Eumetazoa (), also known as diploblasts, Epitheliozoa, or Histozoa, are a proposed basal animal

clade A clade (), also known as a monophyletic group or natural group, is a group of organisms that are monophyletic – that is, composed of a common ancestor and all its lineal descendants – on a phylogenetic tree. Rather than the English ter ...

as a sister group of the Porifera (sponges). The basal eumetazoan clades are the Ctenophora and the ParaHoxozoa. Placozoa is now also seen as a eumetazoan in the ParaHoxozoa. Several other extinct or obscure life forms, such as ''

Iotuba ''Iotuba chengjiangensis'' (sometimes misspelt ''Lotuba'') is a Cambrian fossil with a U-shaped gut and tentacles, known from the Chengjiang biota. Originally interpreted as a phoronid marine animal, its affinity is currently disputed. It, and ...

'' and ''

Thectardis ''Thectardis avalonensis'' is a triangular-shaped member of the Ediacaran biota, dating from . The organism took the form of an elongated cone with a central depression, and its apex was anchored to the substrate. Morphology The fossils take th ...

'', appear to have emerged in the group. Characteristics of eumetazoans include true tissues organized into

germ layer A germ layer is a primary layer of cells that forms during embryonic development. The three germ layers in vertebrates are particularly pronounced; however, all eumetazoans (animals that are sister taxa to the sponges) produce two or three p ...

s, the presence of

neurons A neuron, neurone, or nerve cell is an electrically excitable cell that communicates with other cells via specialized connections called synapses. The neuron is the main component of nervous tissue in all animals except sponges and placozoa ...

and muscles, and an embryo that goes through a gastrula stage. Some phylogenists once speculated the sponges and eumetazoans

evolved Evolution is change in the heritable characteristics of biological populations over successive generations. These characteristics are the expressions of genes, which are passed on from parent to offspring during reproduction. Variati ...

separately from different single-celled organisms, which would have meant that the animal kingdom does not form a

(a complete grouping of all organisms descended from a common ancestor). However, genetic studies and some morphological characteristics, like the common presence of

choanocyte Choanocytes (also known as "collar cells") are cells that line the interior of asconoid, syconoid and leuconoid body types of sponges that contain a central flagellum, or ''cilium,'' surrounded by a collar of microvilli which are connected by a ...

s, now unanimously support a common origin. Traditionally, eumetazoans are a major group of

animal Animals are multicellular, eukaryotic organisms in the biological kingdom Animalia. With few exceptions, animals consume organic material, breathe oxygen, are able to move, can reproduce sexually, and go through an ontogenetic stage ...

s in the ''Five Kingdoms'' classification of Lynn Margulis and K. V. Schwartz, comprising the Radiata and

Bilateria The Bilateria or bilaterians are animals with bilateral symmetry as an embryo, i.e. having a left and a right side that are mirror images of each other. This also means they have a head and a tail (anterior-posterior axis) as well as a belly and ...

– all animals except 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 organisms that have bodies full of pores and channels allowing water to circulate throu ...

s. When treated as a formal taxon Eumetazoa is typically ranked as a subkingdom. The name Metazoa has also been used to refer to this group, but more often refers to the Animalia as a whole. Many classification schemes do not include a subkingdom Eumetazoa.

Taxonomy

A widely accepted hypothesis, based on molecular data (mostly 18S

rRNA Ribosomal ribonucleic acid (rRNA) is a type of non-coding RNA which is the primary component of ribosomes, essential to all cells. rRNA is a ribozyme which carries out protein synthesis in ribosomes. Ribosomal RNA is transcribed from ribos ...

sequences), divides Bilateria into four superphyla:

Deuterostomia Deuterostomia (; in Greek) are animals typically characterized by their anus forming before their mouth during embryonic development. The group's sister clade is Protostomia, animals whose digestive tract development is more varied. Some exa ...

Ecdysozoa Ecdysozoa () is a group of protostome animals, including Arthropoda ( insects, chelicerata, crustaceans, and myriapods), Nematoda, and several smaller phyla. They were first defined by Aguinaldo ''et al.'' in 1997, based mainly on phylogenetic ...

Lophotrochozoa Lophotrochozoa (, "crest/wheel animals") is a clade of protostome animals within the Spiralia. The taxon was established as a monophyletic group based on molecular evidence. The clade includes animals like annelids, molluscs, bryozoans, bra ...

, and

Platyzoa The paraphyletic "Platyzoa" are a group of protostome unsegmented animals proposed by Thomas Cavalier-Smith in 1998. Cavalier-Smith included in Platyzoa the phylum Platyhelminthes (or flatworms), and a new phylum, the Acanthognatha, into which ...

(sometimes included in Lophotrochozoa). The last three groups are also collectively known as Protostomia. However, some skeptics emphasize inconsistencies in the new data. The zoologist Claus Nielsen argues in his 2001 book ''Animal Evolution: Interrelationships of the Living Phyla'' for the traditional divisions of Protostomia and

Evolutionary origins

It has been suggested that one type of

molecular clock The molecular clock is a figurative term for a technique that uses the mutation rate of biomolecules to deduce the time in prehistory when two or more life forms diverged. The biomolecular data used for such calculations are usually nucleo ...

and one approach to interpretation of the fossil record both place the evolutionary origins of eumetazoa in the Ediacaran. However, the earliest eumetazoans may not have left a clear impact on the fossil record and other interpretations of molecular clocks suggest the possibility of an earlier origin. The discoverers of '' Vernanimalcula'' describe it as the fossil of a bilateral

triploblastic Triploblasty is a condition of the gastrula in which there are three primary germ layers: the ectoderm, mesoderm, and endoderm. Germ cells are set aside in the embryo at the blastula stage, which are incorporated into the gonads during organo ...

animal that appeared at the end of the Marinoan glaciation prior to the Ediacaran Period, implying an even earlier origin for eumetazoans.

References

External links

''Bilateria.''
Tree of Life web project, US National Science Foundation. 2002. 6 January 2006.

*Evers, Christine A., Lisa Starr. ''Biology:Concepts and Applications.'' 6th ed. United States:Thomson, 2006. .
''TRICHOPLAX ADHAERENS (PLACOZOA TYPE)''
St. Petersburg. 2005

*Nielsen, C. 2001. Animal Evolution: Interrelationships of the Living Phyla, 2nd edition, 563 pp. Oxford Univ. Press, Oxford. * * {{Taxonbar, from=Q5174 Animal taxa Subkingdoms Ediacaran first appearances