Platyzoa
These were originally considered some of the most primitive Bilateria, but it now appears they developed from more complex ancestors.[74] A number of parasites are included in this group, such as the flukes andtapeworms.[73] Flatworms are acoelomates, lacking a body cavity, as are their closest relatives, the microscopic Gastrotricha.[75]
The other platyzoan phyla are mostly microscopic and pseudocoelomate. The most prominent are the Rotifera or rotifers, which are common in aqueous environments. They also include theAcanthocephala or spiny-headed worms, the Gnathostomulida, Micrognathozoa, and possibly theCycliophora.[76] These groups share the presence of complex jaws, from which they are called theGnathifera.
The other platyzoan phyla are mostly microscopic and pseudocoelomate. The most prominent are the Rotifera or rotifers, which are common in aqueous environments. They also include theAcanthocephala or spiny-headed worms, the Gnathostomulida, Micrognathozoa, and possibly theCycliophora.[76] These groups share the presence of complex jaws, from which they are called theGnathifera.