Editing Flatworm (section)

==Classification and evolutionary relationships==

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

The relationships of Platyhelminthes to other [[Bilateria]] are shown in the [[phylogenetic tree]]:<ref name="Halanych2004NewViewOfAnimalPhylogeny" />

{{clade
|label1=[[Bilateria]]
|1={{clade
   |1=[[Acoelomorpha]] [[File:Proporus sp. (no background).png|55 px]]
   |2={{clade
      |1=[[Deuterostomia]] [[File:Cyprin carpi 090613-0329 tdp.png|60 px]]
      |label2=[[Protostomia]]
      |2={{clade
         |1=[[Ecdysozoa]] [[File:Aptostichus simus Monterey County.jpg|60px]]
         |label2=[[Spiralia]]
         |2={{clade
            |1=[[Gnathifera (clade)|Gnathifera]] <span style="{{MirrorH}}">[[File:Pseudosagitta maxima 31349361.png|70px]]</span>
            |label2=[[Platytrochozoa]]
            |sublabel2=580 mya
            |2={{clade
               |label1=[[Rouphozoa]]
               |1={{clade
                  |1=[[Gastrotricha]] [[File:Diuronotus aspetos 1.png|60px]]
                  |sublabel2=270 mya
                  |2='''Platyhelminthes''' [[File:Pseudobiceros bedfordi 13376124.png|70px]]
                  }}
               |label2=[[Lophotrochozoa]]
               |sublabel2=550 mya
               |2={{clade
                  |1=[[Mollusca]] <span style="{{MirrorH}}">[[File:Grapevinesnail 01a.jpg|65px]]</span>
                  |2=[[Annelida]] [[File:Polychaeta (no) 2.jpg|60px]]
                  }}
               }}
            }}
         }}
      }}
   }}
}}

The internal relationships of Platyhelminthes are shown below. The tree is not fully resolved.<ref name="TimothyEtAl2004ProtostomesAndPlatyhelminthes" /><ref name=Boll2013>{{Cite journal |doi=10.4013/nbc.2013.81.06 |last1=Boll |first1=P. K. |last2=Rossi |first2=I. |last3=Amaral |first3=S. V. |last4=Oliveira |first4=S. M. |last5=Müller |first5=E. S. |last6=Lemos |first6=V. S. |last7=Leal-Zanchet |first7=A. M. |date=2013 |title=Platyhelminthes ou apenas semelhantes a Platyhelminthes? Relações filogenéticas dos principais grupos de turbelários |journal=Neotropical Biology and Conservation |volume=8 |issue=1 |pages=41–52|doi-access=free }}</ref><ref name=Egger2015>{{Cite journal |doi=10.1016/j.cub.2015.03.034 |last1=Egger |first1=B. |last2=Lapraz |first2=F. |last3=Tomiczek |first3=B. |last4=Müller |first4=S. |last5=Dessimoz |first5=C. |last6=Girstmair |first6=J. |last7=Škunca |first7=N. |last8=Rawlinson |first8=K. A. |last9=Cameron |first9=C. B. |last10=Beli |first10=E. |last11=Todaro |first11=M. A. |last12=Gammoudi |first12=M. |last13=Noreña |first13=C. |last14=Telford |first14=M. I. |date=18 May 2015 |title=A Transcriptomic-Phylogenomic Analysis of the Evolutionary Relationships of Flatworms |journal=Current Biology |volume=25 |issue=10 |pages=1347–1353 |pmid=25866392 |pmc=4446793}}</ref>

{{clade
|label1='''Platyhelminthes'''
|1={{clade
  |1=[[Catenulida]]
  |label2=[[Rhabditophora]]
  |2={{clade
    |1=[[Macrostomorpha]]
    |label2=[[Trepaxonemata]]
    |2={{clade
      |label1=[[Amplimatricata]]
      |1={{clade
        |1=[[Prorhynchidae|Prorhynchida]]
        |2=[[Polycladida]]
         }}
      |2={{clade
        |1=[[Gnosonesimidae|Gnosonesimida]]
        |label2=[[Euneoophora]]
        |2={{clade
          |label1=[[Rhabdocoela]]
          |1={{clade
            |1=[[Kalyptorhynchia]]
            |2=[[Dalytyphloplanida]]
             }}
          |2={{clade
            |1=[[Proseriata]]
            |label2=Acentrosomata
            |2={{clade
              |label1=[[Adiaphanida]]
              |1={{clade
                |1=[[Prolecithophora]]
                |2={{clade
                  |1=[[Fecampiida]]
                  |2=[[Tricladida]] (planarians)
                   }}
                 }}
              |label2=[[Bothrioneodermata]]
              |2={{clade
                |label1=Bothrioplanata
                |1=[[Bothrioplanida]] (freshwater)
                |sublabel2=''parasitic''
                |2=[[Neodermata]] ([[fluke (parasite)|fluke]]s, [[tapeworms]])
                 }}
               }}
             }}
           }}
         }}
       }}
     }}
   }}
}}

