Eumetazoa

Eumetazoa (from Ancient Greek (eû)'well', (metá)'after' and (zôion)'animal'), also known as Epitheliozoa or Histozoa, is a proposed animalsubkingdom as a sister group of the basalphylumPorifera (sponges), and is further divided into the more basal clade Ctenophora and the much larger group ParaHoxozoa, which include Cnidaria, Bilateria and Placozoa (the lattermost is now seen as a clade under ParaHoxozoa, based on genetic data). Eumetazoans contrast with poriferans in that they have more organized body plans with differentiatedsystems, presence of nerves or neuron-like excitable cells, and some clades, the ability of macroscopicmotility.

The now-obsolete subkingdom Parazoa, which includes primarily Porifera, was previously designated as sister taxon to Eumetazoa. Agnothozoa were once considered an intermediary clade between Parazoa and Eumetazoa, being the sister clade to Parazoa which Eumetazoa would late derivate from. The group primarily encompassed the clade Placozoa and the proposed clade Mesozoa who groups Orthonectida, Dicyemida and the dubiousMonoblastozoa, but now are largely considered a polyphyletic group of simplified eumetazoans. Though some archaic fossil groups such as Chancelloriidae and Petalonamae, may, based on weaker morphological evidence, still be considered members of intermediate clades before the crown group Eumetazoa.

The competing hypothesis is the Myriazoa clade, which states Ctenophora diverged earlier than Porifera, who instead would make up a clade with ParaHoxozoa, implying either sponges lost complexity or Ctenophores developed complexity independently, and maybe even a mix of both processes. As far as May 2026, this question remains unresolved and the dispute may became combative, with some scientists deciding to not participate into the debate.

Several other extinct or obscure life forms, such as Thectardis, may have emerged in the total group. Key characteristics present in most eumetazoans include true tissues organized into germ layers, the presence of neurons and muscles, and an embryo that goes through a gastrula stage.

Some phylogenists once speculated the sponges and eumetazoans evolved separately from different single-celled organisms, which would have meant that the animal kingdom does not form a clade (a complete grouping of all organisms descended from a common ancestor). However, genetic studies and some morphological characteristics, like the common presence of choanocytes, now unanimously support a common origin.

Traditionally, eumetazoans are a major group of animals in the Five Kingdoms classification of Lynn Margulis and K. V. Schwartz, comprising the Radiata and Bilateria – all animals except the sponges.