Editing Placozoa

{{Short description|Phylum of aquatic animals}}
{{Automatic taxobox
| fossil_range = {{Fossil range|Middle Triassic|Recent}}<ref name="Knaust 2021">{{Cite journal |last=Knaust |first=Dirk |date=2021-10-07 |title=A microbialite with its entombed benthic community from the Middle Triassic (Anisian-Ladinian) Muschelkalk Group of Germany |url=http://www.schweizerbart.de/papers/pala/detail/320/99661/A_microbialite_with_its_entombed_benthic_community?af=crossref |journal=Palaeontographica Abteilung A |language=en |volume=320 |issue=1–3 |pages=1–63 |doi=10.1127/pala/2021/0114 |issn=0375-0442|url-access=subscription }}</ref>
| name = Placozoans
| image = Trichoplax adhaerens photograph.png
| image_caption = ''[[Trichoplax adhaerens]]''
| display_parents = 2
| taxon = Placozoa
| authority = Grell, 1971
| type_species = ''[[Trichoplax adhaerens]]''
| subdivision_ranks = Classes<ref name="Tessler et al 2022">{{cite journal|last1=Tessler|first1=Michael|last2=Neumann|first2=Johannes S.|last3=Kamm|first3=Kai|last4=Osigus|first4=Hans-Jürgen|last5=Eshel|first5=Gil|last6=Narechania|first6=Apurva|last7=Burns|first7=John A.|last8=DeSalle|first8=Rob|last9=Schierwater|first9=Bernd|title=Phylogenomics and the first higher taxonomy of Placozoa, an ancient and enigmatic animal phylum|journal= Frontiers in Ecology and Evolution|date=2022-12-08|volume=10|doi=10.3389/fevo.2022.1016357|doi-access=free}}</ref>
| subdivision =
* Class [[Polyplacotomia]]
** Order [[Polyplacotomea]]
*** Family [[Polyplacotomidae]]
* Class [[Uniplacotomia]]
** Order [[Trichoplacea]]
*** Family [[Trichoplacidae]]
** Order [[Cladhexea]]
*** Undescribed species
** Order [[Hoilungea]]
*** Family [[Cladtertiidae]]
*** Family [[Hoilungidae]]
** Genus {{extinct}}''[[Maculicorpus]]''
}}

'''Placozoa''' ({{IPAc-en|,|p|l|ae|k|@|'|z|ou|@}} {{respell|PLAK|ə|ZOH|ə}}; {{lit|flat animals}})<ref>{{cite book |author1=Rüdiger Wehner |title=Zoologie |author2=Walter Gehring |date=June 2007 |publisher=Thieme |edition=24th |location=Stuttgart |page=696 |language=de |name-list-style=amp}}</ref> is a [[phylum]] of free-living (non-parasitic) [[marine invertebrate]]s.<ref>{{Cite journal |last1=Kamm |first1=Kai |last2=Schierwater |first2=Bernd |last3=DeSalle |first3=Rob |date=2019 |title=Innate immunity in the simplest animals - placozoans |journal=BMC Genomics |volume=20 |issue=1 |pages=5 |doi=10.1186/s12864-018-5377-3 |pmc=6321704 |pmid=30611207 |doi-access=free }}</ref><ref>{{MeshName|Placozoa}}</ref> They are blob-like animals composed of aggregations of cells. Moving in water by [[ciliary motion]], eating food by [[Phagocytosis|engulfment]], reproducing by [[Fission (biology)|fission]] or [[budding]], placozoans are described as "the simplest animals on Earth."<ref>{{Cite journal |last=Pennisi |first=Elizabeth |date=2021 |title=The simplest of slumbers |url=https://pubmed.ncbi.nlm.nih.gov/34709907 |journal=Science |volume=374 |issue=6567 |pages=526–529 |doi=10.1126/science.acx9444 |issn=1095-9203 |pmid=34709907|bibcode=2021Sci...374..526P |s2cid=240154099 }}</ref> Structural and molecular analyses have supported them as among the most basal animals,<ref>{{Cite book |last1=Neumann |first1=Johannes S. |url=https://books.google.com/books?id=Bk4vEAAAQBAJ |title=Invertebrate Zoology: A Tree of Life Approach |last2=DeSalle |first2=Tessler |last3=DeSalle |first3=Rob |last4=Michael |first4=Bernd |date=2021 |publisher=CRC Press |isbn=978-1-4822-3582-1 |editor-last=Schierwater |editor-first=Bernd |page=71 |language=en |chapter=Modern invertebrate systematics |doi=10.1201/9780429159053 |s2cid=260730525 |editor-last2=DeSalle |editor-first2=Rob |chapter-url=https://www.taylorfrancis.com/chapters/edit/10.1201/9780429159053-4/modern-invertebrate-systematics-johannes-neumann-michael-tessler-rob-desalle-bernd-schierwater }}</ref><ref>{{Cite journal |last1=Kawashima |first1=Takeshi |last2=Yoshida |first2=Masa-Aki |last3=Miyazawa |first3=Hideyuki |last4=Nakano |first4=Hiroaki |last5=Nakano |first5=Natumi |last6=Sakamoto |first6=Tatsuya |last7=Hamada |first7=Mayuko |date=2022 |title=Observing Phylum-Level Metazoan Diversity by Environmental DNA Analysis at the Ushimado Area in the Seto Inland Sea |journal=Zoological Science |volume=39 |issue=1 |pages=157–165 |doi=10.2108/zs210073 |issn=0289-0003 |pmid=35107003|s2cid=246297787 |doi-access=free }}</ref> thus, constituting a primitive [[metazoan]] phylum.<ref>{{Citation |last1=Schierwater |first1=Bernd |title=Placozoa |date=2015 |url=https://link.springer.com/10.1007/978-3-7091-1862-7_5 |work=Evolutionary Developmental Biology of Invertebrates 1 |pages=107–114 |editor-last=Wanninger |editor-first=Andreas |access-date=2023-06-02 |place=Vienna |publisher=Springer Vienna |language=en |doi=10.1007/978-3-7091-1862-7_5 |isbn=978-3-7091-1861-0 |last2=Eitel |first2=Michael|url-access=subscription }}</ref>

The first known placozoan, ''[[Trichoplax adhaerens]]'', was discovered in 1883 by the German zoologist [[Franz Eilhard Schulze]] (1840–1921).<ref name=":0">F. E. Schulze "''Trichoplax adhaerens'' n. g., n. s.", ''Zoologischer Anzeiger'' (Elsevier, Amsterdam and Jena) 6 (1883), p. 92.</ref><ref name="IZ" /> Describing the uniqueness, another German, [[Karl Gottlieb Grell]] (1912–1994), erected a new phylum, Placozoa, for it in 1971. Remaining a [[monotypic]] phylum for over a century,<ref>{{cite journal |last=Voigt |first=O |author2=Collins AG |author3=Pearse VB |author4=Pearse JS |author5=Hadrys H |author6=Ender A |date=23 November 2004 |title=Placozoa — no longer a phylum of one |journal=[[Current Biology]] |volume=14 |issue=22 |pages=R944–5 |doi=10.1016/j.cub.2004.10.036 |pmid=15556848 |s2cid=11539852 |doi-access=free|bibcode=2004CBio...14.R944V }}</ref><ref name="Eitel et al 2013">{{cite journal |last1=Eitel |first1=Michael |last2=Osigus |first2=Hans-Jürgen |last3=DeSalle |first3=Rob |last4=Schierwater |first4=Bernd |date=2 April 2013 |title=Global Diversity of the Placozoa |journal=PLOS ONE |volume=8 |issue=4 |pages=e57131 |bibcode=2013PLoSO...857131E |doi=10.1371/journal.pone.0057131 |pmc=3614897 |pmid=23565136 |doi-access=free}}</ref> new species began to be added since 2018. So far, three other extant species have been described, in two distinct classes: Uniplacotomia (''[[Hoilungia hongkongensis]]'' in 2018 and ''[[Cladtertia collaboinventa]]'' in 2022<ref name=":2222" />) and Polyplacotomia (''[[Polyplacotoma mediterranea]]'', the most basal, in 2019<ref>{{Cite journal |last1=Schierwater |first1=Bernd |last2=Kamm |first2=Kai |last3=Herzog |first3=Rebecca |last4=Rolfes |first4=Sarah |last5=Osigus |first5=Hans-Jürgen |date=2019-03-04 |title=Polyplacotoma mediterranea is a new ramified placozoan species |journal=Current Biology |language=en |volume=29 |issue=5 |pages=R148–R149 |doi=10.1016/j.cub.2019.01.068 |issn=0960-9822 |pmid=30836080 |doi-access=free|bibcode=2019CBio...29.R148O }}</ref>). A single putative fossil species is known, the [[Middle Triassic]] ''[[Maculicorpus microbialis]]''.<ref name="Knaust 2021"/>

