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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 Sediment is a naturally occurring material that is broken down by processes of weathering and erosion, and is subsequently transported by the action of wind, water, or ice or by the force of gravity acting on the particles. For example, sa ...
, swimming as
plankton Plankton are the diverse collection of organisms found in water (or air) that are unable to propel themselves against a current (or wind). The individual organisms constituting plankton are called plankters. In the ocean, they provide a cr ...
, or crawling on other organisms, such as
algae Algae ( , ; : alga ) are any of a large and diverse group of photosynthetic, eukaryotic organisms. The name is an informal term for a polyphyletic grouping that includes species from multiple distinct clades. Included organisms range from ...
and
coral Corals are marine invertebrates within the class Anthozoa of the phylum Cnidaria. They typically form compact colonies of many identical individual polyps. Coral species include the important reef builders that inhabit tropical oceans and secre ...
s.Cannon, L. R. G. (1986) ''Turbellaria of the World. A guide to families and genera''. Brisbane, Queensland Museum, 136 p. With the exception of two acoel freshwater species, all known Acoelomorphs are marine.


Systematics


Etymology

The term "acoelomorph" derives 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 language, Greek and in words borrowed therefrom to expr ...
'', expressing negation or absence, (), meaning "cavity", and (), meaning "form". This refers to the fact that acoelomorphs have a structure lacking a fluid-filled body cavity.


Classification

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 of families. * Nemertodermatida comprise millimetre-sized, mostly interstitial worms, distributed into two families.


Phylogeny

The soft bodies of acoelomorphs and the lack of some of the key bilaterian traits make them difficult to classify. Traditionally, based on phenotypic features, acoelomorphs were considered to belong to 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, unsegment ...
, which was long seen as the sister group to all other
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 an ...
n phyla.Conway-Morris, S.; George, J. D.; Gibson R.; Platt, H. M. (1985) ''The Origins and relationships of lower invertebrates''. Oxford, Clarendon Press, 397 p. 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, therefore involving the
polyphyly A polyphyletic group is an assemblage of organisms or other evolving elements that is of mixed evolutionary origin. The term is often applied to groups that share similar features known as homoplasies, which are explained as a result of conver ...
of flatworms. Actually, Acoelomorpha appeared to constitute a separate, deep-branching phylum, kingpin of bilaterian evolution. Yet their evolutionary affinities remain enigmatic as they might be the sister-group either to all other bilateral animals or to all deuterostomes. Resolving this debate would indicate whether acoelomorphs are simple or simplified. If they are the sister group to
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 an ...
, 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. 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. The close evolutionary relationship between Acoelomorpha and ''Xenoturbella'' is supported by the morphology (structure of epidermal cilia), the embryology (direct development without a feeding larval stage), and the concatenation of hundreds of proteins. * 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, and Xenacoelomorpha as sister group to all other bilaterians (i.e., the Nephrozoa hypothesis) on the other. However, the Nephrozoa hypothesis might reflect methodological errors resulting from model violations in the phylogenomic inference.


Anatomy

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. 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. The
nervous system In Biology, biology, the nervous system is the Complex system, highly complex part of an animal that coordinates its Behavior, actions and Sense, sensory information by transmitting action potential, signals to and from different parts of its ...
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. After decapitation, such a "brain" (rather, a ''cerebroid ganglion'') regenerates in a few weeks. The sensory organs include a statocyst – which presumably helps them orient to gravity –, and, in some cases,
ancestral An ancestor, also known as a forefather, fore-elder or a forebear, is a parent or ( recursively) the parent of an antecedent (i.e., a grandparent, great-grandparent, great-great-grandparent and so forth). ''Ancestor'' is "any person from w ...
pigment-spot ocelli capable of detecting light. Acoelomorphs are simultaneous
hermaphrodite In reproductive biology, a hermaphrodite () is an organism that has both kinds of reproductive organs and can produce both gametes associated with male and female sexes. Many taxonomic groups of animals (mostly invertebrates) do not have ...
s, but have no gonads and no ducts associated with the female reproductive system. Instead,
gamete A gamete (; , ultimately ) is a haploid cell that fuses with another haploid cell during fertilization in organisms that reproduce sexually. Gametes are an organism's reproductive cells, also referred to as sex cells. In species that produce ...
s are produced from the mesenchymal cells that fill the body between the epidermis and the digestive vacuole.


References


External links


Phylogeny of Lower Worms of the Meiofauna (Acoelomorpha)

Acoelomorpha at the Turbellarian taxonomic database

Acoelomorpha: A Phylogenetic Headache

Earthling Nature
' {{Taxonbar, from=Q131486 Animal subphyla