Platyzoa
Encyclopedia
The Platyzoa are a group of protostome
animals proposed by Thomas Cavalier-Smith
in 1998. Cavalier-Smith included in Platyzoa the Phylum
Platyhelminthes
or flatworm
s, and a new phylum, Acanthognatha, into which he gathered several previously described phyla of microscopic animals. Subsequent studies have supported Platyzoa as a clade
, a monophyletic
group of organisms with a common ancestor, while differing on the phyla included and on relationships within Platyzoa.
. The other phyla have a pseudocoel, and share characteristics such as the structure of their jaws and pharynx
, although these have been secondarily lost in the parasitic Acanthocephala. They form a monophyletic subgroup called the Gnathifera
.
The name "Platyzoa" is used because most members are flat, though rotifer
s are not.
, and are sometimes included in that group. Together the two make up the Spiralia
.
Protostome
Protostomia are a clade of animals. Together with the deuterostomes and a few smaller phyla, they make up the Bilateria, mostly comprising animals with bilateral symmetry and three germ layers...
animals proposed by Thomas Cavalier-Smith
Thomas Cavalier-Smith
Professor Thomas Cavalier-Smith , FRS, FRSC, NERC Professorial Fellow, is a Professor of Evolutionary Biology in the Department of Zoology, at the University of Oxford...
in 1998. Cavalier-Smith included in Platyzoa the Phylum
Phylum
In biology, a phylum The term was coined by Georges Cuvier from Greek φῦλον phylon, "race, stock," related to φυλή phyle, "tribe, clan." is a taxonomic rank below kingdom and above class. "Phylum" is equivalent to the botanical term division....
Platyhelminthes
Flatworm
The flatworms, known in scientific literature as Platyhelminthes or Plathelminthes are a phylum of relatively simple bilaterian, unsegmented, soft-bodied invertebrate animals...
or flatworm
Flatworm
The flatworms, known in scientific literature as Platyhelminthes or Plathelminthes are a phylum of relatively simple bilaterian, unsegmented, soft-bodied invertebrate animals...
s, and a new phylum, Acanthognatha, into which he gathered several previously described phyla of microscopic animals. Subsequent studies have supported Platyzoa as a clade
Clade
A clade is a group consisting of a species and all its descendants. In the terms of biological systematics, a clade is a single "branch" on the "tree of life". The idea that such a "natural group" of organisms should be grouped together and given a taxonomic name is central to biological...
, a monophyletic
Monophyly
In common cladistic usage, a monophyletic group is a taxon which forms a clade, meaning that it contains all the descendants of the possibly hypothetical closest common ancestor of the members of the group. The term is synonymous with the uncommon term holophyly...
group of organisms with a common ancestor, while differing on the phyla included and on relationships within Platyzoa.
Phyla
One current scheme places the following traditional phyla in Platyzoa:- PlatyhelminthesFlatwormThe flatworms, known in scientific literature as Platyhelminthes or Plathelminthes are a phylum of relatively simple bilaterian, unsegmented, soft-bodied invertebrate animals...
- GastrotrichGastrotrichThe gastrotrichs are a phylum of microscopic animals abundant in fresh water and marine environments. Most fresh water species are part of the periphyton and benthos...
a - GnathiferaGnathifera (phylum)Gnathifera is an assemblage of phyla of metazoans."Acanthognatha" is a similar grouping, including rotifers, acanthocephalans, gastrotrichs, and gnathostomulids....
- RotiferRotiferThe rotifers make up a phylum of microscopic and near-microscopic pseudocoelomate animals. They were first described by Rev. John Harris in 1696, and other forms were described by Anton van Leeuwenhoek in 1703...
a - AcanthocephalaAcanthocephalaAcanthocephala is a phylum of parasitic worms known as acanthocephales, thorny-headed worms, or spiny-headed worms, characterized by the presence of an evertable proboscis, armed with spines, which it uses to pierce and hold the gut wall of its host...
- GnathostomulidGnathostomulidGnathostomulids, or jaw worms, are a small phylum of nearly microscopic marine animals. They inhabit sand and mud beneath shallow coastal waters and can survive in relatively anoxic environments. They were first recognised and described in 1956....
a - MicrognathozoaLimnognathiaLimnognathia maerski is a microscopic animal, discovered living in homothermic springs on Disko Island, Greenland in 1994, that has variously been assigned as a class or subphylum in the phylum Gnathifera or as a phylum in a Gnathifera superphylum, named Micrognathozoa. It is related to the...
- CycliophoraSymbionSymbion is the name of a genus of aquatic animals, less than ½ mm wide, found living attached to the bodies of cold-water lobsters. They have sac-like bodies, and three distinctly different forms in different parts of their two-stage life-cycle...
- Rotifer
Characteristics
The Platyhelminthes and Gastrotricha are acoelomateBody cavity
By the broadest definition, a body cavity is any fluid-filled space in a multicellular organism. However, the term usually refers to the space located between an animal’s outer covering and the outer lining of the gut cavity, where internal organs develop...
. The other phyla have a pseudocoel, and share characteristics such as the structure of their jaws and pharynx
Pharynx
The human pharynx is the part of the throat situated immediately posterior to the mouth and nasal cavity, and anterior to the esophagus and larynx. The human pharynx is conventionally divided into three sections: the nasopharynx , the oropharynx , and the laryngopharynx...
, although these have been secondarily lost in the parasitic Acanthocephala. They form a monophyletic subgroup called the Gnathifera
Gnathifera
Gnathifera may refer to:*Gnathifera , a phylum within the superphylum Platyzoa *a genus of moths...
.
The name "Platyzoa" is used because most members are flat, though rotifer
Rotifer
The rotifers make up a phylum of microscopic and near-microscopic pseudocoelomate animals. They were first described by Rev. John Harris in 1696, and other forms were described by Anton van Leeuwenhoek in 1703...
s are not.
Classification
The Platyzoa are close relatives of the LophotrochozoaLophotrochozoa
The Lophotrochozoa are a major grouping of protostome animals. The taxon was discovered based on molecular data. Molecular evidence such as a result of studies of the evolution of small-subunit ribosomal RNA supports the monophyly of the phyla listed in the infobox shown at right.-Terminology:The...
, and are sometimes included in that group. Together the two make up the Spiralia
Spiralia
Spiralia is a grouping of animals."Lophotrochozoan" and "spiralian" are sometimes considered equivalent.It receives its name from the spiral cleavage found in most members....
.