== History ==
''Trichoplax'' was discovered in 1883 by the German zoologist [[Franz Eilhard Schulze]], in a seawater aquarium at the Zoological Institute in [[Graz, Austria]].<ref name=":0" /><ref>{{Cite journal |last1=Schierwater |first1=Bernd |last2=Osigus |first2=Hans-Jürgen |last3=Bergmann |first3=Tjard |last4=Blackstone |first4=Neil W. |last5=Hadrys |first5=Heike |last6=Hauslage |first6=Jens |last7=Humbert |first7=Patrick O. |last8=Kamm |first8=Kai |last9=Kvansakul |first9=Marc |last10=Wysocki |first10=Kathrin |last11=DeSalle |first11=Rob |display-authors=8 |date=2021 |title=The enigmatic Placozoa part 1: Exploring evolutionary controversies and poor ecological knowledge |journal=BioEssays |volume=43 |issue=10 |pages=e2100080 |doi=10.1002/bies.202100080 |pmid=34472126|s2cid=237387715 |doi-access=free }}</ref> The generic name is derived from the classical Greek {{lang|grc|θρίξ}} (''{{lang|grc-Latn|thrix}}''), meaning "hair", and {{lang|grc|πλάξ}} (''{{lang|grc-Latn|plax}}''), "plate". The specific epithet ''adhaerens'' is Latin meaning "adherent", reflecting its propensity to stick to the glass slides and pipettes used in its examination.<ref name=":1">{{Cite journal |last1=Syed |first1=T. |last2=Schierwater |first2=B. |name-list-style= |date=2002 |title=''Trichoplax adhaerens'': discovered as a missing link, forgotten as a hydrozoan, re-discovered as a key to metazoan evolution |url=https://hal.sorbonne-universite.fr/hal-03198912/document |url-status=dead |journal=Vie Milieu |volume=52 |issue=4 |pages=177–187 |via=[[HAL (open archive)|HAL]] |access-date=2023-06-02 |archive-date=2023-06-02 |archive-url=https://web.archive.org/web/20230602072630/https://hal.sorbonne-universite.fr/hal-03198912/document }}</ref> Schulze realized that the animal could not be a member of any existing phyla, and based on the simple structure and behaviour, concluded in 1891 that it must be an early metazoan. He also observed the reproduction by fission, cell layers and locomotion.<ref name=":3">{{Cite journal |last1=Romanova |first1=Daria Y. |last2=Varoqueaux |first2=Frédérique |last3=Daraspe |first3=Jean |last4=Nikitin |first4=Mikhail A. |last5=Eitel |first5=Michael |last6=Fasshauer |first6=Dirk |last7=Moroz |first7=Leonid L. |date=2021 |title=Hidden cell diversity in Placozoa: ultrastructural insights from Hoilungia hongkongensis |journal=Cell and Tissue Research |volume=385 |issue=3 |pages=623–637 |doi=10.1007/s00441-021-03459-y |pmc=8523601 |pmid=33876313}}</ref> 

In 1893, Italian zoologist [[Francesco Saverio Monticelli]] described another animal which he named ''Treptoplax'', the specimens of which he collected from Naples. He gave the species name ''T. reptans'' in 1896.<ref>{{Cite journal |last1=Cattaneo-Vietti |first1=R. |last2=Russo |first2=G. F. |date=2019-01-01 |title=A brief history of the Italian marine biology |journal=The European Zoological Journal |language=en |volume=86 |issue=1 |pages=294–315 |doi=10.1080/24750263.2019.1651911 |s2cid=202372627 |issn=2475-0263|doi-access=free }}</ref> Monticelli did not preserve them and no other specimens were found again, as a result of which the identification is ruled as doubtful, and the species rejected.<ref name=":2">{{Cite journal |last1=Tessler |first1=Michael |last2=Neumann |first2=Johannes S. |last3=Kamm |first3=Kai |last4=Osigus |first4=Hans-Jürgen |last5=Eshel |first5=Gil |last6=Narechania |first6=Apurva |last7=Burns |first7=John A. |last8=DeSalle |first8=Rob |last9=Schierwater |first9=Bernd |date=2022-12-08 |title=Phylogenomics and the first higher taxonomy of Placozoa, an ancient and enigmatic animal phylum |journal=Frontiers in Ecology and Evolution |volume=10 |doi=10.3389/fevo.2022.1016357 |issn=2296-701X |doi-access=free }}</ref><ref>{{Cite web |title=WoRMS - World Register of Marine Species - Treptoplax reptans Monticelli, 1896 |url=https://www.marinespecies.org/aphia.php?p=taxdetails&id=142022 |access-date=2023-06-02 |website=www.marinespecies.org}}</ref>

Schulze's description was opposed by other zoologists. For instance, in 1890, F.C. Noll argued that the animal was a flat worm (Turbellaria).<ref name=":22">{{Cite journal |last1=Tessler |first1=Michael |last2=Neumann |first2=Johannes S. |last3=Kamm |first3=Kai |last4=Osigus |first4=Hans-Jürgen |last5=Eshel |first5=Gil |last6=Narechania |first6=Apurva |last7=Burns |first7=John A. |last8=DeSalle |first8=Rob |last9=Schierwater |first9=Bernd |date=2022-12-08 |title=Phylogenomics and the first higher taxonomy of Placozoa, an ancient and enigmatic animal phylum |journal=Frontiers in Ecology and Evolution |volume=10 |doi=10.3389/fevo.2022.1016357 |issn=2296-701X |doi-access=free }}</ref> In 1907, Thilo Krumbach published a hypothesis that ''Trichoplax'' is not a distinct animal but that it is a form of the planula larva of the [[Sea anemone|anemone]]-like [[hydrozoan]] ''Eleutheria krohni''. Although this was refuted in print by Schulze and others, Krumbach's analysis became the standard textbook explanation, and nothing was printed in zoological journals about ''Trichoplax'' until the 1960s.<ref name=":1" /> 

The development of [[electron microscopy]] in the mid-20th century allowed in-depth observation of the cellular components of organisms, following which there was renewed interest in ''Trichoplax'' starting in 1966.<ref>{{Cite journal |last1=Kuhl |first1=Willi |last2=Kuhl |first2=Gertrud |date=1966 |title=Untersuchungen über das bewegungsverhalten von Trichoplax adhaerens F. E. Schulze (Zeittransformation: Zeitraffung) |url=http://link.springer.com/10.1007/BF00442291 |journal=Zeitschrift für Morphologie und Ökologie der Tiere |language=de |volume=56 |issue=4 |pages=417–435 |doi=10.1007/BF00442291 |s2cid=20206608 |issn=0720-213X|url-access=subscription }}</ref> The most important descriptions were made by [[Karl Gottlieb Grell]] at the University of Tübingen since 1971.<ref>{{Cite journal |last=Grell |first=K. G. |date=1971 |title=Embryonalentwicklung bei Trichoplax adhaerens F. E. Schulze |url=http://link.springer.com/10.1007/BF00598728 |journal=Die Naturwissenschaften |language=de |volume=58 |issue=11 |pages=570 |bibcode=1971NW.....58..570G |doi=10.1007/BF00598728 |s2cid=40022799 |issn=0028-1042|url-access=subscription }}</ref><ref>{{Cite journal |last=Grell |first=Karl G. |date=1972 |title=Eibildung und furchung von Trichoplax adhaerens F. E. Schulze (Placozoa) |url=http://link.springer.com/10.1007/BF00391925 |journal=Zeitschrift für Morphologie der Tiere |language=de |volume=73 |issue=4 |pages=297–314 |doi=10.1007/BF00391925 |s2cid=22931046 |issn=0720-213X|url-access=subscription }}</ref> That year, Grell revived Schulze's interpretation that the animals are unique and created a new phylum Placozoa.<ref>{{Cite journal |last=G |first=Grell K. |date=1971 |title=Trichoplax adhaerens F. E. Schulze und die Entstehung der Metazoen |url=https://cir.nii.ac.jp/crid/1571417125100030080 |journal=Naturwissenschaftliche Rundschau |volume=24 |pages=160–161}}</ref><ref name=":1" /> Grell derived the name from the placula hypothesis, [[Otto Bütschli]]'s notion on the [[Urmetazoan|origin of metazoans]].<ref>{{Cite journal |last1=Schierwater |first1=Bernd |last2=DeSalle |first2=Rob |date=2018 |title=Placozoa |journal=Current Biology |language=en |volume=28 |issue=3 |pages=R97–R98 |doi=10.1016/j.cub.2017.11.042|pmid=29408263 |s2cid=235331464 |doi-access=free |bibcode=2018CBio...28..R97S }}</ref>

