Editing Placozoa (section)

===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/>