The Parazoa are an ancestral subkingdom of animal

Animal

Animals are a major group of multicellular, eukaryotic organisms of the kingdom Animalia or Metazoa. Their body plan eventually becomes fixed as they develop, although some undergo a process of metamorphosis later on in their life. Most animals are motile, meaning they can move spontaneously and...

s, literally translated as "beside the animals".

Description

Parazoans differ from their choanoflagellate

Choanoflagellate

The choanoflagellates are a group of free-living unicellular and colonial flagellate eukaryotes considered to be the closest living relatives of the animals...

 ancestors in that they are not microscopic and have differentiated cell

Cellular differentiation

In developmental biology, cellular differentiation is the process by which a less specialized cell becomes a more specialized cell type. Differentiation occurs numerous times during the development of a multicellular organism as the organism changes from a simple zygote to a complex system of...

s. However, they are an outgroup of the animal phylogenetic tree being that they do not have tissues

Biological tissue

Tissue is a cellular organizational level intermediate between cells and a complete organism. A tissue is an ensemble of cells, not necessarily identical, but from the same origin, that together carry out a specific function. These are called tissues because of their identical functioning...

. The only surviving parazoans are the sponges

Sea sponge

Sponges are animals of the phylum Porifera . Their bodies consist of jelly-like mesohyl sandwiched between two thin layers of cells. While all animals have unspecialized cells that can transform into specialized cells, sponges are unique in having some specialized cells, but can also have...

, which belong to the phylum Porifera, and one surviving species (Trichoplax adhaerens

Trichoplax

The Placozoa are a basal form of invertebrate. They are the simplest in structure of all non-parasitic multicellular animals . They are generally classified as a single species, Trichoplax adhaerens, although there is enough genetic diversity that it is likely that there are multiple,...

) in the phylum Placozoa.

Parazoa display no body symmetry (are asymmetrical); all other animal groups display some sort of symmetry. There are currently 5000 species, 150 of which are freshwater. Larvae are planktonic and adults are sessile

Sessility (zoology)

In zoology, sessility is a characteristic of animals which are not able to move about. They are usually permanently attached to a solid substrate of some kind, such as a part of a plant or dead tree trunk, a rock, or the hull of a ship in the case of barnacles. Corals lay down their own...

.

Cladistics

The Parazoa-Eumetazoa split has been estimated at 940 million years ago.

The parazoa group is now considered paraphyletic. It is not included in most modern cladistic analyses

Cladistics

Cladistics is a method of classifying species of organisms into groups called clades, which consist of an ancestor organism and all its descendants . For example, birds, dinosaurs, crocodiles, and all descendants of their most recent common ancestor form a clade...

. When referenced, it is sometimes considered an equivalent to Porifera.

Some authors include Placozoa, a phylum that consists of only one species, Trichoplax adhaerens, in the division, but they are also sometimes placed in the subkingdom Agnotozoa

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....

.

External links

• http://www.jochemnet.de/fiu/BSC1011/BSC1011_12/sld001.htm