==Biology==
[[File:Placozoan anatomy.svg|thumb|upright=1.5|right| {{center|'''Trichoplax body structure in cross section'''<br />1 - lipid drop, 2 - cilium, 3 - dorsal layer of cells, 4 - vacuole,<br />5 - fibrous syncytium, 6 - glandular cell, 7 - vacuole,<br />8 - ventral layer of cells, 9 - zones of intercellular contacts}}]]
{{main|Trichoplax}}

Placozoans do not have well-defined body plans, much like [[amoebas]], unicellular eukaryotes. As Andrew Masterson reported: "they are as close as it is possible to get to being simply a little living blob."<ref>{{Cite web |last=Masterson |first=Andrew |date=2018-08-01 |title=Simple organisms not so simple, after all |url=https://cosmosmagazine.com/science/biology/simple-organisms-not-so-simple-after-all/ |access-date=2023-06-02 |website=Cosmos Magazine |language=en-AU}}</ref> An individual body measures about 0.55&nbsp;mm in diameter.<ref name=":12">{{Cite journal |last1=Eitel |first1=Michael |last2=Francis |first2=Warren R. |last3=Varoqueaux |first3=Frédérique |last4=Daraspe |first4=Jean |last5=Osigus |first5=Hans-Jürgen |last6=Krebs |first6=Stefan |last7=Vargas |first7=Sergio |last8=Blum |first8=Helmut |last9=Williams |first9=Gray A. |last10=Schierwater |first10=Bernd |last11=Wörheide |first11=Gert |display-authors=8 |date=2018 |title=Comparative genomics and the nature of placozoan species |journal=PLOS Biology |volume=16 |issue=7 |pages=e2005359 |doi=10.1371/journal.pbio.2005359 |pmc=6067683 |pmid=30063702 |doi-access=free }}</ref> There are no body parts; as one of the researchers Michael Eitel described: "There's no mouth, there's no back, no nerve cells, nothing."<ref name=":02">{{Cite web |last=Wood |first=Charlie |date=2018-10-06 |title=Simplest Animal Reveals Hidden Diversity |url=https://www.scientificamerican.com/article/worlds-simplest-animal-reveals-hidden-diversity/ |access-date=2023-06-02 |website=Scientific American |language=en |via=Quanta Magazine}}</ref> Animals studied in laboratories have bodies consisting of everything from hundreds to millions of cells.<ref>[https://news.stanford.edu/press-releases/2018/10/15/new-mechanism-animal-cells-stay-intact/ Stanford researchers reveal a new mechanism for how animal cells stay intact]</ref> 

Placozoans have only three anatomical parts as tissue layers inside its body: the upper, intermediate (middle) and lower [[Epithelium|epithelia]]. There are at least six different cell types.<ref name=":32">{{Cite journal |last1=Romanova |first1=Daria Y. |last2=Varoqueaux |first2=Frédérique |last3=Daraspe |first3=Jean |last4=Nikitin |first4=Mikhail A. |last5=Eitel |first5=Michael |last6=Fasshauer |first6=Dirk |last7=Moroz |first7=Leonid L. |date=2021 |title=Hidden cell diversity in Placozoa: ultrastructural insights from Hoilungia hongkongensis |journal=Cell and Tissue Research |volume=385 |issue=3 |pages=623–637 |doi=10.1007/s00441-021-03459-y |pmc=8523601 |pmid=33876313}}</ref> The upper epithelium is the thinnest portion and essentially comprises flat cells with their cell body hanging underneath the surface, and each cell having a [[cilium]].<ref name=":122">{{Cite journal |last1=Eitel |first1=Michael |last2=Francis |first2=Warren R. |last3=Varoqueaux |first3=Frédérique |last4=Daraspe |first4=Jean |last5=Osigus |first5=Hans-Jürgen |last6=Krebs |first6=Stefan |last7=Vargas |first7=Sergio |last8=Blum |first8=Helmut |last9=Williams |first9=Gray A. |last10=Schierwater |first10=Bernd |last11=Wörheide |first11=Gert |display-authors=8 |date=2018 |title=Comparative genomics and the nature of placozoan species |journal=PLOS Biology |volume=16 |issue=7 |pages=e2005359 |doi=10.1371/journal.pbio.2005359 |pmc=6067683 |pmid=30063702 |doi-access=free }}</ref> Crystal cells are sparsely distributed near the marginal edge. A few cells have unusually large number of [[mitochondria]].<ref name=":32" /> The middle layer is the thickest made up of numerous fiber cells, which contain mitochondrial complexes, vacuoles and [[Endosymbiont|endosymbiotic bacteria]] in the [[endoplasmic reticulum]]. The lower epithelium consists of numerous monociliated cylinder cells along with a few endocrine-like gland cells and lipophil cells. Each lipophil cell contains numerous middle-sized granules, one of which is a [[secretory granule]].<ref name=":222">{{Cite journal |last1=Tessler |first1=Michael |last2=Neumann |first2=Johannes S. |last3=Kamm |first3=Kai |last4=Osigus |first4=Hans-Jürgen |last5=Eshel |first5=Gil |last6=Narechania |first6=Apurva |last7=Burns |first7=John A. |last8=DeSalle |first8=Rob |last9=Schierwater |first9=Bernd |date=2022-12-08 |title=Phylogenomics and the first higher taxonomy of Placozoa, an ancient and enigmatic animal phylum |journal=Frontiers in Ecology and Evolution |volume=10 |doi=10.3389/fevo.2022.1016357 |issn=2296-701X |doi-access=free }}</ref><ref name=":122" />

The body axes of ''Hoilungia'' and ''Trichoplax'' are overtly similar to the oral–aboral axis of [[cnidarians]],<ref name="DuBuc_2019">{{cite journal |vauthors=DuBuc TQ, Ryan JF, Martindale MQ |date=May 2019 |title="Dorsal-Ventral" Genes Are Part of an Ancient Axial Patterning System: Evidence from Trichoplax adhaerens (Placozoa) |journal=Molecular Biology and Evolution |volume=36 |issue=5 |pages=966–973 |doi=10.1093/molbev/msz025 |pmc=6501881 |pmid=30726986}}</ref> animals from another phylum with which they are most closely related.<ref>{{Cite journal |last1=Laumer |first1=Christopher E. |last2=Fernández |first2=Rosa |last3=Lemer |first3=Sarah |last4=Combosch |first4=David |last5=Kocot |first5=Kevin M. |last6=Riesgo |first6=Ana |last7=Andrade |first7=Sónia C. S. |last8=Sterrer |first8=Wolfgang |last9=Sørensen |first9=Martin V. |last10=Giribet |first10=Gonzalo |date=2019-07-10 |title=Revisiting metazoan phylogeny with genomic sampling of all phyla |journal=Proceedings. Biological Sciences |volume=286 |issue=1906 |pages=20190831 |doi=10.1098/rspb.2019.0831 |issn=1471-2954 |pmc=6650721 |pmid=31288696}}</ref> Structurally, they can not be distinguished from other placozoans, so that identification is purely on genetic (mitochondrial DNA) differences.<ref name=":1222">{{Cite journal |last1=Eitel |first1=Michael |last2=Francis |first2=Warren R. |last3=Varoqueaux |first3=Frédérique |last4=Daraspe |first4=Jean |last5=Osigus |first5=Hans-Jürgen |last6=Krebs |first6=Stefan |last7=Vargas |first7=Sergio |last8=Blum |first8=Helmut |last9=Williams |first9=Gray A. |last10=Schierwater |first10=Bernd |last11=Wörheide |first11=Gert |display-authors=8 |date=2018 |title=Comparative genomics and the nature of placozoan species |journal=PLOS Biology |volume=16 |issue=7 |pages=e2005359 |doi=10.1371/journal.pbio.2005359 |pmc=6067683 |pmid=30063702 |doi-access=free }}</ref> Genome sequencing has shown that each species has a set of unique genes and several uniquely missing genes.<ref name=":2222">{{Cite journal |last1=Tessler |first1=Michael |last2=Neumann |first2=Johannes S. |last3=Kamm |first3=Kai |last4=Osigus |first4=Hans-Jürgen |last5=Eshel |first5=Gil |last6=Narechania |first6=Apurva |last7=Burns |first7=John A. |last8=DeSalle |first8=Rob |last9=Schierwater |first9=Bernd |date=2022-12-08 |title=Phylogenomics and the first higher taxonomy of Placozoa, an ancient and enigmatic animal phylum |journal=Frontiers in Ecology and Evolution |volume=10 |doi=10.3389/fevo.2022.1016357 |issn=2296-701X |doi-access=free }}</ref>

