# Protostome

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Clade of animals whose mouth develops before the anus

Protostomes Temporal range: Ediacaran - Recent PreꞒ Ꞓ O S D C P T J K Pg N Scientific classification Kingdom: Animalia Subkingdom: Eumetazoa Clade: ParaHoxozoa Clade: Bilateria Clade: Nephrozoa Clade: Protostomia Grobben, 1908 Subgroups Ecdysozoa Scalidophora Cryptovermes †Palaeoscolecida †Acosmia †Eolarva †Saccorhytida †Saccus Spiralia Gnathifera Platytrochozoa †Kimberella

**Protostomia** ([/ˌproʊtəˈstoʊmi.ə/](https://en.wikipedia.org/wiki/Help:IPA/English)) is the [clade](/source/Clade) of animals once thought to be characterized by the creature's [blastopore](/source/Blastopore) becoming its mouth during [embryonic development](/source/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](/source/Deuterostomia).[1][2] Well-known examples of **protostomes** are [arthropods](/source/Arthropod), [molluscs](/source/Mollusc), [annelids](/source/Annelid), [flatworms](/source/Flatworm) and [nematodes](/source/Nematode). They are also called **schizocoelomates** since [schizocoely](/source/Schizocoely) typically occurs in them.

Together with the Deuterostomia and [Xenacoelomorpha](/source/Xenacoelomorpha), these form the clade [Bilateria](/source/Bilateria), animals with [bilateral symmetry](/source/Symmetry_(biology)#Bilateral_symmetry), [anteroposterior axis](/source/Anteroposterior_axis) and [three](/source/Triploblastic) [germ layers](/source/Germ_layer).[3]

## Protostomy

See also: [Embryological origins of the mouth and anus](/source/Embryological_origins_of_the_mouth_and_anus)

In animals at least as complex as [earthworms](/source/Earthworms), the first phase in [gut](/source/Gut_(zoology)) development involves the [embryo](/source/Embryo) forming a dent on one side (the [blastopore](/source/Blastopore)) which deepens to become its digestive tube (the [archenteron](/source/Archenteron)). In the sister-clade, the deuterostomes (lit. 'second-mouth'), the original dent becomes the [anus](/source/Anus) while the gut eventually tunnels through to make another opening, which forms the [mouth](/source/Mouth). The protostomes (from Greek πρωτο- *prōto-* 'first' + στόμα *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.[4][1] 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.[1]

Protostome and deuterostome embryos differ in several other ways. Secondary body cavities ([coeloms](/source/Coelom)) generally form by [schizocoely](/source/Schizocoely), where the coelom forms out of a solid mass of embryonic tissue splitting away from the rest, instead of by [enterocoelic pouching](/source/Enterocoely), where the coelom would otherwise form out of in-folded gut walls.[5]

## Evolution

The common ancestor of protostomes and deuterostomes was evidently a worm-like aquatic animal of the [Ediacaran](/source/Ediacaran_biota). 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.[6]

Protostomes are divided into the [Ecdysozoa](/source/Ecdysozoa) (e.g. [arthropods](/source/Arthropoda), [nematodes](/source/Nematoda)) and the [Spiralia](/source/Spiralia) (e.g. [molluscs](/source/Mollusca), [annelids](/source/Annelida), [platyhelminths](/source/Platyhelminthes), and [rotifers](/source/Rotifera)). A modern consensus [phylogenetic tree](/source/Phylogenetic_tree) for the protostomes is shown below.[7][8][9][10][11][12][*[excessive citations](https://en.wikipedia.org/wiki/Wikipedia:Citing_sources#Bundling_citations)*]

Nephrozoa Deuterostomia Protostomia †Kimberella †Kimberella persii †Kimberella quadrata Ecdysozoa †Palaeoscolecida †Acosmia †Eolarva †Saccorhytida †Saccorhytus †Beretella †Saccus †Saccus xixiangensis †Saccus necopinus Scalidophora †Qinscolex †Dahescolex †Shanscolex †Xinliscolex †Zhongpingscolex Kinorhyncha Priapulida Cryptovermes Nematoida Panarthropoda Spiralia Gnathifera Platytrochozoa

