Editing Chaetognatha (section)

== Phylogeny ==
[[File:NS and jaws Chaetognathifera.png|thumb|Hypothesis of the possible homology between the jaw parts and nervous system parts of extant and extinct Chaetognathifera taxa. Putative homologies between jaw parts and nervous system respectively have the same color. Questionable, and/or, disputable homologies are in grey. Non oberved part of the nervous system are in dotted lines. Only the anterior part of each organism is represented for the jaws. Modified from Bekkouche and Gąsiorowski 2022<ref name="Gynognathifera">{{cite journal |last1=Bekkouche |first1=Nicolas |last2=Gąsiorowski |first2=Ludwik |date=2022 |title=Careful amendment of morphological data sets improves phylogenetic frameworks: re-evaluating placement of the fossil ''Amiskwia sagittiformis'' |journal=Journal of Systematic Palaeontology |volume=20 |pages=1–14 |doi=10.1080/14772019.2022.2109217|url=https://hal.sorbonne-universite.fr/hal-03828531/file/Manuscript_Amiskwia_Bekkouche_and_Gasiorowski_Hall_version_compressed.pdf }}</ref>]]
===External===
The evolutionary relationships of chaetognaths have long been enigmatic. [[Charles Darwin]] remarked that arrow worms were "remarkable for the obscurity of their affinities".<ref name="Ball2006">{{cite journal |doi=10.1016/j.cub.2006.07.006 |title=Phylogeny: The Continuing Classificatory Conundrum of Chaetognaths |date=2006 |last1=Ball |first1=Eldon E. |last2=Miller |first2=David J. |journal=Current Biology |volume=16 |issue=15 |pages=R593–R596 |pmid=16890517 |s2cid=18793650 |doi-access=free |bibcode=2006CBio...16.R593B |hdl=1885/26305 |hdl-access=free }}</ref> Chaetognaths in the past have been traditionally, but erroneously, classed as [[deuterostomes]] by [[embryologist]]s due to deuterostome-like features in the embryo. [[Lynn Margulis]] and K. V. Schwartz placed chaetognaths in the deuterostomes in their ''Five Kingdom'' classification.<ref>[http://sn2000.taxonomy.nl/Taxonomicon/TaxonTree.aspx?id=40615 Systema Naturae 2000 Taxon: Phylum Chaetognatha per Margulis and Schwartz] {{webarchive |url=https://web.archive.org/web/20051127153818/http://sn2000.taxonomy.nl/Taxonomicon/TaxonTree.aspx?id=40615 |date=November 27, 2005 }} (select Margulis & Schwartz in 'Classification by')—last retrieved November 25, 2006</ref> However, several developmental features are at odds with deuterostomes and are either akin to [[Spiralia]] or unique to Chaetognatha.<ref name="Perez2021"/>
 