''Trichoplax'' is a small, flattened, animal around {{convert|1|mm|abbr=on}} across. An amorphous multi-celled body, analogous to a single-celled ''[[Amoeba (genus)|amoeba]]'', it has no regular outline, although the lower surface is somewhat concave, and the upper surface is always flattened. The body consists of an outer layer of simple [[epithelium]] enclosing a loose sheet of stellate cells resembling the [[mesenchyme]] of some more complex animals. The epithelial cells bear [[cilia]], which the animal uses to help it creep along the seafloor.<ref name="IZ">
{{cite book
 |last=Barnes  |first=Robert D.
 |year=1982
 |title=Invertebrate Zoology
 |pages=84–85
 |publisher=Holt-Saunders International
 |location=Philadelphia
 |isbn= 978-0-03-056747-6
}}
</ref>

The lower surface engulfs small particles of organic detritus, on which the animal [[Detritivore|feeds]]. All placozoans can reproduce asexually, budding off smaller individuals, and the lower surface may also bud off eggs into the [[mesenchyme]].<ref name=IZ/>
[[Sexual reproduction]] has been reported to occur in one [[clade]] of placozoans,<ref name="pmid16230622">
{{cite journal
 |last1=Signorovitch  |first1=A.Y.
 |last2=Dellaporta    |first2=S.L.
 |last3=Buss          |first3=L.W.
 |year=2005 
 |title=Molecular signatures for sex in the Placozoa
 |journal=[[Proceedings of the National Academy of Sciences of the United States of America]]
 |volume=102  |issue=43  |pages=15518–22
 |pmid=16230622  |doi=10.1073/pnas.0504031102
 |pmc=1266089  |bibcode=2005PNAS..10215518S
|doi-access=free
 }}
</ref><ref name="pmid16552431">
{{cite journal
 |last=Charlesworth  |first=D.
 |year=2006
 |title=Population genetics: Using recombination to detect sexual reproduction: The contrasting cases of Placozoa and ''C.&nbsp;elegans''
 |journal=Heredity {{small|(Edinb.)}}
 |volume=96  |issue=5  |pages=341–342
 |pmid=16552431  |doi=10.1038/sj.hdy.6800809
 |s2cid=44333533
}}
</ref> whose strain H8 was later found to belong to genus ''[[Cladtertia]]'',<ref name="Tessler et al 2022" /> where [[Genetic recombination|intergenic recombination]] was observed as well as other hallmarks of sexual reproduction.

Some ''Trichoplax'' species contain ''[[Rickettsiales]]'' bacteria as [[endosymbiont]]s.<ref>
{{Cite journal
 |first1=Kai    |last1=Kamm 
 |first2=Bernd  |last2=Schierwater
 |first3=Rob    |last3=DeSalle 
 |date=2019-01-05
 |title=Innate immunity in the simplest animals – placozoans
 |journal=BMC Genomics
 |volume=20  |issue=1  |pages=5
 |pmc=6321704  |pmid=30611207  |issn=1471-2164
 |doi=10.1186/s12864-018-5377-3
|doi-access=free 
 }}
</ref>
One of the at least 20&nbsp;described species turned out to have two bacterial endosymbionts; ''[[Grellia]]'' which lives in the animal's endoplasmic reticulum and is assumed to play a role in the protein and membrane production. The other endosymbiont is the first described ''[[Margulisbacteria]]'', that lives inside cells used for [[Algivore|algal digestion]]. It appears to eat the fats and other lipids of the algae and provide its host with vitamins and amino acids in return.<ref>
{{cite press release
 |title=Deceptively simple: Minute marine animals live in a sophisticated symbiosis with bacteria
 |date=10 June 2019
 |publisher=Max Planck Society
 |website=Phys.org
 |url=https://phys.org/news/2019-06-deceptively-simple-minute-marine-animals.html
 |access-date=2021-06-23  }}</ref>
<ref name=Harald>{{cite journal
 |first1=Harald  |last1=Gruber-Vodicka    |first2=Niko     |last2=Leisch 
 |first3=Manuel  |last3=Kleiner           |first4=Tjorven  |last4=Hinzke
 |first5=Manuel  |last5=Liebeke           |first6=Margaret |last6=McFall-Ngai
 |first7=Michael G.   |last7=Hadfield     |first8=Nicole   |last8=Dubilier
 |display-authors=6
 |year=2019
 |title=Two intracellular and cell type-specific bacterial symbionts in the placozoan ''Trichoplax'' H2
 |journal=Nature Microbiology
 |volume=4 |issue=9 |pages=1465–1474 |doi=10.1038/s41564-019-0475-9
 |pmid=31182796 |pmc=6784892
}}
</ref>

Studies suggest that aragonite crystals in crystal cells have the same function as statoliths, allowing it to use gravity for [[Gravitaxis|spatial orientation]].<ref>{{Cite journal |last1=Schierwater |first1=Bernd |last2=Osigus |first2=Hans-Jürgen |last3=Bergmann |first3=Tjard |last4=Blackstone |first4=Neil W. |last5=Hadrys |first5=Heike |last6=Hauslage |first6=Jens |last7=Humbert |first7=Patrick O. |last8=Kamm |first8=Kai |last9=Kvansakul |first9=Marc |last10=Wysocki |first10=Kathrin |last11=DeSalle |first11=Rob |date=2021 |title=The enigmatic Placozoa part 2: Exploring evolutionary controversies and promising questions on earth and in space |url=https://onlinelibrary.wiley.com/doi/10.1002/bies.202100083 |journal=BioEssays |language=en |volume=43 |issue=10 |pages=e2100083 |doi=10.1002/bies.202100083 |pmid=34490659 |issn=0265-9247|doi-access=free }}</ref>

Located in the dorsal epithelium there are lipid granules called shiny spheres which release a cocktail of venoms and toxins as an anti-predator defense, and can induce paralysis or death in some predators. Genes has been found in Trichoplax with a strong resemblance to the venom genes of some poisonous snakes, like the American copperhead and the West African carpet viper.<ref>[https://www.snexplores.org/article/living-mysteries-meet-earths-simplest-animal Living Mysteries: Meet Earth’s simplest animal]</ref><ref>{{Cite journal |last=Cuervo-González |first=Rodrigo |date=September 2017 |title=Rhodope placozophagus (Heterobranchia) a new species of turbellarian-like Gastropoda that preys on placozoans |url=https://linkinghub.elsevier.com/retrieve/pii/S0044523117300694 |journal=Zoologischer Anzeiger |language=en |volume=270 |pages=43–48 |doi=10.1016/j.jcz.2017.09.005|bibcode=2017ZooAn.270...43C|url-access=subscription }}</ref>

