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The Placozoa are a
basal Basal or basilar is a term meaning ''base'', ''bottom'', or ''minimum''. Science * Basal (anatomy), an anatomical term of location for features associated with the base of an organism or structure * Basal (medicine), a minimal level that is neces ...
form of marine free-living (non-parasitic)
multicellular organism A multicellular organism is an organism In biology, an organism () is any organic, life, living system that functions as an individual entity. All organisms are composed of cells (cell theory). Organisms are classified by taxonomy (biol ...
. They are the simplest in structure of all animals. Three genera have been found: the classical ''
Trichoplax adhaerens ''Trichoplax adhaerens'' is one of the three named species in the phylum In biology, a phylum (; plural The plural (sometimes abbreviated An abbreviation (from Latin ''brevis'', meaning ''short'') is a shortened form of a word or p ...
'', ''
Hoilungia hongkongensis ''Hoilungia hongkongensis'' is a species in the phylum Placozoa. The organism appears superficially similar to ''Trichoplax adhaerens'', but genetic analysis of its mitochondrial DNA shows numerous differences. It was discovered in brackish water ...
'', and ''
Polyplacotoma mediterranea ''Polyplacotoma mediterranea'' is a species in the phylum Placozoa. They differ greatly from other species of placozoans with regards to their morphology and genetic makeup. Their bodily structure consists of elongated polytomous body branches, ...
'', where the last appears most basal. The last two have been found only since 2017. Although the Placozoa were first discovered in 1883 by the German zoologist
Franz Eilhard Schulze Franz Eilhard Schulze (22 March 1840 – 2 November 1921) was a German anatomist Anatomy (Greek ''anatomē'', 'dissection') is the branch of biology Biology is the natural science that studies life and living organisms, including their ...
(1840–1921) and since the 1970s more systematically analyzed by the German protozoologist
Karl Gottlieb GrellKarl Gottlieb Grell (28 December 1912, Burg an der Wupper – 4 October 1994) was a German zoologist and protistologist, famous for his work on ''Trichoplax''. Karl Grell received in 1934 his doctorate (Promotion (Germany), Promotion) from the Unive ...
(1912–1994), a common name does not yet exist for the
taxon In biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes, Molecular biology, molecular interactions, Physiology, physiological mechani ...
; the scientific name means "flat animals".


Biology

''Trichoplax'' is a small, flattened, animal around across. An amorphous multi-celled body, analogous to a single-celled ''
Amoeba An amoeba (; less commonly spelt ameba or amœba; plural ''am(o)ebas'' or ''am(o)ebae'' ), often called an amoeboid, is a type of cell or unicellular organism A unicellular organism, also known as a single-celled organism, is an organism ...
'', 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 Epithelium is one of the four basic types of animal Animals (also called Metazoa) are multicellular A multicellular organism is an organism In biology, an organism () is any organic, life, living system that functions as ...
enclosing a loose sheet of stellate cells resembling the
mesenchyme Mesenchyme () is a type of loosely organised animal embryonic ''Embryonic'' is the twelfth studio album by experimental rock band the Flaming Lips released on October 13, 2009, on Warner Bros. Records, Warner Bros. The band's first double album, ...
of some more complex animals. The epithelial cells bear
cilia The cilium (; the plural is cilia) is an organelle In cell biology Cell biology (also cellular biology or cytology) is a branch of biology Biology is the natural science that studies life and living organisms, including their anatomy, ...

cilia
, which the animal uses to help it creep along the seafloor. The lower surface engulfs small particles of organic detritus, on which the animal feeds. All placozoa can reproduce asexually, budding off smaller individuals, and the lower surface may also bud off eggs into the
mesenchyme Mesenchyme () is a type of loosely organised animal embryonic ''Embryonic'' is the twelfth studio album by experimental rock band the Flaming Lips released on October 13, 2009, on Warner Bros. Records, Warner Bros. The band's first double album, ...
.
Sexual reproduction Sexual reproduction is a type of reproduction Reproduction (or procreation or breeding) is the biological process Biological processes are those processes that are vital for an organism In biology, an organism (from Ancient Greek, ...
has been reported to occur in one
clade A clade (), also known as a monophyletic group or natural group, is a group of organisms that are monophyly, monophyletic – that is, composed of a common ancestor and all its lineage (evolution), lineal descendants - on a phylogenetic tree. R ...

