||Small ciliated worms known as acoels and nemertodermatids comprise the Acoelomorpha. Their position among other bilaterian animals is not clear, but a recent genomic study (Phillipe et al., 2011) placed them 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 (Echinodermata + Hemichordata). Earlier proposals that Acoelomorpha was the most primitive bilaterian group are likely to be a result of a statistical error known as long-branch attraction. Relationships of species within the Acoelomorpha are no less certain. Their relatively simple morphology offers few characters to determine relationships and still too few of them have been examined for significant molecular-sequence data. Especially the smaller and probably more primitive species living in sediments are little studied; more is known of the more derived and larger acoels such as the convolutids. The current framework from molecular-sequence data, reflected in the hierarchy shown in this database, comes from Jondelius et al. (2011)
||Tyler S., Artois T. . (2017). WoRMS Acoelomorpha: World list of Acoelomorpha (version 2017-03-01). In: Roskov Y., Abucay L., Orrell T., Nicolson D., Bailly N., Kirk P.M., Bourgoin T., DeWalt R.E., Decock W., De Wever A., Nieukerken E. van, Zarucchi J., Penev L., eds. (2017). Species 2000 & ITIS Catalogue of Life, 22nd March 2017. Digital resource at www.catalogueoflife.org/col. Species 2000: Naturalis, Leiden, the Netherlands. ISSN 2405-8858.