Traditional, pre-cladistic systems placed the flatworms in a single phylum, the Platyhelminthes, subdivided into the classes Turbellaria, Monogenea, Trematoda, and Cestoda (see Hyman, 1951). Turbellarians are the largely free-living flatworms---small soft-bodied worms with a ciliated epidermis by which they swim and creep over substrates and with a sac-like gut whose only opening is the mouth. In contrast, the Monogenea, Trematoda, and Cestoda have a specialized, non-ciliated, syncytial epidermis and are obligate parasites, mostly in or on the bodies of vertebrates. As a taxonomic group, the Turbellaria is, in particular, considered an invalid class because it is not monophyletic. (It is both paraphyletic---that is, having descendants, namely the parasitic classes, that are not classified within it [Ehlers, 1985] and 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, so an alternative name sometimes applied to turbellarians, "free-living Platyhelminthes," is not quite appropriate.
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. Two genomic studies (Bourlat et al., 2006; Phillipé et al., 2011), placed the Acoelomorpha within the Deuterostomia, in a phylum Xenacoelomorpha that includes also the enigmatic sack-like animal Xenoturbella); under this concept, Xenacoelomorpha is placed as sister group to the Ambulacraria (Echinodermata + Hemichordata). Another genomic analysis places the Xenacoelomorpha outside both Protostomia and Deuterostomia, again as the most basal of any bilaterian groups (Cannon et al., 2016). In any case the Platyhelminthes, as defined in these molecular systems, would be only distantly related to the Acoelomorpha.
This taxonomic database covers all turbellarian flatworms, freshwater as well as marine ones, and including parasitic turbellarians. The position of the major parasitic taxa of the Platyhelminthes (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). This means that it is within the turbellarian taxon Eulecithophora, sister group to (or at least within an uncertain polytomy with) the Rhabdocoela and Adiaphanida.
|Louise Bush sorting turbellarians|
|Steve Schilling collecting geographic data|
Data in this listing were initially compiled by Louise Bush. Dr. Bush's last update to the database was in 1991, the year of her death. Since 1991, the database has been updated by S. Tyler, Steve Schilling, Matt Hooge, and Tom Artois, with help from specialists on various groups of turbellarian platyhelminths, including Masaharu Kawakatsu, Ulrich Ehlers, Marco Curini-Galletti, Oleg Timoshkin, Lukas Schärer, Wim Willems, Johannes Achatz, Anatoly Petrov, Julian P. S. Smith III, Eduard Solà, Francisco Brusa, Cristina Damborenea, Miquel Vila Farre.
Images are mostly extracted from scientific papers describing species. Copyrights for images remain with the original print publishers. Copies of many of the images of Rhabdocoela and Proseriata were kindly provided by Tom Artois, Ernest Schockaert, Matt Hooge, and Rick Hochberg. Images of Acoela, Prolecithophora, and other Rhabdocoela were provided by Matt Hooge.
Many of the notes on these turbellarian taxa and references dealing with them are from index cards that Louise Bush maintained, displayed here either as images of those cards themselves or, in the case of Acoelomorpha species, transcribed into text entries. Steve Schilling scanned the Bush's cards and transcribed notes from many of them for this database.
Steve Schilling entered almost all of the geographic-distribution data. He combed the literature for any mention of collections of turbellarians and scrupulously compared geographic names mentioned with names appearing in gazeteers. Some of the early phases of this work on geographic distributions of species was supported by the Ocean Biodiversity Information System (OBIS).
This material is based upon work supported by the National Science Foundation under Grant No. 0118804.