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.