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.[3]
Classification
The Platyzoa are close relatives of the Lophotrochozoa. Together the two make up the Spiralia.
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).[1] The Lophotrochozoa and Rouphozoa are then named the Platytrochozoa.[1] This makes the Platyzoa a paraphyletic group.[clarification needed]
^Sørensen, Martin (14 December 2005). "Limnognathia". Zoological Museum, University of Copenhagen. Archived from the original on 24 December 2018. Retrieved 28 December 2016.
