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).
Acoelomorphs resemble flatworms in many respects, but have a simpler anatomy, even beyond the absence of a gut. Like flatworms, they have no circulatory or respiratory systems, but they also lack an excretory system. They have no true brain organglia, simply a network of nerves beneath the epidermis, although the nerves are slightly more concentrated towards the forward end of the animal. The sensory organs include a statocyst and, in some cases, very primitive pigment-spot ocellicapable of detecting light.
They are simultaneous hermaphrodites, but have no gonads, and no ducts associated with the female reproductive system. Instead, gametes are produced from themesenchymal cells that fill the body between the epidermis and the digestive vacuole.
The Acoelomorpha are a disputed phylum of animals with planula-like features that were considered to belong to the phylumPlatyhelminthes. 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.