{{cladogram|caption=Summary of relationships of gnathiferans in recent studies including Chaetognatha within the clade, with disputed relationships represented as polytomies<ref name=marletaz19>{{Cite journal | last5 = Rokhsar | first5 = Daniel S.| last4 = Satoh | first4 = Noriyuki | last3 = Goto | first3 = Taichiro | last2 = Peijnenburg | first2 = Katja T. C. A. | last1 = Marlétaz | first1 = Ferdinand | year = 2019 | title = A new spiralian phylogeny places the enigmatic arrow worms among gnathiferans | journal = Current Biology | volume = 29 | issue = 2 | pages = 312–318.e3 | doi=10.1016/j.cub.2018.11.042 | pmid = 30639106| doi-access = free | bibcode = 2019CBio...29E.312M| url = https://discovery.ucl.ac.uk/10097250/1/Phylogeny_chaeto_rv9s_pre.pdf }}</ref><ref name=vinther19>{{cite journal | first1 = Jakob | last1 = Vinther | first2 = Luke A. | last2 = Parry | year = 2019 | title = Bilateral jaw elements in Amiskwia sagittiformis bridge the morphological gap between gnathiferans and chaetognaths | journal = Current Biology | volume = 29 | issue = 5 | pages = 881–888.e1 | doi = 10.1016/j.cub.2019.01.052 | pmid = 30799238 | doi-access = free | bibcode = 2019CBio...29E.881V | url = https://research-information.bris.ac.uk/files/183275665/Collated_for_pure.pdf }}</ref><ref name=frobius17>{{cite journal | last1 = Fröbius | first1 = Andreas C. | last2 = Funch | first2 = Peter | year = 2017 | 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><ref name=laumer15>{{cite journal | first1 = Christopher E. | last1 = Laumer | first2 = Nicolas | last2 = Bekkouche | first3 = Alexandra | last3 = Kerbl | first4 = Freya | last4 = Goetz | first5 = Ricardo C. | last5 = Neves | first6 = Martin V. | last6 = Sørensen | first7 = Reinhardt M. | last7 = Kristensen | first8 = Andreas | last8 = Hejnol | first9 = Casey W. | last9 = Dunn | first10 = Gonzalo | last10 = Giribet | first11 = Katrine | last11 = Worsaae | year = 2015 | title = Spiralian phylogeny informs the evolution of microscopic lineages | journal = Current Biology | volume = 25 | issue = 15 | pages = 2000–2006 | doi = 10.1016/j.cub.2015.06.068 | pmid = 26212884 | doi-access = free | bibcode = 2015CBio...25.2000L }}</ref><ref name=sielaff16>{{cite journal | title = Phylogeny of Syndermata (syn. Rotifera): Mitochondrial gene order verifies epizoic Seisonidea as sister to endoparasitic Acanthocephala within monophyletic Hemirotifera | journal = Molecular Phylogenetics and Evolution | volume = 96 | year = 2016 | pages = 79–92 | doi = 10.1016/j.ympev.2015.11.017 | last1 = Sielaff | first1 = Malte | last2 = Schmidt | first2 = Hanno | last3 = Struck | first3 = Torsten H. | last4 = Rosenkranz | first4 = David | last5 = Mark Welch | first5 = David B. | last6 = Hankeln | first6 = Thomas | last7 = Herlyn | first7 = Holger | pmid = 26702959 | bibcode = 2016MolPE..96...79S }}</ref>
|clades={{Clade|style=font-size:100%; line-height:100%
   |label1=[[Spiralia]]
   |1={{clade
      |label1=[[Gnathifera (clade)|Gnathifera]]
      |1={{clade
         |1=[[Gnathostomulida]]
         |2={{Clade
            |1=[[Micrognathozoa]]
            |2='''Chaetognatha'''
            |label3=[[Rotifera]]
            |3={{clade
               |1=[[Seisonida]]
               |2=[[Acanthocephala]]
               |3=[[Bdelloidea]]
               |4=[[Monogononta]]
               }}
            }}
         }}
      |2=[[Platytrochozoa]]
      }}
   }}
|align=right}}

{{cladogram|caption=Chaetognaths in the [[metazoa]]n tree of life, when considered the sister group of Gnathifera.<ref name="Perez2021"/>
|clades={{Clade|style=font-size:100%; line-height:100%
   |label1=Metazoa
   |1={{clade
       |1=[[Ctenophora]]
       |2=[[Porifera]]
       |3=[[Cnidaria]]
       |4=[[Placozoa]]
       |5={{clade
          |1=[[Xenacoelomorpha]]
          |2={{clade
             |1=[[Deuterostomia]]
             |2={{clade
                |1=[[Ecdysozoa]]
                |2={{clade
                   |1=[[Lophotrochozoa]]
                   |2={{clade
                      |1=[[Gnathifera (clade)|Gnathifera]]
                      |2='''Chaetognatha'''
                      }}
                   }} |label2=[[Spiralia]]
                }} | label2=[[Protostomia]]
             }} | label2=[[Nephrozoa]]
          }} |label5=[[Bilateria]]
       }}
   }}
}}