The oldest confidently identified parasitic flatworm fossils are [[cestode]] eggs found in a [[Permian]] shark [[coprolite]], but helminth hooks still attached to [[Devonian]] [[acanthodians]] and [[placoderms]] might also represent parasitic flatworms with simple life cycles.<ref>{{cite book |isbn=978-0-12-804001-0 |author=De Baets, K., P. Dentzien-Dias, I. Upeniece, O. Verneau and P. C. J. Donoghue |doi=10.1016/bs.apar.2015.06.002 |pmid=26597066 |title=Advances in Parasitology |chapter=Chapter Three – Constraining the Deep Origin of Parasitic Flatworms and Host-Interactions with Fossil Evidence |editor=Kenneth De Baets and D. Timothy J Littlewood |volume=90 |pages=93–135 |date=2015-12-15 |s2cid=7278956 |url=https://hal-univ-perp.archives-ouvertes.fr/hal-01257074/file/debaets2015.pdf |access-date=2022-07-15 |archive-date=2022-07-30 |archive-url=https://web.archive.org/web/20220730040408/https://hal-univ-perp.archives-ouvertes.fr/hal-01257074/file/debaets2015.pdf |url-status=live }}</ref> The oldest known free-living platyhelminth specimen is a fossil preserved in [[Eocene]] age [[Baltic amber]] and placed in the monotypic species ''[[Micropalaeosoma|Micropalaeosoma balticus]]'',<ref name="Poinar2003">{{cite journal |doi=10.1111/j.1744-7410.2003.tb00095.x |last1=Poinar |first1=G. |year=2003 |title=A Rhabdocoel Turbellarian (Platyhelminthes, Typhloplanoida) in Baltic Amber with a Review of Fossil and Sub-Fossil Platyhelminths |journal= Invertebrate Biology |volume=122 |issue=4|pages=308–312 |jstor=3227067}}</ref> whilst the oldest subfossil specimens are [[Schistosoma|schistosome]] eggs discovered in ancient Egyptian [[Mummy|mummies]].<ref name="Rhode2001PlatyhelminthesInEncLifeSci" /> The Platyhelminthes have very few [[synapomorphy|synapomorphies]] - distinguishing features that all Platyhelminthes (but no other animals) exhibit. This makes it difficult to work out their relationships with other groups of animals, as well as the relationships between different groups that are described as members of the Platyhelminthes.<ref>{{cite journal |author1=Carranza, S. |author2=Baguñà, J. |author3=Riutort, M.  |name-list-style=amp |title=Are the Platyhelminthes a monophyletic primitive group?
|journal=Molecular Biology and Evolution |volume=14|pages=485–497 |pmid=9159926 |issue=5 |date= May 1, 1997 |doi=10.1093/oxfordjournals.molbev.a025785 |doi-access=free }}</ref>

The "traditional" view before the 1990s was that Platyhelminthes formed the [[sister group]] to all the other bilaterians, which include, for instance, [[arthropod]]s, [[mollusc]]s, [[annelid]]s and [[chordate]]s. Since then, [[molecular phylogenetics]], which aims to work out evolutionary "family trees" by comparing different organisms' [[biochemistry|biochemicals]] such as [[DNA]], [[RNA]] and [[protein]]s, has radically changed scientists' view of evolutionary relationships between animals.<ref name="Halanych2004NewViewOfAnimalPhylogeny" /> Flatworms are now recognized as secondarily simplified bilaterians.<ref>{{Cite journal|title=A global metabarcoding analysis expands molecular diversity of Platyhelminthes and reveals novel early-branching clades|first1=Konstantina|last1=Mitsi|first2=Alicia S.|last2=Arroyo|first3=Iñaki|last3=Ruiz-Trillo|date=September 27, 2019|journal=Biology Letters|volume=15|issue=9|pages=20190182|doi=10.1098/rsbl.2019.0182|pmid=31506037|pmc=6769146}}</ref>

