Animals with embryonic bilateral symmetry

<span class="mw-page-title-main">Bilateria</span>

Bilateria

<p><b>Bilateria</b> (<span class="rt-commentedText nowrap"><span class="IPA nopopups noexcerpt"><a href="/en/Help:IPA/English" title="Help:IPA/English" class="wl">/<span style="border-bottom:1px dotted"><span>ˌ</span><span>b</span><span>aɪ</span><span>l</span><span>ə</span><span>ˈ</span><span>t</span><span>ɪər</span><span>i</span><span>ə</span></span>/</a></span></span>) is a group of <a href="/en/Animal" title="Animal" class="wl">animals</a>, called <b>bilaterians</b>, with <a href="/en/Symmetry_(biology)#Bilateral_symmetry" title="Symmetry (biology)" class="mw-redirect wl">bilateral symmetry</a> as an <a href="/en/Embryo" title="Embryo" class="wl">embryo</a> (i.e. having a left and a right side that are <a href="/en/Mirror_image" title="Mirror image" class="wl">mirror images</a> of each other). This also means they have a head and a tail (anterior–posterior axis), as well as a belly and a back (ventral–dorsal axis).<span class="mw-ref reference" id="cite_ref-Brusca2016_2-0"><a href="#cite_note-Brusca2016-2" style="counter-reset: mw-Ref 2;"><span class="mw-reflink-text">[2]</span></a></span> Nearly all are bilaterally symmetrical as adults as well; the most notable exception is the <a href="/en/Echinoderm" title="Echinoderm" class="wl">echinoderms</a>, which achieve secondary <a href="/en/Pentaradial_symmetry" title="Pentaradial symmetry" class="mw-redirect wl">pentaradial symmetry</a> as adults, but are bilaterally symmetrical during embryonic development.</p>

