Because of their simple organization the Acoela have been considered to be either primitive bilaterians or descendants of coelomates through secondary loss of derived features. Sequence data of 18 S ribosomal DNA genes from non–fast evolving species of acoels and other metazoans reveal that this group does not belong to the Platyhelminthes but represents the extant members of the earliest divergent Bilateria, an interpretation that is supported by recent studies on the embryonic cleavage pattern and nervous system of acoels. This study has implications for understanding the evolution of major body plans, and for perceptions of the Cambrian evolutionary explosion.

Phylogenetic interrelationships of 32 species belonging to 18 genera and four families of the superfamily Microphalloidea were studied using partial sequences of nuclear lsrDNA analysed by Bayesian inference and maximum parsimony. The resulting trees were well resolved at most nodes and demonstrated that the Microphalloidea, as represented by the present data-set, consists of three main clades corresponding to the families Lecithodendriidae, Microphallidae and Pleurogenidae + Prosthogonimidae. Interrelationships of taxa within each clade are considered; as a result of analysis of molecular and morphological data, Floridatrema Kinsella & Deblock, 1994 is synonymised with Maritrema Nicoll, 1907, Candidotrema Dollfus, 1951 with Pleurogenes Looss, 1896, and Schistogonimus Lühe, 1909 with Prosthogonimus Lühe, 1899. The taxonomic value of some morphological features, used traditionally for the differentiation of genera within the Lecithodendriidae and Prosthogonimidae, is reconsidered. Previous systematic schemes are discussed from the viewpoint of present results, and perspectives of future studies are outlined.

We combined nearly complete sequences of large (LSU) and small (SSU) subunit rDNA from 32 flatworm species to estimate the phylogeny of the Platyhelminthes using maximum parsimony, maximum likelihood and Bayesian inference methods. Rooted against the Catenulida, combined evidence trees offered no support for the Revertospermata, which was also rejected by constraint analysis. Generally, nodal support was higher for groupings estimated from the combined data partitions and all methods of analysis provided congruent estimates of phylogeny. The Monogenea and Proseriata were resolved as monophyletic, rejecting previous suggestions of paraphyly based on SSU and partial LSU data sets and thus supporting widely accepted morphological synapomorphies. Monophyly of the Neodermata was supported and its sister group was a clade of neoophoran ‘turbellarians’ to the exclusion of the Proseriata which in turn was more basal. Taxa with similar spermatology to the Neodermata (Ichthyophaga, Notentera, Urastoma and Kronborgia) were the sister group to Tricladida + Prolecithophora, which in turn were sister to the Rhabdocoela. Polycladida + Macrostomida + Lecithoepitheliata was the earliest divergent offshoot of the Rhabditophora. Among the Neodermata, the Cercomeromorphae (Cestoda + Monogenea) was not supported, whereas Cestoda + Trematoda was well supported. Although there is no known synapomorphy for this latter grouping, our data highlight problems associated with the ‘cercomer theory’ and we reject putative homologies regarding neodermatan ‘cercomers’ that have been sustained in the literature without careful scrutiny. © 2003 The Linnean Society of London, Biological Journal of the Linnean Society, 2003, 78, 155–171.

We used a data matrix of 65 morphological characters from 25 ingroup and 6 outgroup taxa, and an alignmcnt comprising complete 18s rDNA scqucnccs from 82 species of parasitic and free-living Platylielminthcs and from 19 species of loivcr invcrtchratcs to analyse phylogenctic relationships ofvarious platyhclminth taxa. Of the 1358 unambiguously aligiiablc molecular positions, 995 wcrc variable and 757 \vcrc phylogcnetically informative (parsimony criterion); complete 18s rDNA sequences ranged in length from 1755 to 2873 lip. Main conclusions arc: Ncodermata are monophyletic, and the Trematoda, hlonogcnca and Cestoda within thcm arc monophylctic as well. The sister group of thc Ncodcrmata is all the otlicr Ncoophora; the Kalyptorhynchia, 'I'yphloplanida, Dalyclliida and Tcmiiocephalida form one clade, and the last three another. Rlonophyly of the Seriata is rejected, but Polycladida/ Rfacrostomida/Haplopharyngida arc monophylctic, as arc the last two taxa. As a consequence, validity of the taxon Trepaxonemata is rejected. Further studies must show the corrcct position of the Acoela and Ncmcrtodermatida. It is strcsscd that morphological and molecular data in some respects lead to contradictory results, for instance concerning the position of the Fccampiidac/ 1 ~r a . r t o n i n / Z~i i t /~~o~h~~~ and the rclativc position of the Lccithoepitlicliata. Denser sampling of taxa for molecular data, complcmcntary scqucnccs from independent gcncs, and inclusion of additional morphological data arc necessary to resolve thcsc contradictions. 0 1:)w 'l'l1r l . i l l 1 l~~~~l Socict) 01 1.011d011