxa, [[Acoela]] and [[Nemertodermatida]] (together called [[Acoelomorpha]]), and the remaining bilaterians.<ref name=Nielsen-2008/><ref name="Hejnol 2009">{{cite book |chapter-url=https://www.researchgate.net/publication/230766195 |chapter=The mouth, the anus, and the blastopore - open questions about questionable openings |title=Animal Evolution — Genomes, Fossils, and Trees |year=2009 |editor=M. J. Telford |editor2=D. T. J. Littlewood |last1=Hejnol |first1= A. |last2=Martindale |first2=M. Q. |pages=33–40}}</ref><ref>{{cite journal |doi=10.1038/s41559-016-0005 |title=The developmental basis for the recurrent evolution of deuterostomy and protostomy |journal=Nature Ecology & Evolution| volume=1 |page=0005 |year=2016 |last1=Martín-Durán |first1=José M. |last2=Passamaneck |first2=Yale J. |last3=Martindale |first3=Mark Q. |last4=Hejnol |first4=Andreas |issue=1 |url=https://qmro.qmul.ac.uk/xmlui/handle/123456789/54816 |pmid=28812551 |s2cid=90795 }}</ref> The latter clade was called Nephrozoa by Jondelius et al. (2002) and [[Eubilateria]] by Baguña and Riutort (2004).<ref name=Nielsen-2008/> The acoelomorph taxa had previously been considered flatworms with secondarily lost characteristics, but the new relationship suggested that the simple acoelomate worm form was the original bilaterian body plan and that the coelom, the digestive tract, excretory organs, and nerve cords developed in the Nephrozoa.<ref name=Nielsen-2008/><ref name=Cannon-2016>{{Cite journal |last1=Cannon |first1=Johanna Taylor |last2=Vellutini |first2=Bruno Cossermelli |last3=Smith |first3=Julian |last4=Ronquist |first4=Fredrik |last5=Jondelius |first5=Ulf |last6=Hejnol |first6=Andreas |title=Xenacoelomorpha is the sister group to Nephrozoa |year=2016 |journal=Nature |volume=530 |issue=7588 |pages=89–93 |doi=10.1038/nature16520|pmid=26842059 |url=http://urn.kb.se/resolve?urn=urn:nbn:se:nrm:diva-1844 |bibcode=2016Natur.530...89C |s2cid=205247296 }}</ref> Subsequently, the acoelomorphs were placed in phylum Xenacoelomorpha, together with the [[xenoturbellid]]s, and the sister relationship between Xenacoelomorpha and Nephrozoa supported in phylogenomic analyses.<ref name=Cannon-2016/>

A [[cladogram]] for Bilateria under the Nephrozoa hypothesis from a 2014 review by Casey Dunn and colleagues, is shown below.<ref name="Dunn Giribet Edgecombe Hejnol 2014">{{cite journal |last1=Dunn |first1=Casey W. |last2=Giribet |first2=Gonzalo |last3=Edgecombe |first3=Gregory D. |last4=Hejnol |first4=Andreas |title=Animal Phylogeny and Its Evolutionary Implications |journal=Annual Review of Ecology, Evolution, and Systematics |volume=45 |issue=1 |date=23 November 2014 |doi=10.1146/annurev-ecolsys-120213-091627 |pages=371–395}}</ref> The cladogram indicates approximately when some clades radiated into newer clades, in millions of years ago (Mya).<ref>{{Cite journal |last1=Peterson |first1=Kevin J. |last2=Cotton |first2=James A.|last3=Gehling |first3=James G. |last4=Pisani |first4=Davide |date=2008-04-27 |title=The Ediacaran emergence of bilaterians: congruence between the genetic and the geological fossil records |journal=[[Philosophical Transactions of the Royal Society B|Philosophical Transactions of the Royal Society of London B: Biological Sciences]] |volume=363 |issue=1496 |pages=1435–1443 |doi=10.1098/rstb.2007.2233 |pmid=18192191 |pmc=2614224 |df=dmy-all}}</ref> 

{{clade |style=font-size:85%;line-height:85%
|label1='''Bilateria''' 
|1={{clade
   |label1=[[Xenacoelomorpha]] 
   |1={{clade
      |1=[[Xenoturbellida]] [[File:Xenoturbella japonica.jpg|70 px]]
      |2=[[Acoelomorpha]] [[File:Proporus sp.png|60 px]]
      }}
   |label2= [[Nephrozoa]] 
   |sublabel2=650 Mya
   |2={{clade
      |label1=[[Deuterostomia]] 
      |sublabel1=''anus before mouth''
      |1={{clade
         |1=[[Ambulacraria]] [[File:Portugal 20140812-DSC01434 (21371237591).jpg|60 px]]
         |2=[[Chordata]] [[File:Blueback herring fish (white background).jpg|60 px]]
         }}
      |label2= [[Protostomia]] 
      |sublabel2=610 mya
      |2={{clade
         |1=[[Ecdysozoa]] [[File:Long nosed weevil edit.jpg|60 px]]
         |2=[[Spiralia]] [[File:Grapevinesnail 01.jpg|60 px]]
         }}
      }}
   }}
}}

