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; 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, which lack

muscles Skeletal muscles (commonly referred to as muscles) are organs of the vertebrate muscular system and typically are attached by tendons to bones of a skeleton. The muscle cells of skeletal muscles are much longer than in the other types of muscle ...

nerves A nerve is an enclosed, cable-like bundle of nerve fibers (called axons) in the peripheral nervous system. A nerve transmits electrical impulses. It is the basic unit of the peripheral nervous system. A nerve provides a common pathway for the e ...

and internal organs, which in many cases resembles a cell colony rather than a multicellular organism itself. All other animals are eumetazoans, which do have differentiated tissues. On occasion, Parazoa reunites Porifera with

Archaeocyatha Archaeocyatha (or archaeocyathids 'ancient cups') is a taxon of extinct, sessile, reef-building marine sponges that lived in warm tropical and subtropical waters during the Cambrian Period. It is believed that the centre of the Archaeocyatha or ...

, a group of extinct sponges sometimes considered a separate phylum. In other cases,

Placozoa The Placozoa are a basal form of marine free-living (non-parasitic) multicellular organism. They are the simplest in structure of all animals. Three genera have been found: the classical ''Trichoplax adhaerens'', ''Hoilungia hongkongensis'', an ...

is included, depending on the authors.

Porifera and Archaeocyatha

Porifera and

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 through t ...

phyla and

—comprising only the ''

Trichoplax adhaerens ''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'' ...

'' 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 (general), taxonomy, a group is paraphyletic if it consists of the group's most recent common ancestor, last common ancestor and most of its descendants, excluding a few Monophyly, monophyletic subgroups. The group is said to be pa ...

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

. 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

, a phylum consisting of a single species, ''

'', in the division, but sometimes it is also placed in the

Agnotozoa Agnotozoa is the name of a taxon of simple animals. The name first appeared in an invertebrate paleontology book as one of the "branches" of the subkingdom Metazoa. The branch contained only one group: Mesozoa. More recently, some have used the ...

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