Eumetazoa (from Ancient Greekεὖ (eû)'well'μετά (metá)'after' and ζῷον (zôion)'animal'), also known asdiploblasts,Epitheliozoa orHistozoa, are a proposedbasalanimalclade as asister group ofPorifera (sponges).[7][8][9][10][11] The basal eumetazoan clades are theCtenophora and theParaHoxozoa.Placozoa is now also seen as a eumetazoan in the ParaHoxozoa. The competing hypothesis is theMyriazoa clade.[12] The SubkingdomParazoa andAgnotozoa are the other taxa, and agnotozoa may be fake or even nonexistent at studies. Parazoa or Agnotozoa are a main sister group to eumetazoans, forming clade Blastozoa/Diploblastozoa. Alternatively,Parazoa is the sister group to Agnotozoa, forming Monoblastozoa/Simplecita.Several other extinct or obscure life forms, such asIotuba andThectardis, appear to have emerged in the group.[13] Characteristics of eumetazoans include truetissues organized intogerm layers, the presence ofneurons andmuscles, and an embryo that goes through agastrula stage.
Somephylogenists once speculated the sponges and eumetazoansevolved separately from different single-celled organisms, which would have meant that the animal kingdom does not form aclade (a complete grouping of all organisms descended from a common ancestor). However, genetic studies and some morphological characteristics, like the common presence ofchoanocytes, now unanimously support a common origin.[14]
Traditionally, eumetazoans are a major group ofanimals in the Five Kingdoms classification ofLynn Margulis and K. V. Schwartz, comprising theRadiata andBilateria – all animals except thesponges.[15] 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.
However, some skeptics[who?] emphasize inconsistencies in the new data. The zoologist Claus Nielsen argues in his 2001 bookAnimal Evolution: Interrelationships of the Living Phyla for the traditional divisions ofProtostomia andDeuterostomia.[citation needed]
It has been suggested that one type ofmolecular clock and one approach to interpretation of the fossil record both place the evolutionary origins of eumetazoa in theEdiacaran.[16] 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.[17] The discoverers ofVernanimalcula describe it as the fossil of abilateraltriploblastic animal that appeared at the end of theMarinoan glaciation prior to theEdiacaran period, implying an even earlier origin for eumetazoans.[18]
^{{citeMikhail A. Fedonkin initially classified Trilobozoa as a class within the phylum Coelenterata, which at that time included both cnidarians and ctenophores-groups that are part of the Eumetazoa subkingdom. Following the division of Coelenterata into separate phyla (Cnidaria and Ctenophora), Trilobozoa was elevated to its own phylum.Therefore, while Fedonkin's initial classification placed Trilobozoa within Eumetazoa, the current understanding of their exact phylogenetic position remains uncertain and subject to ongoing debate though, it is generally considered to be within Eumetazoa.}}
^Lankester, Ray (1877). "Notes on the Embryology and classification of the Animal kingdom: comprising a revision of speculations relative to the origin and significance of the germ-layers".Quarterly Journal of Microscopical Science (N.S.), No. 68: 399–454.
^Beklemishev, V. L.The basis of the comparative anatomy of the invertebrates [Основы сравнительной анатомии беспозвоночных]. 1st ed., 1944; 2nd ed., 1950; 3rd ed. (2 vols.), 1964. English translation, 1969,[1]. Akademia Nauk, Moscow, Leningrad.
^Ulrich, W. (1950). "Begriff und Einteilung der Protozoen". In Grüneberg, H. (ed.).Moderne Biologie. Festschrift zum 60. Geburtstag von Hans Nachtsheim (in German). Berlin: Peters. pp. 241–250.