<div class="shortdescription nomobile noexcerpt noprint searchaux" style="display:none" about="#mwt1">Animals with embryonic bilateral symmetry</div>
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</span><div class='mobile-collapse'><header class="mobile-collapse-header mobile-collapse-btn"><b>Quick facts: Bilaterians Temporal range  Ediacaran–Present...</b> <span class='arrow'>▼</span></header> <table class="infobox biota mobile-collapse-element" style="text-align: left; width: 200px; font-size: 100%">
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<th colspan="2" style="text-align: center; background-color: rgb(235,235,210)" class="notheme">Bilaterians<br><div style="font-size: 85%;">Temporal range: <a href="/en/Ediacaran" title="Ediacaran" class="wl">Ediacaran</a>–<a href="/en/Holocene" title="Holocene" class="wl">Present</a>, <span class="noprint"><span style="display:inline-block;">560–0<span>&nbsp;</span><a href="/en/Megaannum" title="Megaannum" class="mw-redirect wl">Ma</a></span><span class="mw-ref reference" id="cite_ref-1"><a href="#cite_note-1" style="counter-reset: mw-Ref 1;"><span class="mw-reflink-text">[1]</span></a></span> <div id="Timeline-row" style="margin: 4px auto 0; clear:both; width:220px; padding:0px; height:18px; overflow:visible; white-space:nowrap; border:1px #666; border-style:solid none; position:relative; z-index:0; font-size:97%;">
<div style="position:absolute; height:100%; left:0px; width:207.23076923077px; padding-left:5px; text-align:left; background-color:rgb(254,217,106); background-image: linear-gradient(to right, rgba(255,255,255,1), rgba(254,217,106,1) 15%, rgba(254,217,106,1));" class="notheme"><a href="/en/Precambrian" title="Precambrian" class="wl">PreꞒ</a></div>
<div style="position:absolute; height:100%; text-align:center; background-color:rgb(127,160,86); left:37.636923076923px; width:18.073846153846px;" class="notheme"><a href="/en/Cambrian" title="Cambrian" class="wl">Ꞓ</a></div>
<div style="position:absolute; height:100%; text-align:center; background-color:rgb(0,146,112); left:55.710769230769px; width:14.08px;" class="notheme"><a href="/en/Ordovician" title="Ordovician" class="wl">O</a></div>
<div style="position:absolute; height:100%; text-align:center; background-color:rgb(179,225,182); left:69.790769230769px; width:8.3261538461539px;" class="notheme"><a href="/en/Silurian" title="Silurian" class="wl">S</a></div>
<div style="position:absolute; height:100%; text-align:center; background-color:rgb(203,140,55); left:78.116923076923px; width:20.409230769231px;" class="notheme"><a href="/en/Devonian" title="Devonian" class="wl">D</a></div>
<div style="position:absolute; height:100%; text-align:center; background-color:rgb(103,165,153); left:98.526153846154px; width:20.307692307692px;" class="notheme"><a href="/en/Carboniferous" title="Carboniferous" class="wl">C</a></div>
<div style="position:absolute; height:100%; text-align:center; background-color:rgb(240,64,40); left:118.83384615385px; width:15.907015384615px;" class="notheme"><a href="/en/Permian" title="Permian" class="wl">P</a></div>
<div style="position:absolute; height:100%; text-align:center; background-color:rgb(129,43,146); left:134.74086153846px; width:17.092984615385px;" class="notheme"><a href="/en/Triassic" title="Triassic" class="wl">T</a></div>
<div style="position:absolute; height:100%; text-align:center; background-color:rgb(52,178,201); left:151.83384615385px; width:19.089230769231px;" class="notheme"><a href="/en/Jurassic" title="Jurassic" class="wl">J</a></div>
<div style="position:absolute; height:100%; text-align:center; background-color:rgb(127,198,78); left:170.92307692308px; width:26.738461538462px;" class="notheme"><a href="/en/Cretaceous" title="Cretaceous" class="wl">K</a></div>
<div style="position:absolute; height:100%; text-align:center; background-color:rgb(253,154,82); left:197.66153846154px; width:14.543692307692px;" class="notheme"><a href="/en/Paleogene" title="Paleogene" class="wl">Pg</a></div>
<div style="position:absolute; height:100%; text-align:center; background-color:rgb(255,230,25); left:212.20523076923px; width:6.9215384615385px;" class="notheme"><a href="/en/Neogene" title="Neogene" class="wl">N</a></div>
<div id="end-border" style="position:absolute; height:100%; background-color:#666; width:1px; left:219px" class="notheme"></div><div style="margin:0 auto; line-height:0; clear:both; width:220px; padding:0px; height:8px; overflow:visible; background-color:transparent; position:relative; top:-4px; z-index:100;" class="notheme"><div style="position:absolute; height:8px; left:30.461538461538px; width:189.53846153846px; background-color:#360; opacity:0.42; " class="notheme"></div>
<div style="position:absolute; height:8px; left:30.461538461538px; width:189.53846153846px; background-color:#360; opacity:1; " class="notheme"></div>
<div style="position:absolute; height:6px; top:1px; left:31.461538461538px; width:187.53846153846px; background-color:#6c3;" class="notheme"></div>
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<td colspan="2" style="text-align: center"><span class="mw-default-size"><a href="/en/File:Animal_diversity_October_2007_with_Trilobite.jpg" class="mw-file-description image media-thumb" data-hash="File:Animal_diversity_October_2007_with_Trilobite.jpg"><img loading="lazy" src="//upload.wikimedia.org/wikipedia/commons/thumb/4/4b/Animal_diversity_October_2007_with_Trilobite.jpg/220px-Animal_diversity_October_2007_with_Trilobite.jpg" decoding="async" data-file-width="1000" data-file-height="1200" data-file-type="bitmap" height="264" width="220" srcset="//upload.wikimedia.org/wikipedia/commons/thumb/4/4b/Animal_diversity_October_2007_with_Trilobite.jpg/330px-Animal_diversity_October_2007_with_Trilobite.jpg 1.5x, //upload.wikimedia.org/wikipedia/commons/thumb/4/4b/Animal_diversity_October_2007_with_Trilobite.jpg/440px-Animal_diversity_October_2007_with_Trilobite.jpg 2x" alt="Animal_diversity_October_2007_with_Trilobite.jpg"></a></span></td></tr>
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<td colspan="2" style="text-align: center; font-size: 88%">Diversity of bilaterians.</td></tr>


