Eumetazoa

Eumetazoa (from Ancient Greek εὖ (eû) 'well', μετά (metá) 'after' and ζῷον (zôion) 'animal'), also known as Epitheliozoa or Histozoa, is a proposed animal subkingdom as a sister group of the basal phylum Porifera (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 differentiated systems, presence of nerves or neuron-like excitable cells, and some clades, the ability of macroscopic motility. 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 dubious Monoblastozoa, 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...