Phylogenetic analysis of the Rhabdocoela (Platyhelminthes) with emphasis on the Neodermata and relatives

Phylogenetic systematic analysis of 24 taxa representing the rhabdocoel pla tyhelminths, based on a suite of 89 morphological characters, produced two equally parsimonious trees, 181 steps long, with a consistency index (CI) o f 0.69 and a rescaled consistency index (RCI) of 0.56, differing only with respect to that portion of the tree containing Umagillidae, Acholadidae, Gr affillinae, Pseudograffillinae, Pterastericolidae and Hypoblepharinidae. Ou r results accommodate all previously proposed sister taxa to the Neodermata in a single clade in which ((Dalyelliidae + Temnocephalida) Typhloplanidae ) is the sister group of ((Fecampiidae + Urastoma) (Udonella ((Aspidogastre a + Digenea) (Monogenea (Gyrocotylidea (Amphilinidea + Eucestoda)))))). Boo tstrap and jackknife analyses indicate that the groupings of ((Dalyelliidae + Temnocephalida) Typhloplanidae) and of ((Fecampiidae + Urastoma) (Udonel la ((Aspidogastrea + Digenea) (Monogenea (Gyrocotylidea (Amphilinidea + Euc estoda)))))) are highly robust, with the latter clade having a CI of 90% an d RCI of 82%. Disagreements among previous analyses of these taxa have been due to the influence of missing data for critical characters in key taxa a nd differences in the taxa analysed, rather than any inherent weakness in t he morphological data. Non-phylogenetic systematic approaches to homology a ssessment and misconceptions regarding phylogenetic systematic methodology are discussed. Recent analyses combining sequence data with a subset of app roximately 60% of the morphological characters should be re-assessed using the entire morphological database. Even if Udonella is a monogenean, it is most parsimonious to suggest that the common ancestor of the Neodermata had a vertebrate-arthropod two-host life cycle.