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