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<th colspan="2" style="min-width:15em; text-align: center; background-color: rgb(235,235,210)" class="notheme"><a href="/en/Taxonomy_(biology)" title="Taxonomy (biology)" class="wl">Scientific classification</a> <span class="plainlinks" style="font-size:smaller; float:right; padding-right:0.4em; margin-left:-3em;"><span class="mw-default-size"><a href="/en/Template:Taxonomy/Bilateria" title="e" class="image wl"><img loading="lazy"alt="e" src="//upload.wikimedia.org/wikipedia/commons/7/74/Red_Pencil_Icon.png" decoding="async" data-file-width="16" data-file-height="16" data-file-type="bitmap" height="16" width="16"></a></span></span></th></tr>
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<td>Kingdom:</td>
<td><a href="/en/Animal" title="Animal" class="wl">Animalia</a></td></tr>
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<td>Subkingdom:</td>
<td><a href="/en/Eumetazoa" title="Eumetazoa" class="wl">Eumetazoa</a></td></tr>
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<td><i>Clade</i>:</td>
<td><a href="/en/ParaHoxozoa" title="ParaHoxozoa" class="wl">ParaHoxozoa</a></td></tr>
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<td><i>Clade</i>:</td>
<td><a href="/en/Bilateria" title="Bilateria" class="wl">Bilateria</a><br><small><a href="/en/Berthold_Hatschek" title="Berthold Hatschek" class="wl">Hatschek</a>, 1888</small></td></tr>



































































