A different hypothesis is that Ambulacraria is sister to Xenacoelomorpha, together forming [[Xenambulacraria]]. Xenambulacraria may be sister to Chordata or to [[Centroneuralia]] (corresponding to Nephrozoa without Ambulacraria, or, as shown here, to Chordata + Protostomia).<ref>{{Cite journal |last1=Kapli |first1=Paschalia |last2=Natsidis |first2=Paschalis |last3=Leite |first3=Daniel J. |last4=Fursman |first4=Maximilian |last5=Jeffrie |first5=Nadia |last6=Rahman |first6=Imran A. |last7=Philippe |first7=Hervé |last8=Copley |first8=Richard R. |last9=Telford |first9=Maximilian J. |display-authors=5 |date=2021-03-19 |title=Lack of support for Deuterostomia prompts reinterpretation of the first Bilateria |journal=Science Advances |volume=7 |issue=12 |pages=eabe2741 |doi=10.1126/sciadv.abe2741 |pmc=7978419 |pmid=33741592 |bibcode=2021SciA....7.2741K }}</ref> A 2019 study by Hervé Philippe and colleagues presents the tree, cautioning that "the support values are very low, meaning there is no solid evidence to refute the traditional protostome and deuterostome dichotomy".<ref name="PhilippePoustka2019">{{cite journal |last1=Philippe |first1=Hervé |last2=Poustka |first2=Albert J. |last3=Chiodin |first3=Marta |last4=Hoff |first4=Katharina J. |last5=Dessimoz |first5=Christophe |last6=Tomiczek |first6=Bartlomiej |last7=Schiffer |first7=Philipp H. |last8=Müller |first8=Steven |last9=Domman |first9=Daryl |last10=Horn |first10=Matthias |last11=Kuhl |first11=Heiner |last12=Timmermann |first12=Bernd |last13=Satoh |first13=Noriyuki |last14=Hikosaka-Katayama |first14=Tomoe |last15=Nakano |first15=Hiroaki |last16=Rowe |first16=Matthew L. |last17=Elphick |first17=Maurice R. |last18=Thomas-Chollier |first18=Morgane |last19=Hankeln |first19=Thomas |last20=Mertes |first20=Florian |last21=Wallberg |first21=Andreas |last22=Rast |first22=Jonathan P. |last23=Copley |first23=Richard R. |last24=Martinez |first24=Pedro |last25=Telford |first25=Maximilian J. |display-authors=5 |title=Mitigating Anticipated Effects of Systematic Errors Supports Sister-Group Relationship between Xenacoelomorpha and Ambulacraria |journal=Current Biology |volume=29 |issue=11 |date=2019 |doi=10.1016/j.cub.2019.04.009 |doi-access=free |pages=1818–1826.e6|pmid=31104936 |bibcode=2019CBio...29E1818P |hdl=21.11116/0000-0004-DC4B-1 |hdl-access=free }}</ref> As of 2024, the issue of which hypothesis is correct remains unresolved.<ref>{{Cite journal |last1=Abalde |first1=Samuel |last2=Jondelius |first2=Ulf |date=2025-02-10 |editor-last=Whelan |editor-first=Nathan |title=A Phylogenomic Backbone for Acoelomorpha Inferred From Transcriptomic Data |url=https://academic.oup.com/sysbio/article/74/1/70/7841810 |journal=Systematic Biology |volume=74 |issue=1 |pages=70–85 |doi=10.1093/sysbio/syae057 |issn=1063-5157 |pmc=11809588 |pmid=39451056}}</ref><ref>{{Cite journal |last1=Robertson |first1=Helen E. |last2=Sebé-Pedrós |first2=Arnau |last3=Saudemont |first3=Baptiste |last4=Loe-Mie |first4=Yann |last5=Zakrzewski |first5=Anne-C. |last6=Grau-Bové |first6=Xavier |last7=Mailhe |first7=Marie-Pierre |last8=Schiffer |first8=Philipp |last9=Telford |first9=Maximilian J. |last10=Marlow |first10=Heather |date=2024-03-19 |title=Single cell atlas of Xenoturbella bocki highlights limited cell-type complexity |journal=Nature Communications |volume=15 |issue=1 |page=2469 |bibcode=2024NatCo..15.2469R |doi=10.1038/s41467-024-45956-y |issn=2041-1723 |pmc=10951248 |pmid=38503762}}</ref>

