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 app ...

, 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

flatworm 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, unsegment ...

s. 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 more ...

s. 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 peri ...

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 negation or ...

'', 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 ancient Greek, βένθος (bénthos), meaning "t ...

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 epi ...

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 Mesenchyme () is a type of loosely organized animal embryonic connective tissue of undifferentiated cells that give rise to most tissues, such as skin, blood or bone. The interactions between mesenchyme and epithelium help to form nearly every o ...

, 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 nucleus) ...

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 mo ...

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 intercellula ...

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 struc ...

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 Cestodar ...

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 language, Greek πλατύ, ''platy'', meaning "flat" and ἕλμινς (root: ἑλμινθ-), ''helminth-'', meaning "worm") are a Phylum (biology), phylum of relati ...

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

bilateria 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 and ...

n 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 ''Antigonaria'' 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. Histori ...

Dörjes, 1968 * Family

Antroposthiidae Antroposthiidae is a family of acoels, containing three genera and four species: Taxonomy Genera There are three genera in the family Antroposthiidae. *''Antroposthia'' Faubel, 1974 *''Convoluella ''Convoluella'' is a genus of worms belonging ...

Faubel, 1976 * Family

Diopisthoporidae ''Diopisthoporus'' 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. Hist ...

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 Dakuidae 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. Historically ...

Hooge, 2003 *** Family

Isodiametridae Isodiametridae 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 ...

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 Mecynostomidae is a family of acoels. * ''Eumecynostomum altitudi'' Faubel & Regier, 1983 * ''Eumecynostomum asterium'' Hooge & Tyler, 2003 * ''Eumecynostomum bathycolum'' (Westblad, 1948) * ''Eumecynostomum boreale'' (Faubel, 1977) * ''Eumecyn ...

Dörjes, 1968 ** Infraorder Prosopharyngida Jondelius ''et al''., 2011 *** Family

Hallangiidae Hallangiidae is a family of Acoela. It contains 2 species in 2 genera. Genera There are two genera in the family ''Hallangiidae''. *''Aechmalotus'' Beklemischev, 1915 *''Hallangia'' Westblad, 1946 Species There are two species in the family '' ...

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 Solenofilomorphidae 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. Genera There are 5 genera in the family So ...

Dörjes, 1968

References

{{Taxonbar, from=Q2052913 Acoelomorphs