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<th colspan="2" style="text-align: center; background-color: rgb(235,235,210)" class="notheme">Phyla</th></tr>
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<td colspan="2" style="text-align: left">
<ul><li><a href="/en/Proarticulata" title="Proarticulata" class="wl">Proarticulata</a>? <a href="/en/Extinction" title="Extinction" class="wl">†</a></li>
<li><a href="/en/Xenacoelomorpha" title="Xenacoelomorpha" class="wl">Xenacoelomorpha</a>?</li>
<li><a href="/en/Nephrozoa" title="Nephrozoa" class="wl">Nephrozoa</a>:
<ul><li><i><a href="/en/Ikaria_wariootia" title="Ikaria wariootia" class="wl">Ikaria</a></i> <a href="/en/Extinction" title="Extinction" class="wl">†</a></li>
<li><b>Superphylum <a href="/en/Deuterostomia" title="Deuterostomia" class="mw-redirect wl">Deuterostomia</a></b>
<ul><li><a href="/en/Chordata" title="Chordata" class="mw-redirect wl">Chordata</a></li>
<li><a href="/en/Xenacoelomorpha" title="Xenacoelomorpha" class="wl">Xenacoelomorpha</a>?</li>
<li><a href="/en/Ambulacraria" title="Ambulacraria" class="wl">Ambulacraria</a> (unranked)
<ul><li><a href="/en/Hemichordata" title="Hemichordata" class="mw-redirect wl">Hemichordata</a></li>
<li><a href="/en/Echinodermata" title="Echinodermata" class="mw-redirect wl">Echinodermata</a></li>
<li><a href="/en/Cambroernida" title="Cambroernida" class="mw-redirect wl">Cambroernida</a> <a href="/en/Extinction" title="Extinction" class="wl">†</a></li></ul></li></ul></li>
<li><b><a href="/en/Protostomia" title="Protostomia" class="mw-redirect wl">Protostomia</a> (unranked)</b>
<ul><li><b>Superphylum <a href="/en/Ecdysozoa" title="Ecdysozoa" class="wl">Ecdysozoa</a></b>
<ul><li><a href="/en/Saccorhytida" title="Saccorhytida" class="mw-redirect wl">Saccorhytida</a>? <a href="/en/Extinction" title="Extinction" class="wl">†</a></li>
<li><a href="/en/Scalidophora" title="Scalidophora" class="wl">Scalidophora</a> (unranked)
<ul><li><a href="/en/Kinorhyncha" title="Kinorhyncha" class="wl">Kinorhyncha</a></li>
<li><a href="/en/Loricifera" title="Loricifera" class="wl">Loricifera</a></li>
<li><a href="/en/Priapulida" title="Priapulida" class="wl">Priapulida</a></li></ul></li>
<li><a href="/en/Nematoida" title="Nematoida" class="wl">Nematoida</a> (unranked)
<ul><li><a href="/en/Nematoda" title="Nematoda" class="mw-redirect wl">Nematoda</a></li>
<li><a href="/en/Nematomorpha" title="Nematomorpha" class="wl">Nematomorpha</a></li></ul></li>
<li><a href="/en/Panarthropoda" title="Panarthropoda" class="wl">Panarthropoda</a> (unranked)
<ul><li><a href="/en/Onychophora" title="Onychophora" class="wl">Onychophora</a></li>
<li><a href="/en/Arthropod" title="Arthropod" class="wl">Arthropoda</a></li>
<li><a href="/en/Tardigrada" title="Tardigrada" class="mw-redirect wl">Tardigrada</a></li></ul></li></ul></li>
<li><b><a href="/en/Spiralia" title="Spiralia" class="wl">Spiralia</a> (unranked)</b>
<ul><li><b><a href="/en/Chaetognatha" title="Chaetognatha" class="wl">Chaetognatha</a></b></li>
<li><a href="/en/Mesozoa" title="Mesozoa" class="wl">Mesozoa</a> (unranked)
<ul><li><a href="/en/Salinella" title="Salinella" class="wl">Monoblastozoa</a></li></ul></li>
<li><b>Superphylum <a href="/en/Platyzoa" title="Platyzoa" class="wl">Platyzoa</a></b>
<ul><li><a href="/en/Acanthocephala" title="Acanthocephala" class="wl">Acanthocephala</a></li>
<li><a href="/en/Cycliophora" title="Cycliophora" class="mw-redirect wl">Cycliophora</a></li>
<li><a href="/en/Gastrotricha" title="Gastrotricha" class="mw-redirect wl">Gastrotricha</a></li>
<li><a href="/en/Gnathostomulida" title="Gnathostomulida" class="mw-redirect wl">Gnathostomulida</a></li>
<li><a href="/en/Micrognathozoa" title="Micrognathozoa" class="mw-redirect wl">Micrognathozoa</a></li>
<li><a href="/en/Platyhelminthes" title="Platyhelminthes" class="mw-redirect wl">Platyhelminthes</a></li>
<li><a href="/en/Rotifera" title="Rotifera" class="mw-redirect wl">Rotifera</a></li></ul></li>
<li><b>Superphylum <a href="/en/Lophotrochozoa" title="Lophotrochozoa" class="wl">Lophotrochozoa</a></b>
<ul><li><a href="/en/Annelida" title="Annelida" class="mw-redirect wl">Annelida</a></li>
<li><a href="/en/Brachiopoda" title="Brachiopoda" class="mw-redirect wl">Brachiopoda</a></li>
<li><a href="/en/Bryozoa" title="Bryozoa" class="wl">Bryozoa</a></li>
<li><a href="/en/Dicyemida" title="Dicyemida" class="wl">Dicyemida</a></li>
<li><a href="/en/Entoprocta" title="Entoprocta" class="wl">Entoprocta</a></li>
<li><a href="/en/Hyolitha" title="Hyolitha" class="wl">Hyolitha</a> <a href="/en/Extinction" title="Extinction" class="wl">†</a></li>
<li><a href="/en/Mollusca" title="Mollusca" class="wl">Mollusca</a></li>
<li><a href="/en/Nemertea" title="Nemertea" class="wl">Nemertea</a></li>
<li><a href="/en/Orthonectida" title="Orthonectida" class="wl">Orthonectida</a></li>
<li><a href="/en/Phoronida" title="Phoronida" class="mw-redirect wl">Phoronida</a></li></ul></li></ul></li></ul></li></ul></li></ul></td></tr>