[[File:Placozoa.svg|thumb|upright=2|left| {{center|Global distribution{{hsp}}<ref>
{{cite journal
 |last1 = Eitel    |first1 = Michael     |last2 = Osigus    |first2 = Hans-Jürgen
 |last3 = Desalle  |first3 = Rob         |last4 = Schierwater   |first4 = Bernd
 |year = 2013
 |title = Global diversity of the Placozoa
 |journal = PLOS ONE
 |volume = 8  |issue = 4  |page = e57131
 |pmid = 23565136 |pmc = 3614897 |bibcode = 2013PLoSO...857131E
 |doi = 10.1371/journal.pone.0057131  |doi-access = free
}}
</ref>}}]]

{{clear left}}

The Placozoa show substantial evolutionary radiation in regard to [[sodium channel]]s, of which they have 5–7&nbsp;different types, more than any other invertebrate species studied to date.<ref>
{{cite journal
 |last1=Romanova |first1=Daria Y.        |last2=Smirnov  |first2=Ivan V. 
 |last3=Nikitin  |first3=Mikhail A.      |last4=Kohn     |first4=Andrea B. 
 |last5=Borman   |first5=Alisa I.        |last6=Malyshev |first6=Alexey Y. 
 |last7=Balaban  |first7=Pavel M.        |last8=Moroz    |first8=Leonid L. 
 |display-authors=6
 |date=29 October 2020
 |title=Sodium action potentials in placozoa: Insights into behavioral integration and evolution of nerveless animals
 |journal=Biochemical and Biophysical Research Communications
 |volume=532  |issue=1  |pages=120–126
 |doi=10.1016/j.bbrc.2020.08.020
 |pmid=32828537 |pmc=8214824
}}
</ref>

Three modes of population dynamics depended upon feeding sources, including induction of social behaviors, morphogenesis, and reproductive strategies. <ref>{{cite journal| doi=10.3389/fcell.2022.823283 | volume=10 | title=Expanding of life strategies in placozoa: Insights from long-term culturing of ''Trichoplax'' and ''Hoilungia''  | year=2022 | journal=Frontiers in Cell and Developmental Biology | last1 = Romanova | first1 = Daria | last2 = Nikitin | first2 = Mikhail | last3 = Shchenkov | first3 = Sergey | last4 = Moroz | first4 = Leonid | page=823283 | pmid=35223848 | pmc=8864292 | doi-access=free }}</ref>

In addition to fission, representatives of all species produced “swarmers” (a separate vegetative reproduction stage), which could also be formed from the lower epithelium with greater cell-type diversity.<ref>{{cite journal| doi=10.3389/fcell.2022.823283 | volume=10 | title=Expanding of Life Strategies in Placozoa: Insights From Long-Term Culturing of ''Trichoplax'' and ''Hoilungia''  | year=2021 | pages=623–637 |journal=Frontiers in Cell and Developmental Biology | last1 = Romanova | first1 = Daria | last2 = Varoqueaux | first2 = Frederique | last3 = Daraspe | first3 = Jean | last4 = Nikitin | first4 = Mikhail | last5 = Eitel | first5 = Michael | last6 = Fasshauer | first6 = Dirk | last7 = Moroz | first7 = Leonid | pmid=35223848 | pmc=8864292 | doi-access=free }}</ref>

==Evolutionary relationships==
There is no convincing fossil record of the Placozoa, although the [[Ediacaran biota]] (Precambrian, {{ma|550}}) organism ''[[Dickinsonia]]'' appears somewhat similar to placozoans.<ref name=PalAss>
{{PalAss2008
 |last1=Sperling |first1=Erik
 |last2=Vinther  |first2=Jakob
 |last3=Pisani   |first3=Davide
 |last4=Peterson |first4=Kevin
 |title=A placozoan affinity for ''Dickinsonia'' and the evolution of Late Precambrian metazoan feeding modes|
 |page=81
}}
</ref> Knaust (2021) reported preservation of placozoan fossils in a microbialite bed from the [[Middle Triassic]] [[Muschelkalk]] ([[Germany]]).<ref name="Knaust 2021"/>

Traditionally, classification was based on their level of organization, i.e., they possess no tissues or organs. However this may be as a result of secondary loss and thus is inadequate to exclude them from relationships with more complex animals. More recent work has attempted to classify them based on the DNA sequences in their genome; this has placed the phylum between the [[sponge]]s and the [[Eumetazoa]].<ref name=genome>
{{cite journal
 | last1=Srivastava | first1=M.         | last2=Begovic     | first2=Emina
 | last3=Chapman    | first3=Jarrod     | last4=Putnam      | first4=Nicholas H.
 | last5=Hellsten   | first5=Uffe       | last6=Kawashima   | first6=Takeshi
 | last7=Kuo        | first7=Alan       | last8=Mitros      | first8=Therese
 | last9=Salamov    | first9=Asaf       | last10=Carpenter  | first10=Meredith L.
 | last11=Signorovitch | first11=Ana Y. | last12=Moreno     | first12=Maria A.
 | last13=Kamm      | first13=Kai       | last14=Grimwood   | first14=Jane
 | last15=Schmutz   | first15=Jeremy    | last16=Shapiro    | first16=Harris
 | last17=Grigoriev | first17=Igor V.   | last18=Buss       | first18=Leo W.
 | last19=Schierwater  | first19=Bernd  | last20=Dellaporta | first20=Stephen L.
 | last21=Rokhsar   | first21=Daniel S.       | display-authors=6
 | date=21 August 2008
 | title=The ''Trichoplax'' genome and the nature of placozoans
 | journal=[[Nature (journal)|Nature]]
 | volume=454  | issue=7207  | pages=955–960
 | doi=10.1038/nature07191  | doi-access=free
 | pmid=18719581  | s2cid=4415492
 | bibcode=2008Natur.454..955S
 }}
</ref> In such a feature-poor phylum, molecular data are considered to provide the most reliable approximation of the placozoans' phylogeny.

Their exact position on the [[phylogenetic tree]] would give important information about the origin of neurons and muscles. If the absence of these features is an original trait of the Placozoa, it would mean that a nervous system and muscles evolved three times should placozoans and cnidarians be a [[sister group]]; once in the [[Ctenophora]], once in the [[Cnidaria]] and once in the [[Bilateria]]. If they branched off before the Cnidaria and Bilateria split, the neurons and muscles would have the same origin in the two latter groups.

===Functional-morphology hypothesis===
[[File:Gallertoid Model.png|thumb|right|upright=1.8|The Placozoa descending side by side with the sponges, cnidarians and ctenophores from a gallertoid by processes of differentiation]]
[[File:Placozoan.webp|thumb|right|upright=1.8| A placozoan is a small, flattened animal, typically about one mm across and about 25&nbsp;μm thick. Like the [[amoebae]] they superficially resemble, they continually change their external shape. In addition, spherical phases occasionally form which may facilitate movement. ''Trichoplax'' lacks tissues and organs. There is no manifest body symmetry, so it is not possible to distinguish anterior from posterior or left from right. It is made up of a few thousand cells of six types in three distinct layers.<ref name="SmithVaroqueaux2014">{{cite journal | vauthors = Smith CL, Varoqueaux F, Kittelmann M, Azzam RN, Cooper B, Winters CA, Eitel M, Fasshauer D, Reese TS | display-authors = 6 | title = Novel cell types, neurosecretory cells, and body plan of the early-diverging metazoan Trichoplax adhaerens | journal = Current Biology | volume = 24 | issue = 14 | pages = 1565–1572 | date = July 2014 | pmid = 24954051 | pmc = 4128346 | doi = 10.1016/j.cub.2014.05.046 | bibcode = 2014CBio...24.1565S }}</ref>]]

On the basis of their simple structure, the Placozoa were frequently viewed as a model organism for the transition from unicellular organisms to the multicellular animals ([[Metazoa]]) and are thus considered a sister taxon to all other metazoans:

{{clade|label1=Metazoa|1={{clade|
1=Placozoa|
2={{clade|Sponges (Porifera)|2=Animals with tissues (Eumetazoa)}}
}}}}