## See also

- [Embryological origins of the mouth and anus](/source/Embryological_origins_of_the_mouth_and_anus)

- [Urbilaterian](/source/Urbilaterian), a hypothethical common ancestor to Protostomes and Deuterostomes

## References

1. ^ [***a***](#cite_ref-Hejnol2009_1-0) [***b***](#cite_ref-Hejnol2009_1-1) [***c***](#cite_ref-Hejnol2009_1-2) Hejnol, A.; Martindale, M. Q. (2009). ["The mouth, the anus, and the blastopore - open questions about questionable openings"](https://www.researchgate.net/publication/230766195). In M. J. Telford; D. T. J. Littlewood (eds.). *Animal Evolution — Genomes, Fossils, and Trees*. pp. 33–40.

1. **[^](#cite_ref-2)** Martín-Durán, José M.; Passamaneck, Yale J.; Martindale, Mark Q.; Hejnol, Andreas (2016). ["The developmental basis for the recurrent evolution of deuterostomy and protostomy"](https://qmro.qmul.ac.uk/xmlui/handle/123456789/54816). *Nature Ecology & Evolution*. **1** (1): 0005. [doi](/source/Doi_(identifier)):[10.1038/s41559-016-0005](https://doi.org/10.1038%2Fs41559-016-0005). [PMID](/source/PMID_(identifier)) [28812551](https://pubmed.ncbi.nlm.nih.gov/28812551). [S2CID](/source/S2CID_(identifier)) [90795](https://api.semanticscholar.org/CorpusID:90795).

1. **[^](#cite_ref-Hejnol_3-0)** Hejnol, A.; Obst, M.; Stamatakis, A.; Ott, M.; Rouse, G. W.; Edgecombe, G. D.; et al. (2009). ["Assessing the root of bilaterian animals with scalable phylogenomic methods"](https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2817096). *Proceedings of the Royal Society B: Biological Sciences*. **276** (1677): 4261–4270. [doi](/source/Doi_(identifier)):[10.1098/rspb.2009.0896](https://doi.org/10.1098%2Frspb.2009.0896). [PMC](/source/PMC_(identifier)) [2817096](https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2817096). [PMID](/source/PMID_(identifier)) [19759036](https://pubmed.ncbi.nlm.nih.gov/19759036).

1. **[^](#cite_ref-4)** Peters, Kenneth E.; Walters, Clifford C.; Moldowan, J. Michael (2005). *The Biomarker Guide: Biomarkers and isotopes in petroleum systems and Earth history*. Vol. 2. Cambridge University Press. p. 717. [ISBN](/source/ISBN_(identifier)) [978-0-521-83762-0](https://en.wikipedia.org/wiki/Special:BookSources/978-0-521-83762-0).

1. **[^](#cite_ref-5)** Safra, Jacob E. (2003). *The New Encyclopædia Britannica, Volume 1; Volume 3*. Encyclopædia Britannica. p. 767. [ISBN](/source/ISBN_(identifier)) [978-0-85229-961-6](https://en.wikipedia.org/wiki/Special:BookSources/978-0-85229-961-6).

1. **[^](#cite_ref-6)** The Invertebrate tree of life, Giribet & Edgecombe, 2020; p.155

1. **[^](#cite_ref-7)** Edgecombe, Gregory D.; Giribet, Gonzalo; Dunn, Casey W.; Hejnol, Andreas; Kristensen, Reinhardt M.; Neves, Ricardo C.; Rouse, Greg W.; Worsaae, Katrine; Sørensen, Martin V. (June 2011). ["Higher-level metazoan relationships: recent progress and remaining questions"](http://nrs.harvard.edu/urn-3:HUL.InstRepos:27755241). *Organisms, Diversity & Evolution*. **11** (2): 151–172. [Bibcode](/source/Bibcode_(identifier)):[2011ODivE..11..151E](https://ui.adsabs.harvard.edu/abs/2011ODivE..11..151E). [doi](/source/Doi_(identifier)):[10.1007/s13127-011-0044-4](https://doi.org/10.1007%2Fs13127-011-0044-4). [S2CID](/source/S2CID_(identifier)) [32169826](https://api.semanticscholar.org/CorpusID:32169826).

