PLATHELMINTHES AND PLATHELMINTHOMORPHA - PARAPHYLETIC TAXA

The concept of monophyly of Plathelminthes and Plathelminthomorpha (Pl athelminthes and Gnathostomulida) is critically re-analysed. Based on new morphological and molecular evidence, it is concluded that both ta xa are paraphyletic assemblages. The Acoelomorpha (Xenoturbellida?, Ne mertodermatida and Acoela) are considered as the earliest offshoot of the bilaterian stem line, primarily lacking paired cerebral ganglia, a n orthogonal nervous system and ultrafiltration nephridia. They are fo llowed by Plathelminthes sensu stricto (i.e. the rhabditophoran turbel larians and Neodermata), Catenulida, and Gnathostomulida, the latter g roup is probably the sister group of the Syndermata (Rotifera and Acan thocephala). The respective characters, as well as the implication for the proposed stem species of the Bilateria are outlined and discussed .