Detailed [[morphology (biology)|morphological]] analyses of anatomical features in the mid-1980s, as well as molecular phylogenetics analyses since 2000 using different sections of DNA, agree that [[Acoelomorpha]], consisting of [[Acoela]] (traditionally regarded as very simple "[[turbellaria]]ns"<ref name="RuppertBarnes2004Platyhelminthes" />) and [[Nemertodermatida]] (another small group previously classified as "turbellarians"<ref name="JondeliusEtAl2002Nemertodermatida">{{cite journal |author1=Jondelius, U. |author2=Ruiz-Trillo, I. |author3=Baguñà, J. |author4=Riutort, M. |title=The Nemertodermatida are basal bilaterians and not members of the Platyhelminthes |journal=Zoologica Scripta |volume=31 |issue=2 |pages=201–215 |date=April 2002 |doi=10.1046/j.1463-6409.2002.00090.x |s2cid=84015834 }}</ref>) are the sister group to all other bilaterians.<ref name="Halanych2004NewViewOfAnimalPhylogeny" /><ref name="LarssonJondelius2008PhylogenyOfCatenulida">{{cite journal |author1=Larsson, K. |author2=Jondelius, U. |title=Phylogeny of Catenulida and support for Platyhelminthes |journal=Organisms Diversity & Evolution |volume=8 |date=20 December 2008 |pages=378–387 |doi=10.1016/j.ode.2008.09.002 |issue=5 }}</ref> However, a 2007 study concluded that Acoela and Nemertodermatida were two distinct groups of bilaterians.<ref name="Wallberg2007EtAlDismissalOfAcoelomorpha">{{cite journal |author1=Wallberg, A. |author2=Curini-Galletti, M. |author3=Ahmadzadeh, A. |author4=Jondelius, U.  |name-list-style=amp |title=Dismissal of Acoelomorpha: Acoela and Nemertodermatida are separate early bilaterian clades |journal=Zoologica Scripta |volume=36 |issue=5 |pages=509–523 |date= September 2007 |doi=10.1111/j.1463-6409.2007.00295.x
|s2cid=85599100 }}</ref>

''[[Xenoturbella]]'', a bilaterian whose only well-defined organ is a [[statocyst]], was originally classified as a "primitive turbellarian".<ref name="Westblad1949Xenoturbella">{{cite journal |author=Westblad, E. |title=''Xenoturbella bocki'' n.g., n.sp., a peculiar, primitive turbellarian type |journal=Arkiv för Zoologi |year=1949 |volume=1 |pages=3–29 }}</ref> Later studies suggested it may instead be a [[deuterostome]],<ref name="BourlatEtAl2003XenoturbellaPhylum">{{cite journal|title=''Xenoturbella'' is a deuterostome that eats molluscs |journal=Nature |date=21 August 2003 |volume=424 |pages=925–928 |doi=10.1038/nature01851 |vauthors=Bourlat SJ, Nielsen C, Lockyer AE, Littlewood DT, Telford MJ |pmid=12931184
|issue=6951 |bibcode=2003Natur.424..925B |s2cid=4413357 }}</ref><ref>{{cite journal|title=Deuterostome phylogeny reveals monophyletic chordates and the new phylum Xenoturbellida |journal=Nature |year=2006 |volume=444 |pages=85–88 |doi=10.1038/nature05241 |pmid=17051155 |issue=7115 |vauthors=Bourlat SJ, Juliusdottir T, Lowe CJ, Freeman R, Aronowicz J, Kirschner M, Lander ES, Thorndyke M, Nakano H, Kohn AB, Heyland A, Moroz LL, Copley RR, Telford MJ |bibcode=2006Natur.444...85B |s2cid=4366885 }}</ref> but more detailed molecular phylogenetics have led to its classification as sister-group to the Acoelomorpha.<ref name="Cannon2016">{{cite journal|last1=Cannon |first1=J.T. |last2=Vellutini |first2=B.C. |last3=Smith |first3=J. |last4=Ronquist |first4=F. |last5=Jondelius |first5=U. |last6=Hejnol |first6=A. |title=Xenacoelomorpha is the sister group to Nephrozoa |journal=Nature |volume=530 |issue=7588 |date=4 February 2016 |pages=89–93 |pmid=26842059 |doi=10.1038/nature16520|url=http://urn.kb.se/resolve?urn=urn:nbn:se:nrm:diva-1844 |bibcode=2016Natur.530...89C |s2cid=205247296 }}</ref>

