Acoela, or the acoels, is an order of small and simple

invertebrate Invertebrates are animals that neither develop nor retain a vertebral column (commonly known as a ''spine'' or ''backbone''), which evolved from the notochord. It is a paraphyletic grouping including all animals excluding the chordata, chordate s ...

s in the subphylum Acoelomorpha of phylum Xenacoelomorpha, a deep branching

bilaterian Bilateria () is a large clade of animals characterised by bilateral symmetry during embryonic development. This means their body plans are laid around a longitudinal axis with a front (or "head") and a rear (or "tail") end, as well as a left–r ...

group of animals, which resemble flatworms. Historically they were treated as an order of turbellarian flatworms. About 400 species are known, but probably many more not yet described. The

etymology Etymology ( ) is the study of the origin and evolution of words—including their constituent units of sound and meaning—across time. In the 21st century a subfield within linguistics, etymology has become a more rigorously scientific study. ...

of "acoel" is from the

Ancient Greek Ancient Greek (, ; ) includes the forms of the Greek language used in ancient Greece and the classical antiquity, ancient world from around 1500 BC to 300 BC. It is often roughly divided into the following periods: Mycenaean Greek (), Greek ...

words (), the '' alpha privative'', expressing negation or absence, and (), meaning "cavity". This refers to the fact that acoels have a structure lacking a fluid-filled body cavity.

Description

Acoels are very small flattened worms, usually under in length, but some larger species, such as '' Symsagittifera roscoffensis'', may reach up to . They are bilaterally symmetric and microscopic. They are found worldwide in marine and brackish waters, usually having a

benthic The benthic zone is the ecological region at the lowest level of a body of water such as an ocean, lake, or stream, including the sediment surface and some sub-surface layers. The name comes from the Ancient Greek word (), meaning "the depths". ...

lifestyle, although some species are epibionts. Two species, ''Limnoposthia polonica'' and ''Oligochoerus limnophilus'', live in freshwater. Species in the family Convolutidae often form endosymbiotic relationship with microalgae. In one of the genera, '' Waminoa'', the algae are transmitted vertically from parents to offspring. In addition to Convolutidae, there appears to be a potential new and yet unnamed family of acoels that also live in relationships with microalgal

endosymbiont An endosymbiont or endobiont is an organism that lives within the body or cells of another organism. Typically the two organisms are in a mutualism (biology), mutualistic relationship. Examples are nitrogen-fixing bacteria (called rhizobia), whi ...

s. Members of the class Acoela lack a conventional gut, so that the mouth opens directly into the mesenchyme, i.e., the layer of tissue that fills the body. Digestion is accomplished by means of a syncytium that forms a

vacuole A vacuole () is a membrane-bound organelle which is present in Plant cell, plant and Fungus, fungal Cell (biology), cells and some protist, animal, and bacterial cells. Vacuoles are essentially enclosed compartments which are filled with water ...

around ingested food. There are no epithelial cells lining the digestive vacuole, but in the families Diopisthoporidae, Hallangiidae, Hofsteniidae, and Solenofilomorphidae, and the genera Oligochoerus (Convolutidae) and Proporus (Proporidae), there is a short

pharynx The pharynx (: pharynges) is the part of the throat behind the human mouth, mouth and nasal cavity, and above the esophagus and trachea (the tubes going down to the stomach and the lungs respectively). It is found in vertebrates and invertebrates ...

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. As the basal lineage of bilateral animals, the Acoela provide interesting insights into early animal evolution and development. The most thoroughly studied animal in this group is the species '' Isodiametra pulchra''. Acoela used to be classified in the phylum Platyhelminthes. However, Acoela was separated from this phylum after molecular analyses showed that it had diverged before the three main

bilateria Bilateria () is a large clade of animals characterised by bilateral symmetry during embryonic development. This means their body plans are laid around a longitudinal axis with a front (or "head") and a rear (or "tail") end, as well as a left� ...

n clades had formed, making flatworms as traditionally understood an evolutionary grade from which higher animals had evolved.

Taxonomy

The following sub-taxa are recognised in the order Acoela: * Family Actinoposthiidae Hooge, 2001 * Family Antigonariidae Dörjes, 1968 * Family Antroposthiidae Faubel, 1976 * Family Diopisthoporidae Westblad, 1940 * Family Nadinidae Dörjes, 1968 * Family Paratomellidae Dörjes, 1966 * Family Taurididae Kostenko, 1989 * Suborder Bursalia Jondelius ''et al''., 2011 ** Infraorder Crucimusculata Jondelius ''et al''., 2011 *** Family Dakuidae Hooge, 2003 *** Family Isodiametridae Hooge & Tyler, 2005 *** Family Otocelididae Westblad, 1948 *** Family Proporidae Graff, 1882 *** Superfamily Aberrantospermata Jondelius ''et al''., 2011 **** Family Convolutidae Graff, 1905 **** Family Mecynostomidae Dörjes, 1968 ** Infraorder Prosopharyngida Jondelius ''et al''., 2011 *** Family Hallangiidae Westblad, 1946 *** Family Hofsteniidae Bock, 1923 *** Family Solenofilomorphidae Dörjes, 1968

References

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