Plathelminthes and Plathelminthomorpha - paraphyletic taxa

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Abstract
The concept of monophyly of Plathelminthes and Plathelminthomorpha (Plathelminthes and Gnathostomulida) is critically re-analysed. Based on new morphological and molecular evidence, it is concluded that both taxa are paraphyletic assemblages. The Acoelomorpha (Xenoturbellida?, Nemertodermatida and Acoela) are considered as the earliest offshoot of the bilaterian stem line, primarily lacking paired cerebral ganglia, an orthogonal nervous system and ultrafiltration nephridia. They are followed by Plathelminthes sensu stricto (i.e. the rhabditophoran turbellarians and Neodermata), Catenulida, and Gnathostomulida, the latter group is probably the sister group of the Syndermata (Rotifera and Acanthocephala). The respective characters, as well as the implication for the proposed stem species of the Bilateria are outlined and discussed.