Protostomia `--o Lophotrochozoa [or Spiralia] `--+--Bryozoa [Ectoprocta] |--Platyzoa `--o Trochozoa |==Coeloscleritophora† ("Procoelomata†") |--Nemertea [Nemertinea] |--o Eutrochozoa | |?-Tullimonsterida † | |--Mollusca | |--Hyolitha † | |--Sipuncula | `--Annelida `--o Brachiozoa |--Phoronida `--Brachiopoda
The Platyzoa ("flat animals") are a group of protostomia proposed by Thomas Cavalier-Smith in 1998. Cavalier-Smith included here the Platyhelminthes (flatworms), and a new phylum, Acanthognatha, into which he gathered several previously described phyla of microscopic animals. Subsequent studies have supported Platyzoa as a viable clade, while differing on the phyla included and on relationships within Platyzoa.
Due to their lack of coelom (body cavity), the Platyhelminthes were originally and for a long time considered the most primitive Bilateria, but are now thought to be secondarily simple forms that developed from more complex ancestors.
The other platyzoan phyla are microscopic pseudocoelomates. They include the rotifers ("wheel bearers"), microscopic animals common in aqueous environments, which feed via wheel-like bands of cilia, the Acanthocephala or spiny-headed worms, the Gnathostomulida, Micrognathozoa, and possibly the Cycliophora. These groups share characteristics such as the complex structure of their jaws and pharynx, although these have been secondarily lost in the parasitic Acanthocephala. Because of this, they are called the Gnathifera ("jaw bearers"), and constitute a subclade of the Platyzoa.
The Platyzoa are either considered close relatives of the Lophotrochozoa, or more commonly now actually included in that group.
Lophotrochozoa `--Platyzoa |--+--Platyhelminthes | `--Gastrotricha |--+--Cycliophora | `--Entoprocta `--Gnathifera |--Gnathostomulida |--Micrognathozoa `--Syndermata |--Rotifera `--Acanthocephala