The
Parazoa are an ancestral subkingdom of
animalAnimals are a major group of mostly 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...
s, literally translated as "beside the animals".
Parazoans differ from their
choanoflagellateThe 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 macroscopic and have
differentiated cellIn 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 single zygote to a complex system of...
s, but unlike "true animals" (
EumetazoaEumetazoa is a clade comprising all major animal groups except sponges, placozoa and several other little known animals. Characteristics of eumetazoans include true tissues organized into germ layers, and an embryo that goes through a gastrula stage. The clade is usually held to contain at least...
), they do not have
tissuesTissue is a cellular organizational level intermediate between cells and a complete organism. Hence, a tissue is an ensemble of cells, not necessarily identical, but from the same origin, that together carry out a specific function...
. The only surviving parazoans are the
spongesSponges 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 that can transform into...
, which belong to the phylum Porifera, and one surviving species (
Trichoplax adhaerensThe Placozoa are a primitive 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.
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The
Parazoa are an ancestral subkingdom of
animalAnimals are a major group of mostly 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...
s, literally translated as "beside the animals".
Description
Parazoans differ from their
choanoflagellateThe 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 macroscopic and have
differentiated cellIn 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 single zygote to a complex system of...
s, but unlike "true animals" (
EumetazoaEumetazoa is a clade comprising all major animal groups except sponges, placozoa and several other little known animals. Characteristics of eumetazoans include true tissues organized into germ layers, and an embryo that goes through a gastrula stage. The clade is usually held to contain at least...
), they do not have
tissuesTissue is a cellular organizational level intermediate between cells and a complete organism. Hence, a tissue is an ensemble of cells, not necessarily identical, but from the same origin, that together carry out a specific function...
. The only surviving parazoans are the
spongesSponges 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 that can transform into...
, which belong to the phylum Porifera, and one surviving species (
Trichoplax adhaerensThe Placozoa are a primitive 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
sessileIn 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 rock, or the hull of a ship in the case of barnacles. Corals lay down their own substrate....
.
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 analysesCladistics is a form of biological systematics which classifies living organisms on the basis of shared ancestry...
. 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
AgnotozoaAgnotozoa 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