|Waminoa sp. on Plerogyra sp..|
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.
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), and do not have a conventional 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 lack body cavities (acoelomate structure), a hindgut or an anus. Like flatworms they possess neoblasts and biflagellate sperm. They have no true brain or ganglia, simply a network of nerves beneath the ciliated 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 ocelli capable 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 the mesenchymal cells that fill the body between the epidermis and the digestive vacuole.
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