Bedford's flatworm, Pseudobiceros bedfordi
The Platyzoa include the phylum Platyhelminthes, the flatworms. These were originally considered some of the most primitive Bilateria, but it now appears they developed from more complex ancestors.[14] A number of parasites are included in this group, such as the flukes and tapeworms. Flatworms are mates, lacking a body cavity, as are their closest relatives, the microscopic Gastrotricha.[15]
The other platyzoan phyla are mostly microscopic and pseudocoelomate. The most prominentare the Rotifera or rotifers, which are common in aqueous environments. They also include the Acanthocephala or spiny-headed worms, the Gnathostomulida, Micrognathozoa, and possibly the Cycliophora.
[16] These groups share the presence of complex jaws, from which they are called the Gnathifera.
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