clade
of the placozoa, where intergenic recombination was observed as well as other hallmarks of sexual reproduction. Some ''Trichoplax'' species contain ''
Rickettsiales The Rickettsiales, informally called rickettsias, are an order of small Alphaproteobacteria. They are Intracellular parasites#Obligate, obligate intracellular parasites, and some are notable pathogens, including ''Rickettsia'', which causes a var ...
'' bacteria as
endosymbiont An endosymbiont or endobiont is any organism In biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes, Molecular biology, mol ...
s. One of the at least 20 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 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. They show substantial evolutionary radiation in regard to
sodium channel Sodium channels are integral membrane protein An integral membrane protein (IMP) is a type of membrane protein Membrane proteins are common proteins that are part of, or interact with, biological membranes. Membrane proteins fall into several bro ...
s, of which they have 5–7 different types, more than any other invertebrate species studied to date.


Evolutionary relationships

There is no convincing fossil record of the placozoa, although the
Ediacaran biota The Ediacaran (; formerly Vendian) biota is a Taxonomy (biology), taxonomic period classification that consists of all life forms that were present on Earth during the Ediacaran Period (c. 635–541 Year#mya, Mya). These were composed of enig ...
(Precambrian, ) organism ''
Dickinsonia ''Dickinsonia'' is an extinct genus of basal animal that lived during the late Ediacaran period in what is now Australia, China, India, Russia and Ukraine. The individual ''Dickinsonia'' typically resembles a bilaterally symmetrical ribbed oval. ...
'' appears somewhat similar to placozoans. Knaust (2021) reported preservation of placozoan fossils in a microbialite bed from the
Middle Triassic In the geologic timescale The geologic time scale (GTS) is a system of chronological dating that classifies Geology, geological strata (stratigraphy) in time. It is used by geologists, paleontology, paleontologists, and other earth sciences, E ...
Muschelkalk The Muschelkalk (German for "shell-bearing limestone"; french: calcaire coquillier) is a sequence of sedimentary rock, sedimentary rock strata (a lithostratigraphy, lithostratigraphic unit) in the geology of central and western Europe. It has a Mid ...

Muschelkalk
(
Germany ) , image_map = , map_caption = , map_width = 250px , capital = Berlin Berlin (; ) is the Capital city, capital and List of cities in Germany by population, largest city of Germany by both area and population. Its 3,769,495 inh ...

Germany
). 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, so is inadequate to demark a
clade A clade (), also known as a monophyletic group or natural group, is a group of organisms that are monophyly, monophyletic – that is, composed of a common ancestor and all its lineage (evolution), lineal descendants - on a phylogenetic tree. R ...

clade
. More recent work has attempted to classify them based on the DNA sequences in their genome; this has placed the phylum between the
sponge Sponges, the members of the phylum Porifera (; meaning 'pore bearer'), are a basal animal clade as a sister of the Diploblasts. They are Multicellular organism, multicellular organisms that have bodies full of pores and channels allowing water ...

sponge
s and the
eumetazoa Eumetazoa (), also known as Diploblasts, Epitheliozoa, or Histozoa, are a proposed basal animal Marine life, sea life, or ocean life is the s, s, and other s that live in the of the or , or the of coastal . At a fundamental level, marin ...
. 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 A phylogenetic tree (also phylogeny or evolutionary tree Felsenstein J. (2004). ''Inferring Phylogenies'' Sinauer Associates: Sunderland, MA.) is a branching diagram or a tree (graph theory), tree showing the evolutionary relationships among va ...

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 In phylogenetics In biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes, Molecular biology, molecular interactions, Physiology, physi ...
; once in the
Ctenophora Ctenophora (; ctenophore ; ) comprise a phylum In biology, a phylum (; plural The plural (sometimes list of glossing abbreviations, abbreviated ), in many languages, is one of the values of the grammatical number, grammatical category ...

Ctenophora
, once in the
Cnidaria Cnidaria () is a phylum In biology, a phylum (; plural The plural (sometimes list of glossing abbreviations, abbreviated ), in many languages, is one of the values of the grammatical number, grammatical category of number. The plural of ...

