Traditional, pre-cladistic systems placed the flatworms in a single phylum, the Platyhelminthes, subdivided into the classes Turbellaria, Monogenea, Trematoda, and Cestoda. Turbellarians are the largely free-living flatworms---those that don't parasitize other animals---while the other classes encompass the obligate parasites, most of which live in or on the bodies of vertebrates. Turbellaria is, in particular, considered an invalid class because it is not monophyletic. (It is either paraphyletic--that is, having descendants, namely the parasitic classes, that are not classified within it [Ehlers, 1985]; or it could be polyphyletic--that is, having arisen from more than one ancestor, one for the acoels and another for catenulidans and rhabditophorans [Baguñà and Riutort, 2004; Phillipé et al., 2011]). The term "turbellarian" can still be applied to those worms that were formerly classified in the Turbellaria, but the term "Turbellaria" (specifically, the capitalized taxon name) would have to be written in quotation marks to indicate its paraphyletic status. Not all turbellarians are free-living. In recognition of recent proposals based on molecular sequences (particularly of 18s rDNA, some Hox genes, and, most recently, fuller genomic data) the system displayed here treats the Acoelomorpha as separate from the phylum Platyhelminthes. The position and status of Acoelomorpha have been volatile in molecular systematics, some claiming that it itself is paraphyletic, many claiming that it (possibly as separate taxa Acoela and Nemertodermatida) belongs near the base of the animal tree of life, as the most basal bilaterians---that is, the most primitive of any animal phylum outside of the sponges and cnidarians. The most recent genomic study (Phillipé et al., 2011), placed the Acoelomorpha within the Deuterostomia, in a phylum Xenacoelomorpha that includes also the enigmatic sack-like animal Xenoturbella); Xenacoelomorpha is placed as sister group to the Ambulacraria (Ehinoderms + Hemichordata). Significantly, the earlier proposals that Acoelomorpha was the most primitive bilaterian---a controversial claim that did not make sense in terms of the morphology of acoels---are now seen to be a result of a statistical error known as long-branch attraction. The Platyhelminthes, as defined in these molecular systems, would be only distantly related to the Acoelomorpha and classified as a lophotrochozoan phylum. This taxonomic database covers all turbellarian flatworms, including parasitic turbellarians. The position of the major parasitic taxa (the other classes in the traditional system), encompassed now in the taxon Neodermata, is shown as proposed by cladistic systems based on morphological characters (see Baguñà and Riutort, 2004, for summary of other proposed positions of the Neodermata). ... [Information of the supplier]