Cladogram showing Xenambulacraria hypothesis with a [[Paraphyly|paraphyletic]] Deuterostomia:<ref name="Mulhair McCarthy 2022">{{Cite journal |last1=Mulhair |first1=Peter O. |last2=McCarthy |first2=Charley G.P. |last3=Siu-Ting |first3=Karen |last4=Creevey |first4=Christopher J. |last5=O’Connell |first5=Mary J. |date=December 2022 |title=Filtering artifactual signal increases support for Xenacoelomorpha and Ambulacraria sister relationship in the animal tree of life |url=https://linkinghub.elsevier.com/retrieve/pii/S0960982222016840 |journal=Current Biology |volume=32 |issue=23 |pages=5180–5188.e3 |doi=10.1016/j.cub.2022.10.036|pmid=36356574 |bibcode=2022CBio...32E5180M }}</ref>

{{clade
|style=font-size:85%;line-height:85%
|label1= '''Bilateria''' 
|1={{clade
   |label1=[[Xenambulacraria]] 
   |sublabel1=
   |1={{clade
      |1=[[Xenacoelomorpha]] [[File:Xenoturbella japonica.jpg|70 px]]
      |2=[[Ambulacraria]] [[File:Portugal 20140812-DSC01434 (21371237591).jpg|60 px]]
      }}
   |label2=[[Centroneuralia]] 
   |sublabel2=
   |2={{clade
      |1=[[Chordata]] [[File:Blueback herring fish (white background).jpg|60 px]]
      |label2=[[Protostomia]] 
      |2={{clade
         |1=[[Ecdysozoa]] [[File:Long nosed weevil edit.jpg|60 px]]
         |2=[[Spiralia]] [[File:Grapevinesnail 01.jpg|60 px]]
         }}
      }}
   }}
}}

Cladogram showing hypothesis of Xenambulacraria within a monophyletic Deuterostomia:<ref name="Mulhair McCarthy 2022"/>

{{clade
|style=font-size:85%;line-height:85%
|label1='''Bilateria'''
|1={{clade
   |label1=[[Deuterostomia]]
   |1={{clade
      |label1=[[Xenambulacraria]]
      |1={{clade
         |1=[[Xenacoelomorpha]] [[File:Xenoturbella japonica.jpg|70 px]]
         |2=[[Ambulacraria]] [[File:Portugal 20140812-DSC01434 (21371237591).jpg|60 px]]
         }}
      |2=[[Chordata]] [[File:Blueback herring fish (white background).jpg|60 px]]
      }}
   |label2=[[Protostomia]] 
   |2={{clade
      |1=[[Ecdysozoa]] [[File:Long nosed weevil edit.jpg|60 px]]
      |2=[[Spiralia]] [[File:Grapevinesnail 01.jpg|60 px]]
      }}
   }}
}}

== Taxonomic history ==

The Bilateria were named by the Austrian embryologist [[Berthold Hatschek]] in 1888. In his classification, the group included the Zygoneura, Ambulacraria, and Chordonii (the Chordata).<ref name="Nielsen 2009">{{cite journal |last=Nielsen |first=Claus |title=The 'new phylogeny'. What is new about it? |journal=Annual Review of Ecology, Evolution and Systematics |volume=35 |year=2009 |pages=229–256 |url=https://www.palaeodiversity.org/pdf/03Suppl/Supplement_Nielsen.pdf}}</ref><ref>{{cite book |last=Hatschek |first=Berthold |author-link=Berthold Hatschek |year=1888 |title=Lehrbuch der Zoologie |edition=1st |language=de |pages=1–144 |location=Jena |publisher=Gustav Fischer}}</ref> In 1910, the Austrian zoologist [[Karl Grobben]] renamed the Zygoneura to Protostomia, and created the Deuterostomia to encompass the Ambulacraria and Chordonii.<ref name="Nielsen 2009"/><ref>{{cite book |last1=Grobben |first1=Karl |author1-link=Karl Grobben |last2=Claus |first2=Carl Friedrich Wilhelm |author2-link=Carl Friedrich Wilhelm Claus |year=1910 |title=Lehrbuch der Zoologie |edition=2nd |location=Marburg |publisher=Elvert'sche Verlagsbuchhandlung}}</ref>

== See also ==

* [[Embryological origins of the mouth and anus]]

== Notes ==

{{notelist}}

== References ==

{{Reflist|30em}}

== External links ==

* [http://tolweb.org/tree?group=Bilateria&contgroup=Animals Tree of Life web project — Bilateria] {{Webarchive|url=https://web.archive.org/web/20201116111252/http://tolweb.org/tree?group=Bilateria&contgroup=Animals |date=2020-11-16 }}
* [http://www.ucmp.berkeley.edu/phyla/metazoasy.html University of California Museum of Paleontology — Systematics of the Metazoa]

{{Eukaryota classification}}
{{Animalia}}

{{Taxonbar|from=Q5173}}
{{Authority control}}

[[Category:Bilaterians|Bilaterians]]
[[Category:Subkingdoms]]
[[Category:Ediacaran first appearances]]
[[Category:Taxa named by Berthold Hatschek]]