Waminoa sp. on Plerogyra sp..
Scientific classification
Kingdom: Animalia
Phylum: Xenacoelomorpha
Subphylum: Acoelomorpha
Ehlers, 1985

The Acoelomorpha are a disputed

  • Phylogeny of Lower Worms of the Meiofauna (Acoelomorpha)
  • Acoelomorpha at the Turbellarian taxonomic database

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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.[11]

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 statocyst and, in some cases, very primitive pigment-spot ocelli capable of detecting light.[11]

). no body cavity, or a-coel) have a gut lined with epithelial cells. As a result, the acoels appear to be solid-bodied (tapeworms leading from the mouth to the vacuole. All other bilateral animals (apart from pharynx cells lining the digestive vacuole, although there is sometimes a short epithelial around ingested food. There are no vacuole that forms a syncytium Digestion is accomplished by means of a [10].gut about 15 mm), and do not have a conventional Symsagittifera roscoffensisThe Acoela are very small flattened worms, usually under 2 millimetres (0.079 in) in length (


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.[9]

An ongoing (Feb. 2011) collaborative research project has "the researchers … confident that they can reach an agreement about where acoels fit in evolutionary history".[4]

Earlier (2007) work dismissed the phylum as paraphyletic, with Acoela and Nemertodermatida as separate clades.[8]

[7][6].Metazoa + Acoelomorpha more basally among Xenoturbella However, some consider the evidence for a position within deuterostomes weak and favor the placement of [5][4].deuterostomes) may lie near the base of the Xenoturbella, slightly more derived than the cnidaria. Recent (2011) results suggest that they (along with Bilateria among the basal although their position in the tree of life is contentious; most researchers believe them to be [3]. In 2004 molecular studies demonstrated that they are a separate phylum,Platyhelminthes Traditionally, they were considered to belong to the phylum [2]