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<th colspan="2" style="text-align: center; background-color: rgb(235,235,210)" class="notheme"><a href="/en/Synonym_(taxonomy)" title="Synonym (taxonomy)" class="wl">Synonyms</a></th></tr>
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<td colspan="2" style="text-align: left">
<p><a href="/en/Triploblasts" title="Triploblasts" class="mw-redirect wl">Triploblasts</a> <small>Lankester, 1873</small></p></td></tr>


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<p>Most <a href="/en/Animal" title="Animal" class="wl">animals</a> are bilaterians, excluding <a href="/en/Sea_sponge" title="Sea sponge" class="mw-redirect wl">sponges</a>, <a href="/en/Ctenophora" title="Ctenophora" class="wl">ctenophores</a>, <a href="/en/Placozoa" title="Placozoa" class="wl">placozoans</a> and <a href="/en/Cnidaria" title="Cnidaria" class="wl">cnidarians</a>. For the most part, bilateral embryos are <a href="/en/Triploblastic" title="Triploblastic" class="mw-redirect wl">triploblastic</a>, having three <a href="/en/Germ_layer" title="Germ layer" class="wl">germ layers</a>: <a href="/en/Endoderm" title="Endoderm" class="wl">endoderm</a>, <a href="/en/Mesoderm" title="Mesoderm" class="wl">mesoderm</a>, and <a href="/en/Ectoderm" title="Ectoderm" class="wl">ectoderm</a>. Except for a few phyla (i.e. <a href="/en/Flatworm" title="Flatworm" class="wl">flatworms</a> and <a href="/en/Gnathostomulid" title="Gnathostomulid" class="wl">gnathostomulids</a>), bilaterians have complete <a href="/en/Digestive_tract" title="Digestive tract" class="mw-redirect wl">digestive tracts</a> with a separate <a href="/en/Mouth" title="Mouth" class="wl">mouth</a> and <a href="/en/Anus" title="Anus" class="wl">anus</a>. Some bilaterians lack body cavities (<a href="/en/Acoelomata" title="Acoelomata" class="mw-redirect wl">acoelomates</a>, i.e. <a href="/en/Flatworm" title="Flatworm" class="wl">Platyhelminthes</a>, <a href="/en/Gastrotricha" title="Gastrotricha" class="mw-redirect wl">Gastrotricha</a> and <a href="/en/Gnathostomulid" title="Gnathostomulid" class="wl">Gnathostomulida</a>), while others display primary body cavities (deriving from the <a href="/en/Blastocoel" title="Blastocoel" class="wl">blastocoel</a>, as <a href="/en/Pseudocoelomata" title="Pseudocoelomata" class="mw-redirect wl">pseudocoeloms</a>) or secondary cavities (that appear <i>de novo</i>, for example the <a href="/en/Coelomata" title="Coelomata" class="mw-redirect wl">coelom</a>).</p>



Bilateria is a group of animals, called bilaterians, with bilateral symmetry as an embryo . This also means they have a head and a tail , as well as a belly and a back . Nearly all are bilaterally symmetrical as adults as well; the most notable exception is the echinoderms, which achieve secondary pentaradial symmetry as adults, but are bilaterally symmetrical during embryonic development. Most animals are bilaterians, excluding sponges, ctenophores, placozoans and cnidarians. For the most part, bilateral embryos are triploblastic, having three germ layers: endoderm, mesoderm, and ectoderm. Except for a few phyla , bilaterians have complete digestive tracts with a separate mouth and anus. Some bilaterians lack body cavities , while others display primary body cavities or secondary cavities . 