{{Short description|Clade of animals whose mouth develops before the anus}} {{Automatic taxobox | fossil_range = {{Fossil range|570|0|[[Ediacaran]] - Recent}} | name = Protostomes | image = <imagemap> File:Protostomia.jpg|220px rect 0 0 660 660 [[Arthropod]] rect 0 660 660 1320 [[Mollusca]] rect 0 1320 660 1980 [[Annelid]] rect 660 0 1320 660 [[Nematode]] rect 660 660 1320 1320 [[Flatworm]] rect 660 1320 1320 1980 [[Rotifer]] </imagemap> | display_parents = 4 | taxon = Protostomia | authority = [[Karl Grobben|Grobben]], 1908 | subdivision_ranks = Subgroups | subdivision = *[[Ecdysozoa]] **[[Scalidophora]] **[[Cryptovermes]] **{{extinct}}[[Palaeoscolecida]] **{{extinct}}[[Acosmia]] **{{extinct}}[[Eolarva]] **{{extinct}}[[Saccorhytida]] **{{extinct}}[[Saccus (genus)|Saccus]] *[[Spiralia]] **[[Gnathifera (Spiralia)|Gnathifera]] **[[Platytrochozoa]] *{{extinct}}[[Kimberella]] }}
'''Protostomia''' ({{IPAc-en|ˌ|p|r|oʊ|t|ə|'|s|t|oʊ|m|i|.|ə}}) is the [[clade]] of animals once thought to be characterized by the creature's [[blastopore]] becoming its mouth during [[embryonic development]]. This nature has since been discovered to be extremely variable among Protostomia's members, although the reverse is typically true of its sister clade, [[Deuterostomia]].<ref name='Hejnol2009'>{{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 | pages=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> Well-known examples of '''protostomes''' are [[arthropod]]s, [[mollusc]]s, [[annelid]]s, [[flatworm]]s and [[nematode]]s. They are also called '''schizocoelomates''' since [[schizocoely]] typically occurs in them.
Together with the Deuterostomia and [[Xenacoelomorpha]], these form the clade [[Bilateria]], animals with [[symmetry (biology)#Bilateral symmetry|bilateral symmetry]], [[anteroposterior axis]] and [[triploblastic|three]] [[germ layer]]s.<ref name="Hejnol">{{cite journal | last1=Hejnol | first1=A. | last2=Obst | first2=M. | last3=Stamatakis | first3=A. | last4=Ott | first4=M. | last5=Rouse | first5=G. W. | last6=Edgecombe | first6=G. D. | display-authors=etal | year=2009 | title=Assessing the root of bilaterian animals with scalable phylogenomic methods | journal=Proceedings of the Royal Society B: Biological Sciences | volume=276 | issue=1677| pages=4261–4270 | doi=10.1098/rspb.2009.0896 | pmid=19759036 | pmc=2817096}}</ref>
==Protostomy== {{See also|Embryological origins of the mouth and anus}}
In animals at least as complex as [[earthworms]], the first phase in [[Gut (zoology)|gut]] development involves the [[embryo]] forming a dent on one side (the [[blastopore]]) which deepens to become its digestive tube (the [[archenteron]]). In the sister-clade, the deuterostomes ({{lit|second-mouth}}), the original dent becomes the [[anus]] while the gut eventually tunnels through to make another opening, which forms the [[mouth]]. The protostomes (from Greek {{lang|grc|πρωτο-}} {{Transliteration|grc|prōto-}} 'first' + {{lang|grc|στόμα}} {{Transliteration|grc|stóma}} 'mouth') were so named because it was once believed that in all cases the embryological dent formed the mouth while the anus was formed later, at the opening made by the other end of the gut.<ref>{{cite book |last1=Peters |first1=Kenneth E. |last2=Walters |first2=Clifford C. |last3=Moldowan |first3=J. Michael |title=The Biomarker Guide: Biomarkers and isotopes in petroleum systems and Earth history |volume=2 |year=2005 |publisher=Cambridge University Press |isbn=978-0-521-83762-0 |page=717}}</ref><ref name='Hejnol2009'/> It is now known that the fate of the blastopore among protostomes is extremely variable; while the evolutionary distinction between deuterostomes and protostomes remains valid, the descriptive accuracy of the name ''protostome'' is disputable.<ref name="Hejnol2009"/>
Protostome and deuterostome embryos differ in several other ways. Secondary body cavities ([[coelom]]s) generally form by [[schizocoely]], where the coelom forms out of a solid mass of embryonic tissue splitting away from the rest, instead of by [[Enterocoely|enterocoelic pouching]], where the coelom would otherwise form out of in-folded gut walls.<ref>{{cite book |last=Safra |first=Jacob E. |title=The New Encyclopædia Britannica, Volume 1; Volume 3 |year=2003 |publisher=Encyclopædia Britannica |isbn=978-0-85229-961-6 |page=767}}</ref>
