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

invertebrate Invertebrates are a paraphyletic group of animals that neither possess nor develop a vertebral column (commonly known as a ''backbone'' or ''spine''), derived from the notochord. This is a grouping including all animals apart from the chordate ...

s in the subphylum

Acoelomorpha Acoelomorpha is a subphylum of very simple and small soft-bodied animals with planula-like features which live in marine or brackish waters. They usually live between grains of sediment, swimming as plankton, or crawling on other organisms, suc ...

of phylum

Xenacoelomorpha Xenacoelomorpha is a small phylum of bilaterian invertebrate animals, consisting of two sister groups: xenoturbellids and acoelomorphs. This new phylum was named in February 2011 and suggested based on morphological synapomorphies (physical ...

, a deep branching

bilaterian The Bilateria or bilaterians are animals with bilateral symmetry as an embryo, i.e. having a left and a right side that are mirror images of each other. This also means they have a head and a tail (anterior-posterior axis) as well as a belly an ...

group of animals, which resemble flatworms. Historically they were treated as an order of

turbellaria The Turbellaria are one of the traditional sub-divisions of the phylum Platyhelminthes (flatworms), and include all the sub-groups that are not exclusively parasitic. There are about 4,500 species, which range from to large freshwater forms mor ...

n flatworms. The

etymology Etymology ()The New Oxford Dictionary of English (1998) – p. 633 "Etymology /ˌɛtɪˈmɒlədʒi/ the study of the class in words and the way their meanings have changed throughout time". is the study of the history of the Phonological chan ...

of "acoel" is from the

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

words (), the ''

alpha privative An alpha privative or, rarely, privative a (from Latin ', from Ancient Greek ) is the prefix ''a-'' or ''an-'' (before vowels) that is used in Indo-European languages such as Sanskrit and Greek and in words borrowed therefrom to express negat ...

'', 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 lifestyle, although some species are

epibiont An epibiont (from the Ancient Greek meaning "living on top of") is an organism that lives on the surface of another living organism, called the basibiont ("living underneath"). The interaction between the two organisms is called epibiosis. An ep ...

s. Two species, ''Limonoposthia polonica'' and ''Oligochoerus limnophilus'', lives in freshwater. 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 A syncytium (; plural syncytia; from Greek: σύν ''syn'' "together" and κύτος ''kytos'' "box, i.e. cell") or symplasm is a multinucleate cell which can result from multiple cell fusions of uninuclear cells (i.e., cells with a single nucleu ...

that forms a

vacuole A vacuole () is a membrane-bound organelle which is present in plant and fungal cells and some protist, animal, and bacterial cells. Vacuoles are essentially enclosed compartments which are filled with water containing inorganic and organic m ...

around ingested food. There are no

epithelial Epithelium or epithelial tissue is one of the four basic types of animal tissue, along with connective tissue, muscle tissue and nervous tissue. It is a thin, continuous, protective layer of compactly packed cells with a little intercell ...

cells lining the digestive vacuole, but there is sometimes a short

pharynx The pharynx (plural: pharynges) is the part of the throat behind the mouth and nasal cavity, and above the oesophagus and trachea (the tubes going down to the stomach and the lungs). It is found in vertebrates and invertebrates, though its st ...

leading from the mouth to the vacuole. All other bilateral animals (apart from

tapeworm Eucestoda, commonly referred to as tapeworms, is the larger of the two subclasses of flatworms in the class Cestoda (the other subclass is Cestodaria). Larvae have six posterior hooks on the scolex (head), in contrast to the ten-hooked Cesto ...

s) 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 ''Isodiametra'' is a genus of worm Worms are many different distantly related bilateral animals that typically have a long cylindrical tube-like body, no limbs, and no eyes (though not always). Worms vary in size from microscopic to ove ...

''. Acoela used to be classified in the phylum

Platyhelminthes The flatworms, flat worms, Platyhelminthes, or platyhelminths (from the Greek πλατύ, ''platy'', meaning "flat" and ἕλμινς (root: ἑλμινθ-), ''helminth-'', meaning "worm") are a phylum of relatively simple bilaterian, unsegme ...

. However, Acoela was separated from this phylum after molecular analyses showed that it had diverged before the three main bilaterian clades had formed.

Taxonomy

The following sub-taxa are recognised in the order Acoela: * Family

Actinoposthiidae Actinoposthiidae is a family of acoels.Nilsson, K.S., Wallberg, A., & Jondelius, U. (2011). "New species of Acoela from the Mediterranean, the Red Sea, and the South Pacific." ''Zootaxa'' 2867: 1-31. Taxonomy Genera The following genera are rec ...

Hooge, 2001 * Family Antigonariidae Dörjes, 1968 * Family Antroposthiidae Faubel, 1976 * Family Diopisthoporidae Westblad, 1940 * Family

Nadinidae ''Nadina'' is a genus of acoel Acoela, or the acoels, is an order of small and simple invertebrates in the subphylum Acoelomorpha of phylum Xenacoelomorpha, a deep branching bilaterian group of animals, which resemble flatworms. Historically ...

Dörjes, 1968 * Family

Paratomellidae Paratomellidae is a family of acoel Acoela, or the acoels, is an order of small and simple invertebrates in the subphylum Acoelomorpha of phylum Xenacoelomorpha, a deep branching bilaterian group of animals, which resemble flatworms. Histori ...

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 Proporidae is a family of acoel Acoela, or the acoels, is an order of small and simple invertebrates in the subphylum Acoelomorpha of phylum Xenacoelomorpha, a deep branching bilaterian group of animals, which resemble flatworms. Historicall ...

Graff, 1882 *** Superfamily Aberrantospermata Jondelius ''et al''., 2011 **** Family

Convolutidae Convolutidae is a family of acoels. It contains more than a third of all known acoel species.Turbellaria ...

Graff, 1905 **** Family Mecynostomidae Dörjes, 1968 ** Infraorder Prosopharyngida Jondelius ''et al''., 2011 *** Family Hallangiidae Westblad, 1946 *** Family

Hofsteniidae Hofsteniidae is a family of acoels. This family contains seven species in three genera. This worm ranges in size from 100 μm for embryos to 500 μm for adults, and exhibits "whole-body regeneration" capability, where entire body parts regenera ...

Bock, 1923 *** Family Solenofilomorphidae Dörjes, 1968

References

{{Taxonbar, from=Q2052913 Acoelomorphs