Cnidaria
and once in the
Bilateria The Bilateria or bilaterians are animal Animals (also called Metazoa) are multicellular A multicellular organism is an organism In biology, an organism () is any organic, life, living system that functions as an individual ...
. 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

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 Animals (also called Metazoa) are multicellular A multicellular organism is an organism In biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Bioch ...
) and are thus considered a sister taxon to all other metazoans: According to a functional-morphology model, all or most animals are descended from a '' gallertoid'', a free-living (
pelagic The pelagic zone consists of the water column A water column is a concept Concepts are defined as abstract ideas A mental representation (or cognitive representation), in philosophy of mind Philosophy of mind is a branch of philosophy ...
) sphere in seawater, consisting of a single
ciliated The cilium (; the plural is cilia) is an organelle In cell biology Cell biology (also cellular biology or cytology) is a branch of biology Biology is the natural science that studies life and living organisms, including their anatomy, ...
layer of cells supported by a thin, noncellular separating layer, the
basal lamina The basal lamina is a layer of extracellular matrix In biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes, Molecular biology, molecu ...

basal lamina
. The interior of the
sphere A sphere (from Greek#REDIRECT Greek Greek may refer to: Greece Anything of, from, or related to Greece Greece ( el, Ελλάδα, , ), officially the Hellenic Republic, is a country located in Southeast Europe. Its population is appr ...

sphere
is filled with contractile fibrous cells and a gelatinous
extracellular matrix In biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes, Molecular biology, molecular interactions, Physiology, physiological mechanisms ...
. Both the modern Placozoa and all other animals then descended from this multicellular beginning stage via two different processes: * Infolding of the
epithelium Epithelium is one of the four basic types of animal Animals (also called Metazoa) are multicellular A multicellular organism is an organism In biology, an organism () is any organic, life, living system that functions as ...
led to the formation of an internal system of ducts and thus to the development of a modified gallertoid from which the sponges (
Porifera Sponges, the members of the phylum In biology, a phylum (; plural The plural (sometimes list of glossing abbreviations, abbreviated ), in many languages, is one of the values of the grammatical number, grammatical category of number. T ...

Porifera
),
Cnidaria Cnidaria () is a phylum In biology, a phylum (; plural The plural (sometimes list of glossing abbreviations, abbreviated ), in many languages, is one of the values of the grammatical number, grammatical category of number. The plural of ...

Cnidaria
and
Ctenophora Ctenophora (; ctenophore ; ) comprise a phylum In biology, a phylum (; plural The plural (sometimes list of glossing abbreviations, abbreviated ), in many languages, is one of the values of the grammatical number, grammatical category ...

Ctenophora
subsequently developed. * Other gallertoids, according to this model, made the transition over time to a
benthic The benthic zone is the ecological region at the lowest level of a body of water A body of water or waterbody (often spelled water body) is any significant accumulation of water, generally on a planet's surface. The term most often refers to ...
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 Substrate may refer to: Physical layers *Substrate (biology), the natural environment in which an organism lives, or the surface or medium on which an organism grows or is attached **Substrate (locomotion), the surface over which an organism loco ...
, and between their orientation and the vertical direction perpendicular to the substrate. This results naturally in a
selective Selective may refer to: * Selective school, a school that admits students on the basis of some sort of selection criteria ** Selective school (New South Wales) Selective strength: the human body transitions between being weak and strong. This rang ...
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 The Ediacaran Period ( ) is a geological period that spans 94 million years from the end of the Cryogenian, Cryogenian Period 635 million years ago (Mya), to the beginning of the Cambrian, Cambrian Period 541 Mya. It marks the end of the Protero ...
in
geology Geology (from the Ancient Greek Ancient Greek includes the forms of the Greek language Greek ( el, label=Modern Greek Modern Greek (, , or , ''Kiní Neoellinikí Glóssa''), generally referred to by speakers simply as Greek ...

