Acoelomorpha
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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, such as algae and corals.<ref name="Cannon1986">Cannon, L. R. G. (1986) Turbellaria of the World. A guide to families and genera. Brisbane, Queensland Museum, 136 p.</ref> With the exception of two acoel freshwater species, all known acoelomorphs are marine.<ref>Template:Cite book</ref>
SystematicsEdit
EtymologyEdit
The term "acoelomorph" derives from the Ancient Greek words Template:Wikt-lang (Template:Grc-transl), the alpha privative, expressing negation or absence, Template:Wikt-lang (Template:Grc-transl), meaning "cavity", and Template:Wikt-lang (Template:Grc-transl), meaning "form".<ref>Template:Cite book</ref><ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref> This refers to the fact that acoelomorphs have a structure lacking a fluid-filled body cavity.
ClassificationEdit
The subphylum Acoelomorpha is divided into two classes. There are at least 408 described species, with a majority of these falling within the Crucimusculata infraorder in Acoela.
- Acoela comprise small flattened worms, classified into a dozen families.<ref>{{#invoke:citation/CS1|citation
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- Nemertodermatida comprise millimetre-sized, mostly interstitial worms, distributed into two families.<ref>{{#invoke:citation/CS1|citation
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PhylogenyEdit
The soft bodies of acoelomorphs and the lack of some of the key bilaterian traits make them difficult to classify.<ref>Template:Cite journal</ref><ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref> Traditionally, based on phenotypic features, acoelomorphs were considered to belong to the phylum Platyhelminthes, which was long seen as the sister group to all other bilaterian phyla.<ref name="Conway-Morris1985">Conway-Morris, S.; George, J. D.; Gibson R.; Platt, H. M. (1985) The Origins and relationships of lower invertebrates. Oxford, Clarendon Press, 397 p.Template:Page needed</ref> However, a series of molecular phylogenetics studies at the hinge between the 20th and 21st centuries demonstrated that they are fast-evolving organisms not closely related to platyhelminthes,<ref>Template:Cite journal</ref><ref>Template:Cite journal</ref><ref>Template:Cite journal</ref><ref>Template:Cite journal</ref> therefore involving the polyphyly of flatworms.<ref name="BaguñàRiutort2004CJZ">Template:Cite journal</ref><ref name="BaguñàRiutort2004BE">Template:Cite journal</ref><ref name="Ruiz-Trillo2004">Template:Cite journal</ref><ref name="Philippe2007">Template:Cite journal</ref>
Actually, Acoelomorpha appeared to constitute a separate, deep-branching phylum, kingpin of bilaterian evolution.<ref>Template:Cite journal</ref> Yet their evolutionary affinities remain enigmatic as they might be the sister-group either to all other bilateral animals<ref name="BaguñàRiutort2004CJZ" /><ref name="BaguñàRiutort2004BE" /><ref name="Ruiz-Trillo2004" /> or to all deuterostomes.<ref name="Philippe2007" /> Resolving this debate would indicate whether acoelomorphs are simple or simplified. If they are the sister group to Bilateria, it would point to a simple body plan for the first bilaterian. Alternatively, if acoelomorphs are related to deuterostomes, this would imply that their organisation is the result of secondary simplification.<ref name="Ruiz-Trillo2016">Template:Cite journal</ref>
In addition, comparative analyses of morphological, developmental, and molecular characters raised two points.
- Xenoturbellida is the sister group to acoelomorphs, constituting the so-called Xenacoelomorpha clade.<ref name="Philippe2011">Template:Cite journal</ref> The close evolutionary relationship between Acoelomorpha and Xenoturbella is supported by the morphology (structure of epidermal cilia<ref name="Lundin 1998">Template:Cite journal</ref>), the embryology (direct development without a feeding larval stage<ref name="Nakano2013">Template:Cite journal</ref>), and the concatenation of hundreds of proteins.<ref name="Hejnol2009">Template:Cite journal</ref><ref name="Philippe2011" /><ref name="Edgecombe 2011">Template:Cite journal</ref>
- The phylogenetic placement of Xenacoelomorpha among bilaterian animals is not yet well defined, despite increased taxon and gene sampling, (re)-analyses of published data sets, and use of more sophisticated models of sequence evolution in phylogenomic studies. There is a conflict between two evolutionary hypotheses, with Xenacoelomorpha being the sister group to Ambulacraria within deuterostomes (i.e., the Xenambulacraria hypothesis) on the one hand,<ref name="Philippe2019">Template:Cite journal</ref> and Xenacoelomorpha as sister group to all other bilaterians (i.e., the Nephrozoa hypothesis) on the other.<ref name="Edgecombe 2011"/><ref name="Srivastava 2014">Template:Cite journal</ref><ref name="CannonVellutini2016">Template:Cite journal</ref> However, the Nephrozoa hypothesis might reflect methodological errors resulting from model violations in the phylogenomic inference.<ref name="Philippe2019"/>
AnatomyEdit
Acoelomorphs resemble flatworms in many respects, but have a simpler anatomy, not even having 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.<ref name="Cannon1986" />
The epidermal cells of acoelomorphs are unable to proliferate, a feature that is only shared with rhabditophoran flatworms and was for some time considered a strong evidence for the position of Acoelomorpha within Platyhelminthes. In both groups, the epidermis is renewed from mesodermal stem cells.<ref name="MichalakEgger2009">Template:Cite journal</ref>
The nervous system of acoelomorphs is formed by a set of longitudinal nerve bundles beneath the ciliated epidermis. Close to the anterior end, these bundles are united by a ring commissure, but do not form a true brain, although it is hypothesized that such organization was the precursor of the cephalization of the nerve system in more derived bilaterians.<ref name="Perea-AtienzaGavilan2015">Template:Cite journal</ref> After decapitation, such a "brain" (rather, a cerebroid ganglion) regenerates in a few weeks.<ref name="Sprecher2015">Template:Cite journal</ref>
The sensory organs include a statocyst – which presumably helps them orient to gravity –, and, in some cases, ancestral pigment-spot ocelli capable of detecting light.<ref name=Barnes1982 />
Acoelomorphs 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.<ref name=Barnes1982>Template:Cite book</ref>