Parazoa (Parazoa, gr. Παρα-, para, "next to", and ζωα, zoa, "animal") are a taxon with sub-kingdom category that is located at the base of the phylogenetic tree of the animal kingdom in opposition to the sub-kingdom

Eumetazoa Eumetazoa (), also known as diploblasts, Epitheliozoa, or Histozoa, are a proposed basal animal clade as a sister group of the Porifera (sponges). The basal eumetazoan clades are the Ctenophora and the ParaHoxozoa. Placozoa is now also seen ...

; they group together the most primitive forms, characterized by not having proper tissues or that, in any case, these tissues are only partially differentiated. They generally group a single phylum,

Porifera Sponges, the members of the phylum Porifera (; meaning 'pore bearer'), are a basal animal clade as a sister of the diploblasts. They are multicellular organisms that have bodies full of pores and channels allowing water to circulate through t ...

, which lack muscles, nerves and internal

organ Organ may refer to: Biology * Organ (biology), a part of an organism Musical instruments * Organ (music), a family of keyboard musical instruments characterized by sustained tone ** Electronic organ, an electronic keyboard instrument ** Hammond ...

s, which in many cases resembles a cell colony rather than a multicellular organism itself. All other animals are

eumetazoa Eumetazoa (), also known as diploblasts, Epitheliozoa, or Histozoa, are a proposed basal animal clade as a sister group of the Porifera (sponges). The basal eumetazoan clades are the Ctenophora and the ParaHoxozoa. Placozoa is now also seen ...

ns, 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

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, however, the consensus is growing that Archaeocyatha was in fact a type of sponge that can be classified into Porifera.

Porifera and Placozoa

Some authors include in Parazoa the poriferous or

sponge Sponges, the members of the phylum Porifera (; meaning 'pore bearer'), are a basal animal clade as a sister of the diploblasts. They are multicellular organisms that have bodies full of pores and channels allowing water to circulate throu ...

phyla Phyla, the plural of ''phylum'', may refer to: * Phylum, a biological taxon between Kingdom and Class * by analogy, in linguistics, a large division of possibly related languages, or a major language family which is not subordinate to another Phy ...

and Placozoa—comprising only the '' Trichoplax adhaerens'' species – on the basis of shared primitive characteristics: Both are simple, show a lack of true tissues and organs, have both asexual and sexual

reproduction Reproduction (or procreation or breeding) is the biological process by which new individual organisms – "offspring" – are produced from their "parent" or parents. Reproduction is a fundamental feature of all known life; each individual or ...

, 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 In taxonomy, a group is paraphyletic if it consists of the group's last common ancestor and most of its descendants, excluding a few monophyletic subgroups. The group is said to be paraphyletic ''with respect to'' the excluded subgroups. In ...

form. Of this group only surviving sponges, which belong to the phylum

, and ''

Trichoplax ''Trichoplax adhaerens'' is one of the three named species in the phylum Placozoa. The others are '' Hoilungia hongkongensis'' and '' Polyplacotoma mediterranea''. The Placozoa is a basal group of multicellular animals (metazoa). ''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. The Parazoa group is now considered paraphyletic. When referenced, it is sometimes considered an equivalent to the Porifera. Some authors include the Placozoa, a phylum consisting 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 have simplified coelenterates without common characteristics with sponges.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

References

External links

{{taxonbar, from=Q205716 Parazoa