==Evolution== The common ancestor of protostomes and deuterostomes was evidently a worm-like aquatic animal of the [[Ediacaran biota|Ediacaran]]. The two clades diverged over 600 million years ago. Protostomes evolved into over a million species alive today, compared to around 73,000 deuterostome species.<ref>The Invertebrate tree of life, Giribet & Edgecombe, 2020; p.155</ref>
Protostomes are divided into the [[Ecdysozoa]] (e.g. [[arthropoda|arthropods]], [[nematoda|nematodes]]) and the [[Spiralia]] (e.g. [[mollusca|molluscs]], [[annelida|annelids]], [[platyhelminthes|platyhelminths]], and [[rotifera|rotifers]]). A modern consensus [[phylogenetic tree]] for the protostomes is shown below.<ref>{{cite journal |first1=Gregory D. |last1=Edgecombe |first2=Gonzalo |last2=Giribet |first3=Casey W. |last3=Dunn |first4=Andreas |last4=Hejnol |first5=Reinhardt M. |last5=Kristensen |first6=Ricardo C. |last6=Neves |first7=Greg W. |last7=Rouse |first8=Katrine |last8=Worsaae |first9=Martin V. |last9=Sørensen |doi=10.1007/s13127-011-0044-4 |title=Higher-level metazoan relationships: recent progress and remaining questions |date=June 2011 |journal=Organisms, Diversity & Evolution |volume=11 |issue=2 |pages=151–172 |bibcode=2011ODivE..11..151E |s2cid=32169826 |url=http://nrs.harvard.edu/urn-3:HUL.InstRepos:27755241 |url-access=subscription }}</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 |bibcode=2017NatCo...8....9F |pmc=5431905 }}</ref><ref>{{Cite journal |last1=Smith |first1=Martin R.|last2=Ortega-Hernández|first2=Javier |title=Hallucigenia's onychophoran-like claws and the case for Tactopoda |journal=Nature |volume=514|issue=7522 |pages=363–366 |doi=10.1038/nature13576 |pmid=25132546|year=2014|bibcode=2014Natur.514..363S|s2cid=205239797|url=http://dro.dur.ac.uk/19108/1/19108.pdf}}</ref><ref name="Palaeos Metazoa: Ecdysozoa">{{Cite web |url=http://palaeos.com/metazoa/ecdysozoa/ecdysozoa.html |title=Palaeos Metazoa: Ecdysozoa |website=palaeos.com |access-date=2017-09-02}}</ref><ref>{{Cite journal |last1=Yamasaki |first1=Hiroshi |last2=Fujimoto |first2=Shinta |last3=Miyazaki |first3=Katsumi |date=June 2015 |title=Phylogenetic position of Loricifera inferred from nearly complete 18S and 28S rRNA gene sequences |journal=Zoological Letters |volume=1 |article-number=18 |doi=10.1186/s40851-015-0017-0|pmid=26605063 |pmc=4657359 |doi-access=free }}</ref><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 of London B: Biological Sciences |volume=363 |issue=1496 |pages=1435–1443 |doi=10.1098/rstb.2007.2233 |pmid=18192191 |pmc=2614224}}</ref>{{Excessive citations inline|date=July 2025}} {{clade |label1=[[Nephrozoa]] |1={{clade |1=[[Deuterostomia]] [[File:Myxine glutinosa Gervais.jpg|50px]] |label2='''Protostomia''' |2={{clade |label1={{extinct}}[[Kimberella]] |1={{clade |1={{extinct}}[[Kimberella persii]] |2={{extinct}}[[Kimberella quadrata]] [[File:Kimberella NT.jpg|60 px]] }} |label2=[[Ecdysozoa]] |2={{clade |1={{extinct}}[[Palaeoscolecida]] [[File:Cricocosmia jinningensis 2026.jpg|50 px]] |2={{extinct}}[[Acosmia]] |3={{extinct}}[[Eolarva]] |label4={{extinct}}[[Saccorhytida]] |4={{clade |1={{extinct}}[[Saccorhytus]] [[File:Saccorhytus.png|60 px]] |2={{extinct}}[[Beretella]] }} |label5={{extinct}}[[Saccus (genus)|Saccus]] |5={{clade |1={{extinct}}[[Saccus xixiangensis]] |2={{extinct}}[[Saccus necopinus]] }} |label6=[[Scalidophora]] |6={{clade |1={{extinct}}[[Qinscolex]] |2={{extinct}}[[Dahescolex]] |3={{extinct}}[[Shanscolex]] |4={{extinct}}[[Xinliscolex]] |5={{extinct}}[[Zhongpingscolex]] |6=[[Kinorhyncha]] [[File:Pycnophyes zelinkaei.jpg|35 px]] |7=[[Priapulida]] [[File:Adult priapulid 2.jpg|60 px]] }} |label8=[[Cryptovermes]] |8={{Clade |1=[[Nematoida]] [[File:CelegansGoldsteinLabUNC.jpg|60 px]] |2=[[Panarthropoda]] [[File:Velvet worm.png|60px]] }} }} |label3=[[Spiralia]] |3={{clade |1=[[Gnathifera (clade)|Gnathifera]] [[File:Pseudosagitta maxima 31349361.png|60 px]] |2=[[Platytrochozoa]] [[File:Grapevinesnail 01.jpg|60 px]] }} }} }} }}
==See also== * [[Embryological origins of the mouth and anus]] * [[Urbilaterian]], a hypothethical common ancestor to Protostomes and Deuterostomes
==References== {{Reflist}}
==External links== {{Wikispecies|Protostomia}}
* [http://www.taxonomy.nl/Taxonomicon/TaxonTree.aspx?id=198701 The Taxonomicon] for [[Karl Grobben]] *{{Commons category-inline|Protostomia}}
{{Animalia}} {{Taxonbar|from=Q5171}} {{Authority control}}
[[Category:Protostomes| ]] [[Category:Ediacaran first appearances]] [[Category:Infrakingdoms]] [[Category:Taxa named by Karl Grobben]]