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Bedford's Flatworm.jpg
Pseudobiceros bedfordi (Bedford's Flatworm)
Scientific classification
Kingdom: Animalia
Subkingdom: Eumetazoa
(unranked): Bilateria
(unranked): Protostomia
Superphylum: Platyzoa
Cavalier-Smith, 1998

The Platyzoa /ˌplætɨˈz.ə/ are a group of protostome animals proposed by Thomas Cavalier-Smith in 1998. Cavalier-Smith included in Platyzoa the Phylum Platyhelminthes or flatworms, and a new phylum, Acanthognatha, into which he gathered several previously described phyla of microscopic animals. Subsequent studies have supported Platyzoa as a clade,[1] a monophyletic group of organisms with a common ancestor, while differing on the phyla included and on relationships within Platyzoa.


One current scheme places the following traditional phyla in Platyzoa:


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 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 rotifers are not.[2]


The Platyzoa are close relatives of the Lophotrochozoa, and are sometimes included in that group. Together the two make up the Spiralia.


  1. ^ Passamaneck Y, Halanych KM (July 2006). "Lophotrochozoan phylogeny assessed with LSU and SSU data: evidence of lophophorate polyphyly". Mol. Phylogenet. Evol. 40 (1): 20–8. doi:10.1016/j.ympev.2006.02.001. PMID 16556507. 
  2. ^ "Explanations.html". Retrieved 2009-06-28.