The Acoelomorpha are a disputed phylum of animals with planula-like features that 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.

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

An ongoing (Feb. 2011) collaborative research project has "the researchers ... confident that they can reach an agreement about where acoels fit in evolutionary history.

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.

The Acoela are very small flattened worms, usually under 2 millimetres (0.079 in) in length (Symsagittifera roscoffensis about 15 mm), that do not have a gut. Digestion is accomplished by means of a syncytium that forms a vacuole around ingested food. There are no epithelial cells lining the digestive vacuole, although there is sometimes a short pharynx leading from the mouth to the vacuole. All other bilateral animals (apart from tapeworms) have a gut lined with epithelial cells. As a result, the acoels appear to be solid-bodied (a-coel, or no body cavity).