Platyzoa
Superphylum of unsegmented animals
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Through Animal and Parasitism
Phyla
One scheme placed the following phyla in Platyzoa:
- Rouphozoa
- Gnathifera
- Syndermata
- Rotifera
- Seisonida
- Acanthocephala
- Gnathostomulida
- Micrognathozoa
- Cycliophora
- Syndermata
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Characteristics
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.
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.
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). The Lophotrochozoa and Rouphozoa are then named the Platytrochozoa. This makes the Platyzoa a paraphyletic group.