The Platyzoa are a group of protostome animals. They include the Platyhelminthes or flatworms, together with several exclusively microscopic phyla:
The Cycliophora may also belong here. The Platyhelminthes and Gastrotricha are acoelomate. The other phyla have pseudocoeloms, 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 Platyzoa are close relatives of the Lophotrochozoa, and are sometimes included in that group. Together the two make up the Spiralia.