The paraphyletic "Platyzoa" are a group of

protostome Protostomia () is the clade of animals once thought to be characterized by the formation of the organism's mouth before its anus during embryogenesis, embryonic development. This nature has since been discovered to be extremely variable among Pro ...

unsegmented animals proposed by

Thomas Cavalier-Smith Thomas (Tom) Cavalier-Smith, FRS, FRSC, NERC Professorial Fellow (21 October 1942 – 19 March 2021), was a professor of evolutionary biology in the Department of Zoology, at the University of Oxford. His research has led to discov ...

in 1998. Cavalier-Smith included in Platyzoa the phylum

Platyhelminthes The flatworms, flat worms, Platyhelminthes, or platyhelminths (from the Greek πλατύ, ''platy'', meaning "flat" and ἕλμινς (root: ἑλμινθ-), ''helminth-'', meaning "worm") are a phylum of relatively simple bilaterian, unsegme ...

(or flatworms), and a new phylum, the Acanthognatha, into which he gathered several previously described phyla of microscopic animals. Later it has been described as paraphyletic, containing the Rouphozoa and the Gnathifera.

Phyla

One scheme placed the following phyla in Platyzoa: * Rouphozoa **

Gastrotrich The gastrotrichs (phylum Gastrotricha), commonly referred to as hairybellies or hairybacks, are a group of microscopic (0.06-3.0 mm), worm-like, acoelomate animals, and are widely distributed and abundant in freshwater and marine environmen ...

a * Gnathifera **

Syndermata Syndermata is a clade of animals that, in some systems, is considered synonymous with Rotifera. Older systems separate Rotifera and Acanthocephala Acanthocephala (Greek , ', thorn + , ', head) is a phylum of parasitic worms known as acant ...

*** Rotifera *** Seisonida **

Acanthocephala Acanthocephala (Greek , ', thorn + , ', head) is a phylum of parasitic worms known as acanthocephalans, thorny-headed worms, or spiny-headed worms, characterized by the presence of an eversible proboscis, armed with spines, which it uses to p ...

Gnathostomulid Gnathostomulids, 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 **

Micrognathozoa ''Limnognathia maerski'' is a microscopic freshwater animal, discovered living in warm springs on Disko Island, Greenland, in 1994. Since then, it was also found in Crozet Islands of Antarctica.). With an average length of 100 micrometers ...

** Cycliophora

Characteristics

None of the Platyzoa groups have a respiration or circulation system because of their small size, flat body or parasitic lifestyle. The Platyhelminthes and Gastrotricha are

acoelomate The coelom (or celom) is the main body cavity in most animals and is positioned inside the body to surround and contain the digestive tract and other organs. In some animals, it is lined with mesothelium. In other animals, such as molluscs, it ...

. The other phyla have a pseudocoel, and share characteristics such as the structure of their jaws and

pharynx The pharynx (plural: pharynges) is the part of the throat behind the mouth and nasal cavity, and above the oesophagus and trachea (the tubes going down to the stomach and the lungs). It is found in vertebrates and invertebrates, though its st ...

, although these have been secondarily lost in the

parasitic Parasitism is a close relationship between species, where one organism, the parasite, lives on or inside another organism, the host, causing it some harm, and is adapted structurally to this way of life. The entomologist E. O. Wilson ha ...

Acanthocephala. They form a monophyletic subgroup called the Gnathifera. The name "Platyzoa" is used because most members are flat, though rotifers are not.

Classification

The Platyzoa are close relatives of the

Lophotrochozoa Lophotrochozoa (, "crest/wheel animals") is a clade of protostome animals within the Spiralia. The taxon was established as a monophyletic group based on molecular evidence. The clade includes animals like annelids, molluscs, bryozoans, brachi ...

. Together the two make up the Spiralia. Syndermata was a proposed clade that included Acanthocephala and rotifers, but as it appears they are not sister groups after all, the clade has been abandoned.Gnathifera - Richard C. Brusca
/ref> A recent possible cladogram is shown which would show that the Lophotrochozoa emerged within Platyzoa as a sister group of the Rouphozoa (the Gastrotricha and Platyhelminthes). The Lophotrochozoa and Rouphozoa are then named the Platytrochozoa. This makes the Platyzoa a paraphyletic group.

References

The Taxonomicon - Taxon: Infrakingdom Platyzoa Cavalier-Smith, 1998
- retrieved January 31, 2006
Triploblastic Relationships with Emphasis on the Acoelomates and the Position of Gnathostomulida, Cycliophora, Plathelminthes, and Chaetognatha: A Combined Approach of 18S rDNA Sequences and Morphology
- retrieved January 31, 2006
Myzostomida Are Not Annelids: Molecular and Morphological Support for a Clade of Animals with Anterior Sperm Flagella
- retrieved January 31, 2006
Current_advances_in_the_phylogenetic_reconstruction_of_metazoan
_evolution._A_new_paradigm_for_the_Cambrian_explosion?.html" ;"title="metazoan">Current advances in the phylogenetic reconstruction of metazoan">Current_advances_in_the_phylogenetic_reconstruction_of_metazoan
_evolution._A_new_paradigm_for_the_Cambrian_explosion?-_retrieved_January_31,_2006 {{Taxonbar.html" ;"title="metazoan
evolution. A new paradigm for the Cambrian explosion?">metazoan">Current advances in the phylogenetic reconstruction of from=Q46851 Platyzoa.html" ;"title="metazoan
evolution. A new paradigm for the Cambrian explosion?- retrieved January 31, 2006 {{Taxonbar">from=Q46851 Platyzoa"> Superphyla