Parazoa

The parazoans (Parazoa, Gr. παρα-, para, "next to", and ζωα, zoa, "animal") are a subkingdom taxon that sits at the base of the phylogenetic tree of the animal kingdom as opposed to the subkingdom Eumetazoa; groups the most primitive forms, characterized by not having proper tissues or, in any case, these tissues are only partially differentiated. In general, it groups together a single phylum, Porifera, which lack muscles, nerves and internal organs, which in many cases resembles a cell colony rather than a proper multicellular organism. All other animals are eumetazoans, which do have differentiated tissues.

Parazoa sometimes conflates Porifera with Archaeocyatha, a group of extinct sponges sometimes considered a separate phylum. In other cases Placozoa are included, depending on the authors.

Porifera and Archaeocyatha

Porifera and Archaeocyatha present similarities such as the benthic, sessile habitat and the presence of pores, with differences such as the presence of internal walls and septa in Archaeocyatha. They have been considered separate phyla, however there is a growing consensus that Archaeocyatha was in fact a type of sponge that can be classified under Porifera.

Porifera and Placozoa

Some authors include poriferous phyla or sponges and placozoans in Parazoa —which includes only the species Trichoplax adhaerens— on the basis of shared primitive characteristics: Both are simple, show a lack of true tissues and organs, have both asexual and sexual reproduction and are invariably aquatic. As animals, they are a group that in several studies are at the base of the phylogenetic tree, although in a paraphyletic way. Only sponges survive from this group, belonging to the phylum Porifera, and Trichoplax in the Placozoa phylum.

Parazoa do not show any body symmetry (they are asymmetrical); all other groups of animals show some kind of symmetry. There are currently 5,000 species, 150 of which are freshwater. The larvae are planktonic and the adults are sessile. The Parazoa-Eumetazoa split has been estimated at 940 million years ago.

The Parazoa group is now considered paraphyletic. When referred to, it is sometimes considered an equivalent to the Porifera.

Some authors include the Placozoa, a phylum consisting of a single species, Adhaerens trichoplax, in the division, but it is sometimes also placed in the subkingdom Agnotozoa.

Phylogeny

According to the most up-to-date phylogeny, Porifera would not be directly related to Placozoa. In any case, the placozoans would be simplified coelenterates and without common characteristics with sponges.