Review of data for a morphological look on Xenacoelomorpha (Bilateria incertae sedis)

Recent investigations by means of high-tech morphology, evo-devo studies and molecular data suggest that the taxon Xenacoelomorpha (Nemertodermatida and Acoela plus Xenoturbella), formerly considered as primitive flatworms (Plathelminthes) or even bivalve Mollusca, represents either a quite plesiomorphic grouping as the earliest bilaterian offshoot or but is a substantially reduced and simplified sidebranch of ambulacralian Deuterostomia. Herein, I provide a compilation and review of the current morphological data and possible interpretations of the various character states. Phenotypic and genotypic data suggest monophyly of Xenacoelomorpha. There is no specific similarity between xenacoelmorphs and deuerostome larvae, and reduction appears improbable in free-living and predatory animals. Accordingly, Xenacoelomorpha are more likely similar to Urbilateria rather than degenerated and simplified coelomate deuterostomes. If so, the ground pattern of Bilateria has been retained only partially in the remaining main bilaterian clades (Nephrozoa) after the deviation of the Xenacoelomorpha, namely the nervous system in the Deuterostomia and the body cavity conditions in the acoelomate Lophotrochozoa (particularly Platyzoa), Gastrotricha and cycloneuralian Ecdysozoa.