The Acoelomorpha are a disputed phylum of marine soft-bodied animals with planula-like features. They have also been proposed as a superphylum including the Acoela, Nemertodermatida and Xenoturbella. Most species are free-living, some live on the surface of other organisms. Traditionally, they were considered to belong to the phylum Platyhelminthes. In 2004, molecular studies demonstrated that they are a separate phylum, although their position in the tree of life is contentious; most researchers believe them to be basal among the Bilateria, slightly more derived than the cnidaria. Recent results suggest that they may lie near the base of the deuterostomes. However, some consider the evidence for a position within deuterostomes weak and favor the placement of Xenoturbella and Acoelomorpha more basally among Metazoa.