Molecular [[phylogeny]] shows that Chaetognatha are, in fact, [[protostomes]]. [[Thomas Cavalier-Smith]] places them in the protostomes in his ''Six Kingdom'' classification.<ref>[http://sn2000.taxonomy.nl/Taxonomicon/TaxonTree.aspx?id=40615 Systema Naturae 2000 Taxon: Phylum Chaetognatha per Cavalier-Smith] {{webarchive |url=https://web.archive.org/web/20051127153818/http://sn2000.taxonomy.nl/Taxonomicon/TaxonTree.aspx?id=40615 |date=November 27, 2005 }} (select Cavalier-Smith in 'Classification by')—last retrieved November 25, 2006</ref> The similarities between chaetognaths and nematodes mentioned above may support the protostome thesis—in fact, chaetognaths are sometimes regarded as a basal [[ecdysozoa]]n or [[lophotrochozoa]]n.<ref>{{cite journal | vauthors = Matus DQ, Copley RR, Dunn CW, Hejnol A, Eccleston H, Halanych KM, Martindale MQ, Telford MJ | title = Broad taxon and gene sampling indicate that chaetognaths are protostomes | journal = Current Biology | volume = 16 | issue = 15 | pages = R575–R576 | date = August 2006 | pmid = 16890509 | doi = 10.1016/j.cub.2006.07.017 | s2cid = 11284138 | doi-access = free | bibcode = 2006CBio...16.R575M }}</ref> Chaetognatha appears close to the base of the protostome tree in most studies of their molecular phylogeny.<ref>{{cite journal | vauthors = Marlétaz F, Martin E, Perez Y, Papillon D, Caubit X, Lowe CJ, Freeman B, Fasano L, Dossat C, Wincker P, Weissenbach J, Le Parco Y | title = Chaetognath phylogenomics: a protostome with deuterostome-like development | journal = Current Biology | volume = 16 | issue = 15 | pages = R577-8 | date = August 2006 | pmid = 16890510 | doi = 10.1016/j.cub.2006.07.016 | s2cid = 18339954 | doi-access = free | bibcode = 2006CBio...16.R577M }}</ref> This may explain their deuterostome embryonic characters. If chaetognaths branched off from the protostomes before they evolved their distinctive protostome embryonic characters, they might have retained deuterostome characters inherited from early [[bilateria]]n ancestors. Thus chaetognaths may be a useful model for the ancestral bilaterian.<ref>{{cite journal | vauthors = Papillon D, Perez Y, Caubit X, Le Parco Y | 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 = 2122–9 | date = November 2004 | pmid = 15306659 | doi = 10.1093/molbev/msh229 | doi-access = free }}</ref>
Studies of arrow worms' nervous systems suggests they should be placed within the protostomes.<ref>{{cite journal | vauthors =Rieger V, Perez Y, Müller CH, Lipke E, Sombke A, Hansson BS, Harzsch S | date = February 2010 |title=Immunohistochemical analysis and 3D reconstruction of the cephalic nervous system in Chaetognatha: Insights into the evolution of an early bilaterian brain? |journal=Invertebrate Biology |volume=129 |issue=1 |pages=77–104 |doi=10.1111/j.1744-7410.2010.00189.x}}</ref><ref>{{cite journal | vauthors = Harzsch S, Müller CH | title = A new look at the ventral nerve centre of Sagitta: implications for the phylogenetic position of Chaetognatha (arrow worms) and the evolution of the bilaterian nervous system | journal = Frontiers in Zoology | volume = 4 | pages = 14 | date = May 2007 | pmid = 17511857 | pmc = 1885248 | doi = 10.1186/1742-9994-4-14 | doi-access = free }}</ref> According to 2017 and 2019 papers, chaetognaths either belong to<ref>{{cite journal | vauthors = Fröbius AC, Funch P | title = Rotiferan Hox genes give new insights into the evolution of metazoan bodyplans | journal = Nature Communications | volume = 8 | issue = 1 | pages = 9 | date = April 2017 | pmid = 28377584 | pmc = 5431905 | doi = 10.1038/s41467-017-00020-w | bibcode = 2017NatCo...8....9F }}</ref><ref>{{cite journal | vauthors = Marlétaz F, Peijnenburg KT, Goto T, Satoh N, Rokhsar DS | title = A New Spiralian Phylogeny Places the Enigmatic Arrow Worms among Gnathiferans | language = en | journal = Current Biology | volume = 29 | issue = 2 | pages = 312–318.e3 | date = January 2019 | pmid = 30639106 | doi = 10.1016/j.cub.2018.11.042 | doi-access = free | bibcode = 2019CBio...29E.312M | url = https://discovery.ucl.ac.uk/10097250/1/Phylogeny_chaeto_rv9s_pre.pdf }}</ref> or are the sister group of  [[Gnathifera (clade)|Gnathifera]].<ref name="Perez2021"/>

===Internal===
Below is a consensus evolutionary tree of extant Chaetognatha, based on both morphological and molecular data, as of 2021.<ref name="Perez2021">{{cite book | last1=Perez | first1=Yvan | last2=Müller | first2=Carsten H.G. | last3=Harzsch | first3=Steffen |editor-last=Schierwater | editor-first=Bernd | editor-last2=DeSalle | editor-first2=Rob | title=Invertebrate Zoology: A Tree of Life Approach | publisher=CRC Press | year=2021 | chapter=Chapter 15: Chaetognatha | isbn=978-1-4822-3582-1 | chapter-url=https://books.google.com/books?id=Bk4vEAAAQBAJ&pg=PA231 | access-date=2023-08-14 | page=231}}</ref>

{{clade |style=line-height:100%;
|label1=Chaetognatha
|1={{clade
   |1={{clade
      |1=[[Heterokrohniidae]]
      |2=[[Eukrohniidae]]
      }}
   |2={{clade
      |1=[[Spadellidae]]
      |2={{clade
         |1=[[Krohnittidae]]
         |2=[[Sagittidae]]
         }}
      }}
   }}
}}