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 chorda ...

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, ...

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

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, unsegmen ...

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 mo ...

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 form of words ...

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 pe ...

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 ''Symsagittifera roscoffensis'', also called the Roscoff worm, the mint-sauce worm, or the shilly-shally worm, is a marine flatworm belonging to the phylum Xenacoelomorpha. The origin and nature of the green color of this worm stimulated the curi ...

'', 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 " ...

lifestyle, although some species are epibionts. 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 ...

, 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 intercellu ...

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 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 ...

, 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 worms belonging to the family Isodiametridae Isodiametridae is a family of acoels. Taxonomy Genera The following genera are recognised in the family Isodiametridae: * ''Alluna'' Faubel & Regier, 1983 * ''Ancyloc ...

''. 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, unsegm ...

. 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 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. Histor ...

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 Dörjes, 1968 * Family Paratomellidae Dörjes, 1966 * Family

Taurididae ''Taurida fulvomaculata'' is a species 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 flatwo ...

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 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

{{Taxonbar, from=Q2052913 Acoelomorphs