geology
. These are usually attributed to
bilaterally symmetrical Symmetry in biology refers to the symmetry observed in organism In biology, an organism (from Ancient Greek, Greek: ὀργανισμός, ''organismos'') is any individual contiguous system that embodies the Life#Biology, properties o ...
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 Algae (; singular alga ) is an informal term for a large and diverse group of photosynthetic Photosynthesis is a process used by plants and other organisms to convert Conversion or convert may refer to: Arts, entertainment, and media * Co ...

algae
as photosynthetically active
endosymbionts An endosymbiont or endobiont is any organism that lives within the body or cells of another organism most often, though not always, in a mutualism (biology), mutualistic relationship. (The term endosymbiosis is from the Greek language, Greek: ἔ ...

endosymbionts
, i.e. primarily obtaining their nutrients from their partners in
symbiosis Symbiosis (from Ancient Greek, Greek , , "living together", from , , "together", and , bíōsis, "living") is any type of a close and long-term biological interaction between two different Organism, biological organisms, be it Mutualism (biolog ...

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 The Paleozoic (or Palaeozoic) Era ( ; from the Greek#REDIRECT Greek Greek may refer to: Greece Anything of, from, or related to Greece Greece ( el, Ελλάδα, , ), officially the Hellenic Republic, is a country located in Southeast Eu ...
. Recent work has shown that some of the Ediacaran assemblages (e.g.
Mistaken Point Mistaken Point Ecological Reserve is a wilderness area and a UNESCO World Heritage Site located at the southeastern tip of Newfoundland's Avalon Peninsula in the Canadian Canadians (french: Canadiens) are people identified with the countr ...
) were in deep water, below the
photic zone The photic zone, euphotic zone, epipelagic zone, or sunlight zone is the uppermost layer of a body of water that receives sunlight, allowing phytoplankton to perform photosynthesis. It undergoes a series of physical, chemical, and biological proc ...
, and that the organisms were not dependent on endosymbiotic . * Animals that grazed on
algal mat Algal mats are one of many types of microbial mat that forms on the surface of water or rocks. They are typically composed of blue-green cyanobacteria Cyanobacteria , also known as Cyanophyta, are a phylum of Gram-negative bacteria Gram-neg ...
s 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 In biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes, Molecular biology, molecular interactions, Physiology, physiological mechanisms ...
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 The ectoderm is one of the three primary germ layer A germ layer is a primary layer of cell (biology), cells that forms during embryonic development. The three germ layers in vertebrates are particularly pronounced; however, all eumetazoans ( ...

ectoderm
and
endoderm Endoderm is the innermost of the three primary germ layer A germ layer is a primary layer of cell (biology), cells that forms during embryonic development. The three germ layers in vertebrates are particularly pronounced; however, all eumetazo ...
, 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 basal lamina is a layer of extracellular matrix In biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes, Molecular biology, molecu ...

basal lamina
, the development potential of these animals, very successful in their
ecological niche In ecology Ecology (from el, οἶκος, "house" and el, -λογία, label=none, "study of") is the study of the relationships between living organisms, including humans, and their physical environment. Ecology considers organisms ...

ecological niche
, was of course limited, which would explain the low rate of evolution, referred to as ''bradytely'', of their
phenotype In genetics Genetics is a branch of biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes, Molecular biology, molecular inter ...

phenotype
, their outward form as adults. This hypothesis was supported by a recent analysis of the ''Trichoplax adhaerens''
mitochondrial A mitochondrion (, plural mitochondria) is a double membrane-bound organelle In cell biology, an organelle is a specialized subunit, usually within a cell (biology), cell, that has a specific function. The name ''organelle'' comes from the ide ...

mitochondrial
genome In the fields of molecular biology Molecular biology is the branch of biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes, M ...

genome
in comparison to those of other animals, 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.


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): The above view could be correct, although there is some evidence that the
ctenophore Ctenophora (; singular ctenophore, or ; from grc, κτείς, kteis, comb and , ''pherō'', 'to carry'; commonly known as comb jellies) comprise a phylum of invertebrate Invertebrates are animals that neither possess nor develop a vertebr ...

ctenophore
s, traditionally seen as Eumetazoa, may be the sister to all other animals. This is now a disputed classification. The principal support for such a relationship comes from special cell/cell junctions, the belt
desmosomes A desmosome (; "binding body"), also known as a macula adherens (plural: maculae adherentes) (Latin Latin (, or , ) is a classical language belonging to the Italic languages, Italic branch of the Indo-European languages. Latin was originally s ...
, 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 , 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 () 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.


