Parazoa

Parazoa (Parazoa, gr. Παρα-, para, "next to", and ζωα, zoa, "animals") is an obsolete subkingdom that is located at the base of the phylogenetic tree of the animal kingdom in opposition to the subkingdom Eumetazoa; they group together the most primitive forms, characterized by not having proper tissues or where, in any case, these tissues are only partially differentiated. It generally includes a single phylum, Porifera, which lack muscles, nerves and internal organs, which in many cases resembles a cell colony rather than a multicellular organism itself. All other animals are eumetazoans and agnotozoans (Agnotozoans are possibly paraphyletic or even nonexistent in studies), which do have differentiated tissues.

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

Porifera and Archaeocyatha

Porifera and Archaeocyatha show similarities such as benthic and 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,<ref>Whittaker, R. H. (1969). New concepts of kingdoms or organisms. Science 163 (3863): 150–160.</ref> however, the consensus is growing that Archaeocyatha was in fact a type of sponge that can be classified into Porifera.<ref>Stephen M. Rowland 2001, Archaeocyaths: A History of Phylogenetic Interpretation Journal of Paleontology Vol. 75, No. 6, 75th Anniversary Issue (Nov., 2001), pp. 1065-1078</ref>

Porifera and Placozoa

Some authors include in Parazoa the poriferous or sponge phyla and Placozoa 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 various studies are at the base of the phylogenetic tree, albeit in a paraphyletic form. Of this group only surviving sponges, which belong to the phylum Porifera, and Trichoplax in the phylum Placozoa.

Parazoa do not show any body symmetry (they are asymmetric); all other groups of animals show some kind of symmetry. There are currently 5000 species, 150 of which are freshwater. The larvae are planktonic and the adults are sessile. The Parazoa–Eumetazoa division has been estimated to be 940 million years ago.<ref>Template:Cite journal</ref>

The Parazoa group is now considered paraphyletic.Template:Cn When referenced, it is sometimes considered an equivalent to the Porifera.Template:Cn

Some authors include the Placozoa,<ref>"Reviews glossary". Retrieved 2009-03-20.</ref> a phylum long thought to consist of a single species, Trichoplax adhaerens, in the division, but sometimes it is also placed in the Agnotozoa subkingdom.

Phylogeny

According to the most up-to-date phylogeny, Porifera should not have a direct relationship with Placozoa. In any case, placozoans are likely simplified “coelenterates” without common characteristics with sponges.<ref>Template:Cite journal</ref><ref>Template:Cite journal</ref><ref>Roberto Feuda et al. 2017, Improved Modeling of Compositional Heterogeneity Supports Sponges as Sister to All Other Animals Current Biology, Volume 27, Issue 24, p3864–3870.e4</ref>