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Scientific classification
Kingdom: Animalia
Phylum: Placozoa
Grell, 1971

The Placozoa are a primitive form of invertebrate.[1] They are the simplest in structure of all non-parasitic multicellular animals (Metazoa). They are generally classified as a single species, Trichoplax adhaerens, although there is enough genetic diversity that it is likely that there are multiple, morphologically similar species.[2] A common name does not yet exist for the taxon; the scientific name literally means "flat animals".[3]


Evolutionary relationships

Phylogenetic position of the Placozoa

Some sponges

Other sponges



There is no convincing fossil record of the placozoa, although the Ediacaran (Precambrian, 550 million years ago) organism Dickinsonia appears to be closely allied with this phylum.[4]

Traditionally, classification has been based on their level of organization: i.e. they possess no tissues or organs. However this may be as a result of secondary loss, so is inadequate to demark a clade. More recent work has attempted to classify them based on the DNA sequences in their genome; this has placed the phylum between the (paraphyletic) sponges and the eumetazoa.[5] In such a feature-poor phylum, molecular data are considered to provide the most reliable approximation of the placozoans' phylogeny. Historically, some other hypotheses have been posited.


Functional-morphology hypothesis

The Placozoa descending side by side with the sponges, cnidarians and ctenophores from a gallertoid by processes of differentiation

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:



Sponges (Porifera)

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 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:

  • 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). 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 sides facing toward and away from the substrate, and between their orientation and the vertical direction perpendicular to the substrate. This results naturally in a selective advantage for flattening of the body, as of course can be seen in many benthic species. In the proposed functional-morphology model, the Placozoa, and possibly also several organisms known only from the fossil state, are descended from such a life form, which is now termed placuloid. Three different life strategies have accordingly led to three different 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. Recent work has shown that some of the Ediacaran assemblages (e.g. Mistaken Point) were in deep water, below the photic zone, and that the organisms were not dependent on endosymbiotic photosynthesisers.
    • Animals that grazed on algal mats were ultimately 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 ventral surface facing the substrate and the dorsal, facing away from it, accordingly led to the physiologically distinct cell layers of Trichoplax adhaerens that can still be seen today. Consequently, these are analogous, but not 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 the analysis presented above turn out to be correct, Trichoplax adhaerens would be the oldest branch of the multicellular animals and a relic of the Ediacara fauna, or even the pre-Ediacara fauna. Due to the absence of extracellular matrix and basal lamina, the development potential of these animals, very successful in their ecological niche, was of course limited, which would explain the low rate of evolution, referred to as bradytely, of their phenotype, their outward form as adults.

This hypothesis was supported by a recent analysis of the Trichoplax adhaerens mitochondrial genome in comparison to those of other animals,[6] 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 the p=0.07 level, which indicates a marginal level of statistical significance.[5]

Epitheliozoa hypothesis

Functional-morphology hypotheses are not undisputed among scientists and are often rejected because of their highly theoretical character, which is not directly accessible to empirical study. Cladistics, a modern form of systematics research, is based exclusively on demonstrable features of living and fossil animal groups (taxa) for reconstructing the genealogy of a species or group.

The most important 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):






The principle support for such a relationship comes from special cell/cell junctions, the 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 apomorphies, i.e. evolutionarily derived features, and thus form the basis of a common taxon for all animals that possess them.

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 (choanocytes) of sponges as the 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.

In contrast to the model based on functional morphology described earlier, in the Epitheliozoa concept 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 Bilateria may have developed from endoderm, whereas ectoderm is, among other things, the precursor to the external skin layer (epidermis). The interior space pervaded by a fiber syncytium in the Placozoa would then correspond to connective tissue in the other animals. It is uncertain whether the calcium ions stored in the syncytium are related to the lime skeletons of many cnidarians.

As noted above, this hypothesis was supported 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.[5]

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.

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:








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 known only for parasitic organisms but would be difficult to explain functionally in a free-living species like Trichoplax adhaerens.

All versions of this hypothesis were rejected with high confidence 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.[5]


  1. ^ MeSH Placozoa
  2. ^ Voigt, O; Collins AG, Pearse VB, Pearse JS, Hadrys H, Ender A (2004). al.Current.Biology.2004.pdf "Placozoa — no longer a phylum of one". Current Biology 14 (22): R944–5. PMID 15556848. al.Current.Biology.2004.pdf.  
  3. ^ Rüdiger Wehner & Walter Gehring (June 2007) (in German). Zoologie (24th Edition ed.). Stuttgart: Thieme. p. 696.  
  4. ^ Sperling, Erik; Vinther, Jakob; Pisani, Davide; Peterson, Kevin (2008). "A placozoan affinity for Dickinsonia and the evolution of Late Precambrian metazoan feeding modes". in Cusack, M; Owen, A; Clark, N. Palaeontological Association Annual Meeting. Programme with Abstracts. 52. Glasgow, UK. p. 81.  
  5. ^ a b c d Srivastava et al. (2008). "The Trichoplax genome and the nature of placozoans". Nature 454: 955–960. doi:10.1038/nature07191.  
  6. ^ Dellaporta et al. (2006). "'Mitochondrial genome of Trichoplax adhaerens supports Placozoa as the basal lower metazoan phylum'". Proceedings of the National Academy of Sciences 103 (23): 8751–6. doi:10.1073/pnas.0602076103. PMID 16731622.  