According to a functional-morphology model, all or most animals are descended from a ''[[gallertoid]]'', a free-living ([[pelagic]]) sphere in seawater, consisting of a single [[cilium|ciliated]] layer of cells supported by a thin, noncellular separating layer, the [[basal lamina]]. The interior of the [[sphere]] is filled with contractile fibrous cells and a gelatinous [[extracellular matrix]]. Both the modern Placozoa and all other animals then descended from this multicellular beginning stage via two different processes:<ref name="Grasshoff Gudo 2002 pp. 295–314">{{cite journal | last1=Grasshoff | first1=Manfred | last2=Gudo | first2=Michael | year=2002 | title=The origin of metazoa and the main evolutionary lineages of the animal Kingdom: The gallertoid hypothesis in the light of modern research | journal=Senckenbergiana Lethaea | volume=82 | issue=1 | pages=295–314 | issn=0037-2110 | doi=10.1007/bf03043790 | s2cid=84989130 | publisher=Springer Science and Business Media LLC}}</ref>

* Infolding of the [[epithelium]] led to the formation of an internal system of ducts and thus to the development of a modified gallertoid from which the sponges ([[Porifera]]), [[Cnidaria]] and [[Ctenophora]] subsequently developed.
* Other gallertoids, according to this model, made the transition over time to a [[benthic]] mode of life; that is, their habitat has shifted from the open ocean to the floor (benthic zone). This results naturally in a [[natural selection|selective advantage]] for flattening of the body, as of course can be seen in many benthic species.

[[File:Exodigestion in Trichoplax adhaerens.jpg|thumb|upright=1.8|right| {{center|Crawling motility and food uptake by ''[[Trichoplax adhaerens]]''}}]]
While the probability of encountering food, potential sexual partners, or predators is the same in all directions for animals floating freely in the water, there is a clear difference on the seafloor between the functions useful on body sides facing toward and away from the [[substrate (biology)|substrate]], leading their sensory, defensive, and food-gathering cells to differentiate and orient according to the vertical – the direction perpendicular to the substrate. In the proposed functional-morphology model, the Placozoa, and possibly several similar organisms only known from the fossils, are descended from such a life form, which is now termed ''placuloid''. 

Three different life strategies have accordingly led to three different possible lines of development:
# Animals that live interstitially in the sand of the ocean floor were responsible for the fossil crawling traces that are considered the earliest evidence of animals; and are detectable even prior to the dawn of the [[Ediacaran Period]] in [[geology]]. These are usually attributed to [[bilaterally symmetrical]] worms, but the hypothesis presented here views animals derived from placuloids, and thus close relatives of ''Trichoplax adhaerens'', to be the producers of the traces.
# Animals that incorporated [[algae]] as photosynthetically active [[endosymbionts]], i.e. primarily obtaining their nutrients from their partners in [[symbiosis]], were accordingly responsible for the mysterious creatures of the Ediacara fauna that are not assigned to any modern animal taxon and lived during the Ediacaran Period, before the start of the [[Paleozoic]]. However, recent work has shown that some of the Ediacaran assemblages (e.g. [[Mistaken Point]]) were in deep water, below the [[photic zone]], and hence those individuals could not dependent on endosymbiotic [[photosynthesis|photosynthesisers]].
# Animals that grazed on [[algal mat]]s would ultimately have been the direct ancestors of the Placozoa. The advantages of an amoeboid multiplicity of shapes thus allowed a previously present basal lamina and a gelatinous [[extracellular matrix]] to be lost ''secondarily''. Pronounced differentiation between the surface facing the substrate ([[ventral]]) and the surface facing away from it ([[Dorsal (anatomy)|dorsal]]) accordingly led to the physiologically distinct cell layers of ''Trichoplax adhaerens'' that can still be seen today. Consequently, these are ''analogous'', but not [[homology (biology)|''homologous'']], to [[ectoderm]] and [[endoderm]] – the "external" and "internal" cell layers in eumetazoans – i.e. the structures corresponding functionally to one another have, according to the proposed hypothesis, no common evolutionary origin.

Should any of the analyses presented above turn out to be correct, ''Trichoplax adhaerens'' would be the oldest branch of the multicellular animals, and a relic of the [[Ediacaran fauna]], or even the pre-Ediacara fauna. Although very successful in their [[ecological niche]], due to the absence of extracellular matrix and [[basal lamina]], the development potential of these animals was of course limited, which would explain the low rate of evolution of their [[phenotype]] (their outward form as adults) – referred to as ''bradytely''.{{cn|date=January 2022}} 

This hypothesis was supported by a recent analysis of the ''Trichoplax adhaerens'' [[mitochondrial]] [[genome]] in comparison to those of other animals.<ref>{{cite journal |last1=Dellaporta |first1=S.L. |last2=Xu |first2=A. |last3=Sagasser |first3=S. |last4=Jakob |first4=W. |last5=Moreno |first5=M.A. |last6=Buss |first6=L.W. |last7=Schierwater |first7=B. |display-authors=etal |date=6 June 2006 |title=Mitochondrial genome of ''Trichoplax adhaerens'' supports Placozoa as the basal lower metazoan phylum |journal=[[Proceedings of the National Academy of Sciences of the United States of America]] |volume=103 |issue=23 |pages=8751–8756 |pmid=16731622 |doi=10.1073/pnas.0602076103 |doi-access=free |pmc=1470968 |bibcode=2006PNAS..103.8751D}}</ref> The hypothesis was, however, rejected in a statistical analysis of the ''Trichoplax adhaerens'' whole genome sequence in comparison to the whole genome sequences of six other animals and two related non-animal species, but only at {{nobr|the  {{mvar|p}} {{=}} 0.07  level,}} which indicates a marginal level of statistical significance.<ref name=genome/>

===Epitheliozoa hypothesis===
A concept based on purely morphological characteristics pictures the Placozoa as the nearest relative of the animals with true tissues ([[Eumetazoa]]). The taxon they share, called the [[Epitheliozoa]], is itself construed to be a sister group to the sponges ([[Porifera]]):

{{clade
   |label1= [[Metazoa]] 
   |1={{clade
      |1= [[Porifera]]
      |label2= [[Epitheliozoa]] 
      |2={{clade
         |label1=  
         |1= [[Placozoa]]
         |2= [[Eumetazoa]]
      }}
   }}
}}

The above view could be correct, although there is some evidence that the [[ctenophore]]s, traditionally seen as [[Eumetazoa]], may be the sister to all other animals.<ref>
{{cite journal
 |last1=Whelan |first1=Nathan V.     |last2=Kocot   |first2=Kevin M.
 |last3=Moroz  |first3=Tatiana P.    |last4=Mukherjee |first4=Krishanu 
 |last5=Williams |first5=Peter       |last6=Paulay  |first6=Gustav
 |last7=Moroz  |first7=Leonid L.     |last8=Halanych |first8=Kenneth M.
 |display-authors=6
 |date=2017-10-09
 |title=Ctenophore relationships and their placement as the sister group to all other animals
 |journal=Nature Ecology & Evolution
 |volume=1  |issue=11  |pages=1737–1746
 |doi=10.1038/s41559-017-0331-3 |pmid=28993654
 |pmc=5664179 |bibcode=2017NatEE...1.1737W |issn=2397-334X  |lang=en
}}
</ref>
This is now a disputed classification.<ref>
{{cite press release
 |title=Sponges and comb jellies
 |date=November 2015
 |department=News and features 
 |publisher=University of Bristol
 |website=www.bristol.ac.uk
 |url=https://www.bristol.ac.uk/news/2015/november/sponges-comb-jellies.html#:~:text=Recent%20genomic%20studies%20have%20suggested,are%20the%20oldest%20animal%20phylum.
 |access-date=2023-03-11
 |lang=en-GB
}}
</ref> Placozoans are estimated to have emerged 750–800 million years ago, and the first modern neuron to have originated in the common ancestor of cnidarians and bilaterians about 650&nbsp;million years ago (many of the genes expressed in modern neurons are absent in ctenophores, although some of these missing genes are present in placozoans).<ref>[https://www.eurekalert.org/news-releases/1001544 Tiny sea creatures reveal the ancient origins of neurons]</ref><ref>{{cite journal | url=https://doi.org/10.1016/j.cell.2023.08.027 | doi=10.1016/j.cell.2023.08.027 | title=Stepwise emergence of the neuronal gene expression program in early animal evolution | date=2023 | last1=Najle | first1=Sebastián R. | last2=Grau-Bové | first2=Xavier | last3=Elek | first3=Anamaria | last4=Navarrete | first4=Cristina | last5=Cianferoni | first5=Damiano | last6=Chiva | first6=Cristina | last7=Cañas-Armenteros | first7=Didac | last8=Mallabiabarrena | first8=Arrate | last9=Kamm | first9=Kai | last10=Sabidó | first10=Eduard | last11=Gruber-Vodicka | first11=Harald | last12=Schierwater | first12=Bernd | last13=Serrano | first13=Luis | last14=Sebé-Pedrós | first14=Arnau | journal=Cell | volume=186 | issue=21 | pages=4676–4693.e29 | pmid=37729907 | pmc=10580291 | hdl=10230/58738 | hdl-access=free }}</ref>

