The Acoelomorpha are a disputed phylum of marine, soft-bodied animals with planula-like features. Most species are free-living, some live on the surface of other organisms (ectocommensals) (Mwinyi et al. 2010). Traditionally, they were considered to belong to the phylum Platyhelminthes. In 2004 molecular studies demonstrated that they are a separate phylum (Baguñà & Riutort 2004), 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 (Maxmen 2011, Philippe at al. 2011). However, some consider the evidence for a position within deuterostomes weak and favor the placement of Xenoturbella + Acoelomorpha more basally among Metazoa (Edgecombe 2011).

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

An ongoing (Feb. 2011) collaborative research project has "the researchers ... confident that they can reach an agreement about where acoels fit in evolutionary history" (Maxmen 2011). Acoels are almost entirely marine, living between grains of sediment, swimming as plankton, or crawling on algae. Acoels have a statocyst, which presumably helps them orient to gravity. Their soft bodies make them difficult to classify (Petrov et al. 2006).