ParaHoxozoa


The ParaHoxozoa are a proposed basal Diploblast/Eumetazoa clade as sister of the Ctenophora. It consists of the Triploblasts/Bilateria as well as the Placozoa and Cnidaria.

Characteristics

The Parahoxozoa group was defined based on the presence of several gene classes, as well as Hox/ParaHox-ANTP from which the name of this clade originated. It was later proposed and contested that a gene of the same class as the Hox/ParaHox, the NK gene and the Cdx Parahox gene, is also present in Porifera. Nevertheless, the Parahoxozoa as originally defined without Porifera may be a monophyletic clade, and continues to be used as such.
The position of Placozoa is not clear, as it may be sister the remaining Parahoxozoa or sister to the Cnidaria.

Planula-acoel, triploblasty, and bilaterian similarities

The original bilateria are hypothesized to be a bottom dwelling worm with a single body opening. A through-gut may already have developed with the ctenophora however. The through-gut may have developed from the corners of a single opening with lips fusing. E.g. Acoela resemble the planula larvae of some cnidaria, which exhibit some bilaterian symmetry. They are vermiformes, just as the cnidarian Buddenbrockia is. Placozoa has been noted to resemble planula. Usually, "Planulozoa" is to the exclusion of Placozoa, but not necessarily. In this case it appears synonymous with ParaHoxozoa. Triploblasty developed before the Cnidara-Bilateria radiation as well.