The principal support for such a relationship comes from special cell to cell junctions – belt [[desmosomes]] – that occur not just in the Placozoa but in all animals ''except'' the sponges: They enable the cells to join together in an unbroken layer like the epitheloid of the Placozoa. ''[[Trichoplax adhaerens]]'' also shares the ventral gland cells with most eumetazoans. Both characteristics can be considered evolutionarily derived features ([[apomorphies]]), and thus form the basis of a common taxon for all animals that possess them.{{cn|date=January 2022}} 

One possible scenario inspired by the proposed hypothesis starts with the idea that the monociliated cells of the epitheloid in ''[[Trichoplax adhaerens]]'' evolved by reduction of the collars in the collar cells ([[Choanocyte|choanocytes]]) of sponges as the hypothesized ancestors of the Placozoa abandoned a filtering mode of life. The epitheloid would then have served as the precursor to the true epithelial tissue of the eumetazoans.{{cn|date=January 2022}}

In contrast to the model based on functional morphology described earlier, in the Epitheliozoa hypothesis, the ventral and dorsal cell layers of the Placozoa are homologs of endoderm and ectoderm — the two basic embryonic cell layers of the eumetazoans. The digestive ''[[gastrodermis]]'' in the Cnidaria or the gut epithelium in the bilaterally symmetrical animals ([[Bilateria]]) may have developed from endoderm, whereas ectoderm is the precursor to the external skin layer ([[epidermis]]), among other things. The interior space pervaded by a fiber syncytium in the Placozoa would then correspond to connective tissue in the other animals. It is unclear whether the calcium ions stored in the syncytium would be related to the lime skeletons of many cnidarians.{{cn|date=January 2022}}

As noted above, this hypothesis was supported in a statistical analysis of the ''Trichoplax adhaerens'' whole genome sequence, as compared to the whole-genome sequences of six other animals and two related non-animal species.<ref name=genome/>

===Eumetazoa hypothesis===
A third hypothesis, based primarily on molecular genetics, views the Placozoa as highly simplified [[eumetazoans]]. According to this, ''Trichoplax adhaerens'' is descended from considerably more complex animals that already had muscles and nerve tissues. Both tissue types, as well as the basal lamina of the [[epithelium]], were accordingly lost more recently by radical secondary simplification.<ref>{{cite book |last=Pechenik |first=Jan |year=2015 |title=Biology of the Invertebrates |chapter=The Poriferans and Placozoans |edition=7 |page=90 |publisher=McGraw-Hill Education |isbn=978-0073524184 }}</ref>

Various studies in this regard so far yield differing results for identifying the exact sister group: In one case, the Placozoa would qualify as the nearest relatives of the [[Cnidaria]], while in another they would be a sister group to the [[Ctenophora]], and occasionally they are placed directly next to the [[Bilateria]]. Currently, they are typically placed according to the cladogram below:<ref>{{cite book |last=Layden |first=Michael J. |year=2018 |publication-date=14 February 2018 |chapter=Cnidarian Zic genes |department=Zic family in animal evolution and development |editor-first=Jun |editor-last=Aruga |title=Zic Family: Evolution, development, and disease |edition=1st |series=Advances in Experimental Medicine and Biology |volume=1046 |pages=27–39 |publisher=Springer Nature Singapore |place=Singapore |lang=en |doi=10.1007/978-981-10-7311-3_2 |pmid=29442315 |issn=0065-2598 |isbn=978-981-10-7310-6 }}</ref>

{{clade
  |label1= [[Metazoa]] 
  |sublabel1= 
  |1={{clade
     |1=[[Porifera]]
     |label2= [[Eumetazoa]] 
     |sublabel2= [[Diploblast]]s / <br/> [[Epitheliozoa]] 
     |2={{clade
        |1=[[Ctenophora]]
        |label2= [[ParaHoxozoa]] 
        |2={{clade
           |1=[[Placozoa]]
           |label2= [[Planulozoa]] 
           |2={{clade
              |1=[[Cnidaria]]
              |2=[[Bilateria]] / [[Bilateria|Triploblasts]]
           }}
        }}
     }}
  }}
}}

In this cladogram the [[Epitheliozoa]] and Eumetazoa are synonyms to each other and to the [[Diploblast]]s, and the [[Ctenophora]] are basal to them.

An argument raised against the proposed scenario is that it leaves morphological features of the animals completely out of consideration. The extreme degree of simplification that would have to be postulated for the Placozoa in this model, moreover, is only known for parasitic organisms, but would be difficult to explain functionally in a free-living species like ''Trichoplax adhaerens''.{{cn|date=January 2022}}

This version is supported by statistical analysis of the ''Trichoplax adhaerens'' whole genome sequence in comparison to the whole genome sequences of six other animals and two related non-animal species. However, Ctenophora was not included in the analyses, placing the placozoans outside of the sampled Eumetazoans.<ref name=genome/><ref>{{cite journal |last1=Wallberg |first1=Andreas |last2=Thollesson |first2=Mikael |last3=Farris |first3=James S. |last4=Jondelius |first4=Ulf |date=2004-12-01 |title=The phylogenetic position of the comb jellies (Ctenophora) and the importance of taxonomic sampling |journal=Cladistics |lang=en |volume=20 |issue=6 |pages=558–578 |doi=10.1111/j.1096-0031.2004.00041.x |doi-access=free |pmid=34892961 |s2cid=86185156 |issn=1096-0031 }}</ref>

