Acoelomorpha

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, though some live on the surface of other organisms (ectocommensals). 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 (2011) results suggest that they (along with Xenoturbella) 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 + Acoelomorpha more basally among Metazoa.

Earlier (2007) work dismissed the phylum as paraphyletic, with Acoela and Nemertodermatida as separate clades.