1. **[^](#cite_ref-Fröbius_8-0)** Fröbius, Andreas C.; Funch, Peter (2017-04-04). ["Rotiferan Hox genes give new insights into the evolution of metazoan bodyplans"](https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5431905). *Nature Communications*. **8** (1): 9. [Bibcode](/source/Bibcode_(identifier)):[2017NatCo...8....9F](https://ui.adsabs.harvard.edu/abs/2017NatCo...8....9F). [doi](/source/Doi_(identifier)):[10.1038/s41467-017-00020-w](https://doi.org/10.1038%2Fs41467-017-00020-w). [PMC](/source/PMC_(identifier)) [5431905](https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5431905). [PMID](/source/PMID_(identifier)) [28377584](https://pubmed.ncbi.nlm.nih.gov/28377584).

1. **[^](#cite_ref-9)** Smith, Martin R.; Ortega-Hernández, Javier (2014). ["Hallucigenia's onychophoran-like claws and the case for Tactopoda"](http://dro.dur.ac.uk/19108/1/19108.pdf) (PDF). *Nature*. **514** (7522): 363–366. [Bibcode](/source/Bibcode_(identifier)):[2014Natur.514..363S](https://ui.adsabs.harvard.edu/abs/2014Natur.514..363S). [doi](/source/Doi_(identifier)):[10.1038/nature13576](https://doi.org/10.1038%2Fnature13576). [PMID](/source/PMID_(identifier)) [25132546](https://pubmed.ncbi.nlm.nih.gov/25132546). [S2CID](/source/S2CID_(identifier)) [205239797](https://api.semanticscholar.org/CorpusID:205239797).

1. **[^](#cite_ref-Palaeos_Metazoa:_Ecdysozoa_10-0)** ["Palaeos Metazoa: Ecdysozoa"](http://palaeos.com/metazoa/ecdysozoa/ecdysozoa.html). *palaeos.com*. Retrieved 2017-09-02.

1. **[^](#cite_ref-11)** Yamasaki, Hiroshi; Fujimoto, Shinta; Miyazaki, Katsumi (June 2015). ["Phylogenetic position of Loricifera inferred from nearly complete 18S and 28S rRNA gene sequences"](https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4657359). *Zoological Letters*. **1** 18. [doi](/source/Doi_(identifier)):[10.1186/s40851-015-0017-0](https://doi.org/10.1186%2Fs40851-015-0017-0). [PMC](/source/PMC_(identifier)) [4657359](https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4657359). [PMID](/source/PMID_(identifier)) [26605063](https://pubmed.ncbi.nlm.nih.gov/26605063).

1. **[^](#cite_ref-12)** Peterson, Kevin J.; Cotton, James A.; Gehling, James G.; Pisani, Davide (2008-04-27). ["The Ediacaran emergence of bilaterians: congruence between the genetic and the geological fossil records"](https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2614224). *Philosophical Transactions of the Royal Society of London B: Biological Sciences*. **363** (1496): 1435–1443. [doi](/source/Doi_(identifier)):[10.1098/rstb.2007.2233](https://doi.org/10.1098%2Frstb.2007.2233). [PMC](/source/PMC_(identifier)) [2614224](https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2614224). [PMID](/source/PMID_(identifier)) [18192191](https://pubmed.ncbi.nlm.nih.gov/18192191).

## External links

[Wikispecies](/source/Wikispecies) has information related to ***[Protostomia](https://species.wikimedia.org/wiki/Protostomia)***.

