Acoelomorpha | |
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The acoelomorph Waminoa sp. (orange structure) on the Plerogyra sp. coral (whitish bubbles). | |
Scientific classification | |
Domain: | Eukaryota |
Kingdom: | Animalia |
Phylum: | Xenacoelomorpha |
Subphylum: | Acoelomorpha Ehlers, 1985 |
Classes | |
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. [1] With the exception of two acoel freshwater species, all known acoelomorphs are marine. [2]
The term "acoelomorph" derives from the Ancient Greek words ἀ (a), the alpha privative , expressing negation or absence, κοιλία (koilía), meaning "cavity", and μορφή (morphḗ), meaning "form". [3] [4] This refers to the fact that acoelomorphs have a structure lacking a fluid-filled body cavity.
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.
The soft bodies of acoelomorphs and the lack of some of the key bilaterian traits make them difficult to classify. [8] [9] 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. [10] 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, [11] [12] [13] [14] therefore involving the polyphyly of flatworms. [15] [16] [17] [18]
Actually, Acoelomorpha appeared to constitute a separate, deep-branching phylum, kingpin of bilaterian evolution. [19] Yet their evolutionary affinities remain enigmatic as they might be the sister-group either to all other bilateral animals [15] [16] [17] or to all deuterostomes. [18] 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. [20]
In addition, comparative analyses of morphological, developmental, and molecular characters raised two points.
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. [1]
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. [29]
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. [30] After decapitation, such a "brain" (rather, a cerebroid ganglion) regenerates in a few weeks. [31]
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. [32]
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. [32]
The flatworms, flat worms, Platyhelminthes, or platyhelminths are a phylum of relatively simple bilaterian, unsegmented, soft-bodied invertebrates. Unlike other bilaterians, they are acoelomates, and have no specialised circulatory and respiratory organs, which restricts them to having flattened shapes that allow oxygen and nutrients to pass through their bodies by diffusion. The digestive cavity has only one opening for both ingestion and egestion ; as a result, the food cannot be processed continuously.
Bilateria is a large clade/infrakingdom of animals called bilaterians, characterized by bilateral symmetry during embryonic development. This means their body plans are laid around a longitudinal axis with a front and a rear end, as well as a left–right–symmetrical belly (ventral) and back (dorsal) surface. Nearly all bilaterians maintain a bilaterally symmetrical body as adults; the most notable exception is the echinoderms, which achieve secondary pentaradial symmetry as adults, but are bilaterally symmetrical as an embryo. Cephalization is also a characteristic feature among most bilaterians, where the special sense organs and central nerve ganglia become concentrated at the front/rostral end.
The Trochozoa are a proposed Lophotrochozoa clade that is a sister clade of Bryozoa. The clade would include animals in six phyla: the Nemertea, the Annelida, the Cycliophora, the Mollusca, and the two Brachiozoan phyla, Brachiopoda and Phoronida.
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 1 mm (0.039 in) to large freshwater forms more than 500 mm (20 in) long or terrestrial species like Bipalium kewense which can reach 600 mm (24 in) in length. All the larger forms are flat with ribbon-like or leaf-like shapes, since their lack of respiratory and circulatory systems means that they have to rely on diffusion for internal transport of metabolites. However, many of the smaller forms are round in cross section. Most are predators, and all live in water or in moist terrestrial environments. Most forms reproduce sexually and with few exceptions all are simultaneous hermaphrodites.
Xenoturbella is a genus of very simple bilaterians up to a few centimeters long. It contains a small number of marine benthic worm-like species.
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 they were treated as an order of turbellarian flatworms.
Ambulacraria, or Coelomopora, is a clade of invertebrate phyla that includes echinoderms and hemichordates; a member of this group is called an ambulacrarian. Phylogenetic analysis suggests the echinoderms and hemichordates separated around 533 million years ago. The Ambulacraria are part of the deuterostomes, a clade that also includes the many Chordata, and the few extinct species belonging to the Vetulicolia.
Deuterostomes are bilaterian animals of the superphylum Deuterostomia, typically characterized by their anus forming before the mouth during embryonic development. The three major clades of extant deuterostomes include chordates, echinoderms and hemichordates.
Protostomia is the clade of animals once thought to be characterized by the formation of the organism's mouth before its anus during embryonic development. This nature has since been discovered to be extremely variable among Protostomia's members, although the reverse is typically true of its sister clade, Deuterostomia. Well known examples of protostomes are arthropods, molluscs, annelids, flatworms and nematodes. They are also called schizocoelomates since schizocoely typically occurs in them.
The embryological origin of the mouth and anus is an important characteristic, and forms the morphological basis for separating bilaterian animals into two natural groupings: the protostomes and deuterostomes.
Nephrozoa is a major clade of bilaterians, divided into the protostomes and the deuterostomes, containing almost all animal phyla and over a million extant species. Its sister clade is the Xenacoelomorpha. The Ambulacraria are occasionally thought to be sister to the Xenacoelomorpha, forming the Xenambulacraria as basal Deuterostomia, or basal Bilateria invalidating Nephrozoa and Deuterostomia in multiple studies. The coelom, the digestive tract and excretory organs (nephridia), and nerve cords developed in the Nephrozoa. It has been argued that, because protonephridia are only found in protostomes, they cannot be considered a synapomorphy of this group. This would make Nephrozoa an improper name, leaving Eubilateria as this clade's name.
Schmidtea is a genus of freshwater triclads. Species of the genus Schmidtea are widely used in regeneration and developmental studies.
Kenkiidae is a family of freshwater triclads. Their species can be found sporadically in caves, groundwater, and deep lakes in Central Asia, Far East and North America.
Cura is a genus of freshwater flatworm (triclads) belonging to the family Dugesiidae.
Neppia is a genus of dugesiid triclad that is found in South America, Subantarctic region, Africa, Tasmania and New Zealand.
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, which was then confirmed by phylogenomic analyses of molecular data.
Geoplaninae is a subfamily of land planarians endemic to the Neotropical region. Members of this family are sometimes referred to as the Neotropical land planarians. However, one species, Obama nungara has been introduced in Europe.
Gonzalo Giribet is a Spanish-American invertebrate zoologist and Alexander Agassiz Professor of zoology working on systematics and biogeography at the Museum of Comparative Zoology in Harvard University. He is a past president of the International Society for Invertebrate Morphology, of the Willi Hennig Society, and vice-president of the Sociedad Española de Malacología.
Xenoturbella bocki is a marine benthic worm-like species from the genus Xenoturbella. It is found in saltwater sea floor habitats off the coast of Europe, predominantly Sweden. It was the first species in the genus discovered. Initially it was collected by Swedish zoologist Sixten Bock in 1915, and described in 1949 by Swedish zoologist Einar Westblad. The unusual digestive structure of this species, in which a single opening is used to eat food and excrete waste, has led to considerable study and controversy as to its classification. It is a bottom-dwelling, burrowing carnivore that eats mollusks.
Xenambulacraria is a proposed clade of animals with bilateral symmetry as an embryo, consisting of the Xenacoelomorpha and the Ambulacraria.