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 Epithelium is one of the four basic types of animal Animals (also called Metazoa) are multicellular A multicellular organism is an organism In biology, an organism () is any organic, life, living system that functions as ...
, 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 Cnidaria () is a phylum In biology, a phylum (; plural The plural (sometimes list of glossing abbreviations, abbreviated ), in many languages, is one of the values of the grammatical number, grammatical category of number. The plural of ...

Cnidaria
, while in another they would be a sister group to the
Ctenophora Ctenophora (; ctenophore ; ) comprise a phylum In biology, a phylum (; plural The plural (sometimes list of glossing abbreviations, abbreviated ), in many languages, is one of the values of the grammatical number, grammatical category ...

Ctenophora
, and occasionally they are placed directly next to the
Bilateria The Bilateria or bilaterians are animal Animals (also called Metazoa) are multicellular A multicellular organism is an organism In biology, an organism () is any organic, life, living system that functions as an individual ...
. Currently, they are typically placed according to the cladogram below: In this cladogram the Epitheliozoa and Eumetazoa are synonyms to each other and to the
Diploblast Eumetazoa (), also known as Diploblasts, Epitheliozoa, or Histozoa, are a proposed basal animal clade as a sister group of the Sponge, Porifera. The basal Eumetazoan clades are the Ctenophora and the ParaHoxozoa. Placozoa is now also seen as an E ...
s, and the
Ctenophora Ctenophora (; ctenophore ; ) comprise a phylum In biology, a phylum (; plural The plural (sometimes list of glossing abbreviations, abbreviated ), in many languages, is one of the values of the grammatical number, grammatical category ...

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 known only for parasitic organisms but would be difficult to explain functionally in a free-living species like ''Trichoplax adhaerens''. 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 placozoas outside of the sampled Eumetazoans.


Cnidaria-sister hypothesis

DNA studies suggests that these organisms are related to
Cnidaria Cnidaria () is a phylum In biology, a phylum (; plural The plural (sometimes list of glossing abbreviations, abbreviated ), in many languages, is one of the values of the grammatical number, grammatical category of number. The plural of ...

Cnidaria
, derived from
planula A planula is the free-swimming, flattened, ciliated The cilium (; the plural is cilia) is an organelle In cell biology Cell biology (also cellular biology or cytology) is a branch of biology Biology is the natural science that studi ...

planula
larva (as seen in some Cnidaria). The
Bilateria The Bilateria or bilaterians are animal Animals (also called Metazoa) are multicellular A multicellular organism is an organism In biology, an organism () is any organic, life, living system that functions as an individual ...
also are thought to be derived from planuloids. The Cnidaria and Placozoa body axis are overtly similar, and Placozoan and Cnidarian cells are responsive to the same
neuropeptide Neuropeptides are chemical messengers made up of small chains of amino acid Amino acids are organic compound In , organic compounds are generally any s that contain - . Due to carbon's ability to (form chains with other carbon s), mil ...
antibodies An antibody (Ab), also known as an immunoglobulin (Ig), is a large, Y-shaped protein used by the immune system to identify and neutralize foreign objects such as pathogenic bacteria and Viral disease, viruses. The antibody recognizes a unique mo ...

antibodies
despite extant Placozoa not developing any neurons.


References


External links


The ''Trichoplax adhaerens'' Grell-BS-1999 v1.0 Genome Portal at the DOE Joint Genome Institute





Research articles from the ITZ, TiHo Hannover


* –
Mitochondrial DNA Mitochondrial DNA (mtDNA or mDNA) is the DNA Deoxyribonucleic acid (; DNA) is a molecule File:Pentacene on Ni(111) STM.jpg, A scanning tunneling microscopy image of pentacene molecules, which consist of linear chains of five car ...

Mitochondrial DNA
and
16S rRNA 16S rRNA may refer to: * 16S ribosomal RNA, the prokaryotic ribosomal subunit * Mitochondrially encoded 16S RNA, the eukaryotic ribosomal subunit {{Short pages monitor