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 Animals are multicellular, eukaryotic organisms in the Biology, biological Kingdom (biology), kingdom Animalia (). With few exceptions, animals heterotroph, consume organic material, Cellular respiration#Aerobic respiration, breathe oxygen, ...

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 Sponges or sea sponges are primarily marine invertebrates of the animal phylum Porifera (; meaning 'pore bearer'), a Basal (phylogenetics) , basal clade and a sister taxon of the Eumetazoa , diploblasts. They are sessility (motility) , sessile ...

, which lack

muscles Muscle is a soft tissue, one of the four basic types of animal tissue. There are three types of muscle tissue in vertebrates: skeletal muscle, cardiac muscle, and smooth muscle. Muscle tissue gives skeletal muscles the ability to muscle contra ...

, 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 Placozoa ( ; ) is a phylum of free-living (non-parasitic) marine invertebrates. They are blob-like animals composed of aggregations of cells. Moving in water by ciliary motion, eating food by Phagocytosis, engulfment, reproducing by Fission (biol ...

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 or sea sponges are primarily marine invertebrates of the animal phylum Porifera (; meaning 'pore bearer'), a basal clade and a sister taxon of the diploblasts. They are sessile filter feeders that are bound to the seabed, and a ...

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

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. There are two forms of reproduction: Asexual reproduction, asexual and Sexual ...

, 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 Paraphyly is a taxonomic term describing a grouping that consists of the grouping's last common ancestor and some but not all of its descendant lineages. The grouping is said to be paraphyletic ''with respect to'' the excluded subgroups. In co ...

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

, and '' 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 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.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 Animals