External links


Up to date as of January 15, 2010

Definition from Wiktionary, a free dictionary

See also placozoa




Greek πλακός (flat) + ζῷον (animal).

Proper noun

Wikipedia has an article on:



  1. A taxonomic phylum, within subkingdom Agnotozoa - simple balloon-like marine animals, the placozoans.

See also

Wikispecies has information on:



Up to date as of January 23, 2010

From Wikispecies

Nuvola apps important.svg The status, affinity, scope or nomenclature of this taxon is disputed.

Please see discussion on the talk page.


Main Page
Cladus: Eukaryota
Supergroup: Unikonta
Cladus: Opisthokonta
Regnum: Animalia
Subregnum: incertae sedis
Phylum: Placozoa
Familiae: Trichoplacidae


Placozoa Grell, 1971


  • Dellaporta, S.L.; Xu, A.; Sagasser, S.; Jakob, W.; Moreno, M.A.; Buss, L.W.; Schierwater, B. 2006: Mitochondrial genome of Trichoplax adhaerens supports Placozoa as the basal lower metazoan phylum. PNAS, 103: 8751-8756. [1]
  • Ender, A.; Schierwater, B. 2003: Placozoa are not derived cnidarians: evidence from molecular morphology. Molecular biology and evolution , 20(1): 130-134. [2]
  • Gordon, D.P. 2009: 2. Placozoa: placozoans, trichoplaxes. Pp. 47-48 in Gordon, D.P. (ed.) New Zealand inventory of biodiversity. Volume 1. Kingdom Animalia. Radiata, Lophotrochozoa, Deuterostomia. Canterbury University Press, Christchurch, New Zealand.
  • Grell, K.G. 1971: Trichoplax adhaerens: F.E. Schulze und die Entstehung der Metazoen. Naturwiss. Rundschau, 24: 160-161.
  • Schierwater, B.; de Jong, D.; DeSalle, R. 2009: Placozoa and the evolution of Metazoa and intrasomatic cell differentiation. International journal of biochemistry and cell biology, 41(2): 370-379. efirst version
  • Schierwater, B.; Kamm, K.; Srivastava, M.; Rokhsar, D.; Rosengarten, R.D.; Dellaporta, S.L. 2008: The early ANTP gene repertoire: insights from the placozoan genome. PLoS ONE, 3(8): e2457. doi:10.1371/journal.pone.0002457 [3]
  • da Silva, F.B., Muschner, V.C., Bonatto, S.L. 2007. Phylogenetic position of Placozoa based on large subunit (LSU) and small subunit (SSU) rRNA genes. Genetics and molecular biology, 30: 127-132. [4]
  • Voigt, O.; Collins, A.G.; Buchsbaum Pearse, V.; Pearse, J.S.; Ender, A.; Hadrys, H.; Schierwater, B. 2004: Placozoa: no longer a phylum of one. Current biology, 14: R944-R945. [5]

Vernacular names

Česky: Vločkovci
Español: Placozoos
日本語: 板形動物門
Português: Placozoários
Русский: Плакозои/Пластинчатые
Suomi: Laakkoeläimet
Svenska: Placozoer
Türkçe: En ilkel çokhücreliler
Wikimedia Commons For more multimedia, look at Placozoa on Wikimedia Commons.

Simple English


The Placozoa are an animal phylum, a primitive form of invertebrate.[1] They are the simplest in structure of all (metazoa).

The Placozoa has only one species, Trichoplax adhaerens. There is enough genetic diversity that it is possible that there are multiple, morphologically similar species.[2]

Although they were first discovered in 1883, a common name does not exist for the taxon. The scientific name literally means "flat animals".[3]

Trichoplax is a small, flattened, animal around a millimetre across. Like an Amoeba, it has no regular outline, although the upper surface is always flattened. The body consists of an outer layer of simple epithelium enclosing a loose sheet of cells. The epithelial cells bear flagella, which the animal uses to help it creep along the seafloor. [4] Between these layers is the fiber syncytium, a liquid-filled cavity strutted open by star-like fibers.

Trichoplax feed by absorbing food particles—mainly microbes—with their underside. They generally reproduce asexually, by dividing or budding, but can also reproduce sexually. Though Trichoplax has a small genome in comparison to other animals, nearly 87% of its 11,514 protein-coding genes are similar to known genes in other animals.

In the 1960s and 1970s renewed interest among researchers led to the discovery that the animals people had been studying are adults, not larvae. This newfound interest also included study of the organism in nature (as opposed to aquariums).[5]


  1. Barnes, Robert D. 1982. Invertebrate zoology. Holt-Saunders, Philadelphia, PA. p84 ISBN 0-03-056747-5.
  2. Voigt O et al (2004). al.Current.Biology.2004.pdf "Placozoa — no longer a phylum of one". Current Biology 14: R944–5. doi:10.1016/j.cub.2004.10.036. PMID 15556848. al.Current.Biology.2004.pdf. 
  3. Rüdiger Wehner & Walter Gehring (2007) (in German). Zoologie (24th ed ed.). Stuttgart: Thieme. p. p696. 
  4. UC Berkeley: Introduction to Placozoa
  5. T. Syed and B. Schierwater (2002). "Trichoplax adhaerens: discovered as a missing link, forgotten as a hydrozoan, re-discovered as a key to metazoan evolution". Vie Milieu 52 (4): 177–187. 


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