The Platyhelminthes excluding Acoelomorpha contain two main groups - [[Catenulida]] and [[Rhabditophora]] - both of which are generally agreed to be monophyletic (each contains all and only the descendants of an ancestor that is a member of the same group).<ref name="LarssonJondelius2008PhylogenyOfCatenulida" /><ref name="TimothyEtAl2004ProtostomesAndPlatyhelminthes">{{cite book |author1=Timothy, D. |author2=Littlewood, J. |author3=Telford, M. J. |author4=Bray, R. A. |name-list-style=amp |chapter=Protostomes and Platyhelminthes |pages=209–223 |editor1=Cracraft, J. |editor2=Donoghue, M. J. |title=Assembling the Tree of Life |publisher=Oxford University Press US |year=2004 |isbn=978-0-19-517234-8 |chapter-url=https://books.google.com/books?id=_jLl8zIRzucC&q=neodermata+phylogeny&pg=PA217 |access-date=2020-11-11 |archive-date=2023-03-23 |archive-url=https://web.archive.org/web/20230323154500/https://books.google.com/books?id=_jLl8zIRzucC&q=neodermata+phylogeny&pg=PA217 |url-status=live }}</ref> Early molecular phylogenetics analyses of the Catenulida and Rhabditophora left uncertainties about whether these could be combined in a single monophyletic group; a study in 2008 concluded that they could, therefore Platyhelminthes could be redefined as Catenulida plus Rhabditophora, excluding the Acoelomorpha.<ref name="LarssonJondelius2008PhylogenyOfCatenulida" />

Other molecular phylogenetics analyses agree the redefined Platyhelminthes are most closely related to [[Gastrotricha]], and both are part of a grouping known as [[Platyzoa]]. Platyzoa are generally agreed to be at least closely related to the [[Lophotrochozoa]], a super[[phylum]] that includes molluscs and [[annelid]] worms. The majority view is that Platyzoa are part of Lophotrochozoa, but a significant minority of researchers regard Platyzoa as a sister group of Lophotrochozoa.<ref name="Halanych2004NewViewOfAnimalPhylogeny">{{cite journal |author=Halanych, K. M. |year=2004 |title=The New View of Animal Phylogeny |url=http://mugue.narod.ru/supporting_materials/Halanych_2004.pdf |journal=Annual Review of Ecology, Evolution, and Systematics |volume=35 |pages=229–256 |doi=10.1146/annurev.ecolsys.35.112202.130124 |access-date=2010-10-28 |archive-date=2017-06-11 |archive-url=https://web.archive.org/web/20170611044006/http://mugue.narod.ru/supporting_materials/Halanych_2004.pdf |url-status=live }}</ref>

It has been agreed since 1985 that each of the wholly parasitic platyhelminth groups ([[Cestoda]], [[Monogenea]] and [[Trematoda]]) is monophyletic, and that together these form a larger monophyletic grouping, the [[Neodermata]], in which the adults of all members have [[syncytium|syncytial]] skins.<ref name="WillemsWallbergEtAl2005FillingGap">{{cite journal |author1=Willems, W.R. |author2=Wallberg, A. |author3=Jondelius, U. |title=Filling a gap in the phylogeny of flatworms: relationships within the Rhabdocoela (Platyhelminthes), inferred from 18S ribosomal DNA sequences |journal=Zoologica Scripta |volume=35 |issue=1 |pages=1–17 |year=2005 |doi=10.1111/j.1463-6409.2005.00216.x |url=https://doclib.uhasselt.be/dspace/bitstream/1942/1609/1/Filling%20the%20gap.pdf |access-date=2008-12-23 |display-authors=etal |hdl=1942/1609 |s2cid=85917387 |archive-url=https://web.archive.org/web/20111006005758/https://doclib.uhasselt.be/dspace/bitstream/1942/1609/1/Filling%20the%20gap.pdf |archive-date=2011-10-06 |url-status=dead |hdl-access=free }}</ref> However, there is debate about whether the [[Cestoda]] and [[Monogenea]] can be combined as an intermediate monophyletic group, the [[Cercomeromorpha]], within the Neodermata.<ref name="WillemsWallbergEtAl2005FillingGap" /><ref>{{cite journal |author1=Lockyer, A.E. |author2=Olson, P.D. |author3=Littlewood, D.T.J.  |name-list-style=amp |title=Utility of complete large and small subunit rRNA genes in resolving the phylogeny of the Neodermata (Platyhelminthes): implications and a review of the cercomer theory |journal=Biological Journal of the Linnean Society |volume=78 |issue=2 |pages=155–171 |doi=10.1046/j.1095-8312.2003.00141.x |year=2003|doi-access=free }}</ref> It is generally agreed that the Neodermata are a sub-group a few levels down in the "family tree" of the Rhabditophora.<ref name="LarssonJondelius2008PhylogenyOfCatenulida" /> Hence the traditional sub-phylum "[[Turbellaria]]" is [[paraphyletic]], since it does not include the Neodermata although these are descendants of a sub-group of "turbellarians".<ref>{{cite journal |author=Ehlers, U. |title=Comments on a phylogenetic system of the Platyhelminthes |journal=Hydrobiologia |volume=132 |issue=1 |pages=1–12 |year= 1986 |doi=10.1007/BF00046222  |s2cid=6018712 }}</ref>