Editing Placozoa (section)

==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]]
          }}
       }}
     }}
  }}
}}