==== Cnidaria-sister hypothesis ====
DNA comparisons suggest that placozoans are related to [[Cnidaria]], derived from [[planula]] larva (as seen in some Cnidaria).<ref name=Laumer2019>{{cite journal |last1=Laumer |first1=C.E. |last2=Fernández |first2=R. |last3=Lemer |first3=S. |last4=Combosch |first4=D. |last5=Kocot |first5=K.M. |last6=Riesgo |first6=A. |last7=Andrade |first7=S.C.S. |last8=Sterrer |first8=W. |last9=Sørensen |first9=M.V. |last10=Giribet |first10=G. |display-authors=6 |year=2019 |title=Revisiting metazoan phylogeny with genomic sampling of all phyla |journal=Proc. Biol. Sci. |volume=286 |issue=1906 |page=20190831 |doi=10.1098/rspb.2019.0831 |pmc=6650721 |pmid=31288696}}</ref> The [[Bilateria]] also are thought to be derived from planuloids.<ref name=Aleshin2002>{{cite journal |last1=Aleshin |first1=V.V. |last2=Petrov |first2=N.B. |year=2002 |title=Molecular evidence of regression in evolution of metazoa |journal=Zh. Obshch. Biol. |volume=63 |issue=3 |pages=195–208|pmid=12070939 }}</ref><ref>{{cite journal |last1=Laumer |first1=Christopher E. |last2=Gruber-Vodicka |first2=Harald |last3=Hadfield |first3=Michael G. |last4=Pearse |first4=Vicki B. |last5=Riesgo |first5=Ana |last6=Marioni |first6=John C. |last7=Giribet |first7=Gonzalo |date=2018-10-30 |df=dmy-all |title=Support for a clade of Placozoa and Cnidaria in genes with minimal compositional bias |journal=eLife |lang=en |volume=7 |doi=10.7554/elife.36278 |pmid=30373720 |pmc=6277202 |issn=2050-084X |doi-access=free }}</ref><ref>{{cite journal |last1=Syed |first1=Tareq |last2=Schierwater |first2=Bernd |date=June 2002 |title=The evolution of the placozoa: A new morphological model |journal=Senckenbergiana Lethaea |lang=en |volume=82 |issue=1 |pages=315–324 |doi=10.1007/bf03043791 |s2cid=16870420 |issn=0037-2110}}</ref><ref>{{cite journal |last1=Hejnol |first1=Andreas |last2=Martindale |first2=Mark Q. |date=2008-04-27 |df=dmy-all |title=Acoel development supports a simple planula-like urbilaterian |journal=Philosophical Transactions of the Royal Society B: Biological Sciences |lang=en |volume=363 |issue=1496 |pages=1493–1501 |doi=10.1098/rstb.2007.2239 |issn=0962-8436 |pmc=2614228 |pmid=18192185}}</ref><ref>{{cite journal |last1=Alzugaray |first1=María Eugenia |last2=Bruno |first2=María Cecilia |last3=Villalobos Sambucaro |first3=María José |last4=Ronderos |first4=Jorge Rafael |year=2019 |title=The evolutionary history of the orexin / allatotropin GPCR family: From Placozoa and Cnidaria to Vertebrata |journal=Scientific Reports |volume=9 |issue=1 |page=10217 |biorxiv=10.1101/403709 |doi=10.1038/s41598-019-46712-9 |pmid=31308431 |pmc=6629687 |bibcode=2019NatSR...910217A |s2cid=256990037 }}</ref><ref>{{cite journal |last1=da&nbsp;Silva |first1=Fernanda Britto |last2=Muschner |first2=Valéria C. |last3=Bonatto |first3=Sandro L. |year=2007 |title=Phylogenetic position of Placozoa based on large subunit (LSU) and small subunit (SSU) rRNA genes |journal=Genetics and Molecular Biology |volume=30 |issue=1 |pages=127–132 |doi=10.1590/S1415-47572007000100022 |doi-access=free |issn=1415-4757 }}</ref><ref>{{cite journal |last1=Adl |first1=Sina M. |last2=Bass |first2=David |last3=Lane |first3=Christopher E. |last4=Lukeš |first4=Julius |last5=Schoch |first5=Conrad L. |last6=Smirnov |first6=Alexey |last7=Agatha |first7=Sabine |last8=Berney |first8=Cedric |last9=Brown |first9=Matthew W. |display-authors=6 |date=2018-09-26 |df=dmy-all |title=Revisions to the classification, nomenclature, and diversity of eukaryotes |journal=Journal of Eukaryotic Microbiology |volume=66 |issue=1 |pages=4–119 |lang=en |doi=10.1111/jeu.12691 |pmid=30257078 |pmc=6492006 |issn=1066-5234}}</ref><ref>{{cite book |last1=Giribet |first1=Gonzalo |last2=Edgecombe |first2=Gregory D. |date=2020-03-03 |df=dmy-all |title=The Invertebrate Tree of Life |publisher=Princeton University Press |isbn=978-0-691-19706-7 |lang=en |url=https://books.google.com/books?id=anetDwAAQBAJ&q=cnidaria+placozoa&pg=PP7 }}</ref> The Cnidaria and Placozoa body axis are overtly similar, and placozoan and cnidarian cells are responsive to the same [[neuropeptide]] [[Antibody|antibodies]] despite extant placozoans not developing any neurons.<ref>{{Cite journal |last1=duBuc |first1=Timothy Q. |last2=Ryan |first2=Joseph |last3=Martindale |first3=Mark Q. |date=2019-02-06 |df=dmy-all <!-- |editor-last=True |editor-first=John --> |title="Dorsal-ventral" genes are part of an ancient axial patterning system: Evidence from ''Trichoplax adhaerens'' (Placozoa) |journal=Molecular Biology and Evolution |volume=36 |issue=5 |pages=966–973 |lang=en |doi=10.1093/molbev/msz025 |pmid=30726986 |pmc=6501881 |issn=0737-4038}}</ref><ref>{{cite journal |last=Schuchert |first=Peter |date=1993-03-01 |df=dmy-all |title=''Trichoplax adhaerens'' (phylum Placozoa) has cells that react with antibodies against the neuropeptide RFamide |journal=Acta Zoologica |lang=en |volume=74 |issue=2 |pages=115–117 |doi=10.1111/j.1463-6395.1993.tb01227.x |issn=1463-6395 }}</ref>

{{clade
<!-- |style=font-size:80%; line-height:80% -->
 |label1= [[Choanozoa]] 
 |sublabel1=950 mya
 |1={{clade
    |1=[[Choanoflagellate|Choanoflagellata]] [[File:Desmarella moniliformis.jpg|60 px]]
    |label2= [[Animalia]] 
    |sublabel2=760 mya<br/> 
    |2={{clade
       |1=[[Porifera]] [[File:Reef3859 - Flickr - NOAA Photo Library.jpg|60 px]]
       |label2= [[Eumetazoa]] 
       |2={{clade
          |1=[[Ctenophora]] [[File:Comb jelly.jpg|60 px]]
          |label2= [[ParaHoxozoa]] 
          |sublabel2=680 mya
          |2={{clade
             |label1=  
             |1={{clade
                |label1=  
                |1=[[Placozoa]] [[File:Trichoplax adhaerens photograph.png|60 px]]
                |2=[[Cnidaria]] [[File:Cauliflour Jellyfish, Cephea cephea at Marsa Shouna, Red Sea, Egypt SCUBA.jpg|60 px]]
                }}
             |2=[[Bilateria]] / [[Triploblasts]] [[File:Sorocelis reticulosa.jpg|60 px]]
          }}
       }}
     }}
  }}
}}

==References==
{{reflist}}

==External links==
{{Wikispecies|Placozoa}}
{{Commons category}}
* [https://web.archive.org/web/20080728121614/http://genome.jgi-psf.org/Triad1/ The ''Trichoplax adhaerens'' Grell-BS-1999 v1.0 Genome Portal at the DOE Joint Genome Institute]
* [https://web.archive.org/web/20120204144650/http://peabody.yale.edu/collections/invertebrate-zoology/trichoplax-genome-project The ''Trichoplax'' Genome Project at the Yale Peabody Museum]
* [http://www.microscopy-uk.org.uk/mag/artoct98/tricho.html A Weird Wee Beastie: Trichoplax adhaerens]
* [https://web.archive.org/web/20110718232710/http://www.ecolevol.de/index.php?option=com_content&task=category&sectionid=5&id=57&Itemid=93 Research articles from the ITZ, TiHo Hannover]
* [https://ucmp.berkeley.edu/phyla/placozoa/placozoa.html Information page from the University of California at Berkeley]
* {{cite journal |vauthors=Ender A, Schierwater B |title=Placozoa are not derived cnidarians: evidence from molecular morphology |journal=Mol. Biol. Evol. |volume=20 |issue=1 |pages=130–4 |date=January 2003 |pmid=12519915 |doi= 10.1093/molbev/msg018|doi-access=free }} – [[Mitochondrial DNA]] and [[MT-RNR2|16S rRNA]] analysis and phylogeny of ''Trichoplax adhaerens''
* [https://web.archive.org/web/20070927231041/http://ecolevol.de/pubs/2005/syed_schie_vie_milieu.pdf Historical overview of ''Trichoplax'' research]
* [https://www.sciencedaily.com/releases/2008/08/080820163002.htm Science Daily: Genome Of Simplest Animal Reveals Ancient Lineage, Confounding Array Of Complex Capabilities]
* [https://archive.today/20130415130142/http://icb.oxfordjournals.org/cgi/content/full/icm015v1 Vicki Buchsbaum Pearse, and Oliver Voigt, 2007. "Field biology of placozoans (Trichoplax): distribution, diversity, biotic interactions. Integrative and Comparative Biology"], {{doi|10.1093/icb/icm015}}.

{{Life on Earth}}
{{Eukaryota}}
{{Animalia}}
{{Placozoa}}
{{Taxonbar|from=Q131040}}

[[Category:Placozoa| ]]
[[Category:ParaHoxozoa]]
[[Category:Animal phyla]]
[[Category:Parazoa]]
[[Category:Ediacaran first appearances]]