- [The Taxonomicon](http://www.taxonomy.nl/Taxonomicon/TaxonTree.aspx?id=198701) for [Karl Grobben](/source/Karl_Grobben)

- Media related to [Protostomia](https://commons.wikimedia.org/wiki/Category:Protostomia) at Wikimedia Commons

v t e Extant animal phyla Domain Archaea Bacteria Eukaryota (major groups Metamonada Discoba Diaphoretickes Hacrobia Cryptista Rhizaria Alveolata Stramenopiles Plants Amorphea Amoebozoa Opisthokonta Animalia Fungi Mesomycetozoea) Animalia Porifera (sponges) Ctenophora (comb jellies) ParaHoxozoa (Planulozoa) Placozoa (Trichoplax and relatives) Cnidaria (jellyfish and relatives) Bilateria (Triploblasts) (see below↓) The phylogeny of the animal root is disputed; see also Eumetazoa Benthozoa Bilateria Bilateria Xenacoelomorpha (acoels and relatives) Chordata (vertebrates and relatives) Ambulacraria Echinodermata (starfish and relatives) Hemichordata (acorn worms and relatives) Protostomia Ecdysozoa Scalidophora Kinorhyncha (mud dragons) Priapulida (penis worms) Loricifera (corset animals) Cryptovermes Nematoida Nematoda (roundworms) Nematomorpha (horsehair worms) Panarthropoda Onychophora (velvet worms) Arthropoda (insects and relatives) Tardigrada (waterbears) Spiralia Gnathifera Chaetognatha (arrow worms) Gnathostomulida (jaw worms) M+R Micrognathozoa (Limnognathia) Rotifera (wheel animals inc. acanthocephalans) Platytrochozoa C+E Cycliophora (Symbion) Entoprocta or Kamptozoa Rouphozoa Platyhelminthes (flatworms) Gastrotricha (hairybacks) Mesozoa Orthonectida Dicyemida or Rhombozoa ?Monoblastozoa (Salinella) Lophotrochozoa Annelida (earth worms and relatives) Mollusca (snails and relatives) Nemertea (ribbon worms) Lophophorata Brachiopoda (lamp shells) B+P Bryozoa or Ectoprocta (moss animals) Phoronida (horseshoe worms) The phylogeny of Bilateria is disputed; see also Nephrozoa Deuterostomia Xenambulacraria Centroneuralia Major groups within phyla Sponges Demosponges Glass sponges Calcareous sponges Cnidarians Anthozoans inc. corals Medusozoans inc. jellyfish Myxozoans Chordates Lancelets Tunicates Vertebrates Echinoderms Sea lilies Asterozoans inc. starfish Echinozoans inc. sea urchins Hemichordates Acorn worms Pterobranchs Nematodes Chromadorea Enoplea Secernentea Arthropods Chelicerates inc. arachnids Myriapods Pancrustaceans inc. hexapods Rotifera Bdelloidea Monogononta Seisonidae Acanthocephala Platyhelminths Turbellaria Trematoda Monogenea Cestoda Ectoproctans Phylactolaemata Stenolaemata Gymnolaemata Annelids Polychaetes Clitellata Sipuncula Molluscs Gastropods Cephalopods Bivalves Chitons Tusk shells Phyla with ≥1000 extant species bolded Potentially dubious phyla †

Taxon identifiers Protostomia Wikidata: Q5171 Wikispecies: Protostomia ADW: Protostomia BioLib: 14936 EoL: 10459935 ITIS: 914155 NCBI: 33317 Paleobiology Database: 85324 TaiCOL: t0000028 Taxonomicon: 198701

Authority control databases International GND National Czech Republic

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Adapted from the Wikipedia article [Protostome](https://en.wikipedia.org/wiki/Protostome) by Wikipedia contributors ([contributor history](https://en.wikipedia.org/wiki/Protostome?action=history)). Available under [Creative Commons Attribution-ShareAlike 4.0 International](https://creativecommons.org/licenses/by-sa/4.0/). Changes may have been made.