The "Platyzoa" /ˌplætɪˈzoʊ.ə/ are a group of protostome unsegmented animals proposed by Thomas Cavalier-Smith in 1998.

One scheme placed the following 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 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.

Gnathifera Gastrotricha Platyhelminthes Mesozoa Lophotrochozoa 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.

[4] 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).