30-503: Xenacoelomorpha ( / ˌ z ɛ n ə ˌ s ɛ l oʊ ˈ m ɔːr f ə / ) 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 appearances shared by the animals in the clade ), which was then confirmed by phylogenomic analyses of molecular data (similarities in
60-410: A central front-to-back body axis with mirror image right and left sides. They are triploblasts (meaning they have the three germ layers : ectoderm , endoderm , and mesoderm ). Their body plan is acoelomate – they lack a coelom – do not have a true body cavity. Also an excretory system is absent, yet all genes related to the excretory system are present except for Osr, which is essential for
90-432: A coelom, those technically do not have an acoelomate body plan because they lack the mesoderm germ layer. In acoels , the mouth opens directly into a large endodermal syncytium , while in nemertodermatids and xenoturbellids there is a sack-like gut lined by unciliated cells. A defining feature is a digestive system lacking nerve cells. Because an enteric nervous system , also called the stomatogastric nervous system,
120-446: A head (anterior) end and a tail (posterior) end as well as a back (dorsal) and a belly (ventral); therefore they also have a left side and a right side. Having a front end means that this part of the body encounters stimuli, such as food, favouring cephalisation , the development of a head with sense organs and a mouth. The body stretches back from the head, and many bilaterians have a combination of circular muscles that constrict
150-425: A left and a right side that are mirror images of each other) during embryonic development . This means their body plans are laid around a longitudinal axis ( rostral – caudal axis) with a front (or "head") and a rear (or "tail") 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
180-468: A more complex ancestor. Having a larger number of species within this group would allow for better conclusions and analysis to be made within the phylum and in groups closely related to the phylum. Ambulacraria [REDACTED] Xenacoelomorpha [REDACTED] Chordata [REDACTED] Ecdysozoa [REDACTED] Spiralia [REDACTED] For multiple decades, the genus Xenoturbella contained only one species, X. bocki . In 2016 however,
210-448: A team reported the discovery of four new species from the Gulf of California and sequenced each new species' mitogenome and upon analysis found that the two species that lived in shallow water ( X. bocki and X. hollandorum ) formed a "shallow" clade and that three deep water species formed a "deep" clade. The following year another team discovered a sixth species, X. japonica , found off
240-859: A tentative clade of various species from Actinoposthiidae and Isodiametridae which is not shown in the below cladogram. Several small basal families were not included in their study and their position is still uncertain. X. japonica X. bocki X. hollandorum X. monstrosa X. churro X. profunda Actinoposthiidae Antigonariidae Antroposthiidae Diopisthoporidae Nadinidae Paratomellidae Taurididae Paratomellidae Hallangiidae Hofsteniidae Solenofilomorphidae Proporidae Isodiametridae Dakuidae Convolutidae Mecynostomidae Ascoparia Flagellophora Nemertoderma Nemertinoides Meara Sterreria The phylum consists of small, flat and worm-like creatures found in marine and sometimes brackish water environments , on
270-416: Is also found in many cnidarians, its absence is most likely a derived trait. Bilaterian Triploblasts Lankester, 1873 Bilateria ( / ˌ b aɪ l ə ˈ t ɪər i ə / BY -lə- TEER -ee-ə ) is a large clade or infrakingdom of animals called bilaterians ( / ˌ b aɪ l ə ˈ t ɪər i ə n / BY -lə- TEER -ee-ən ), characterized by bilateral symmetry (i.e. having
300-400: Is not clear if the similarities are ancestral or derived. However, some studies point out that their basal placement may be caused by high mutation rates leading to long branch attraction (LBA). These analyses suggest that the xenacoelomorphs are instead the sister group of Ambulacraria forming the clade Xenambulacraria and that despite their simple body plans, they actually derive from
330-590: Is termed the " Urbilaterian ". The nature of the first bilaterian is a matter of debate. One side suggests that acoelomates gave rise to the other groups (planuloid–aceloid hypothesis by Ludwig von Graff , Elie Metchnikoff , Libbie Hyman , or Luitfried von Salvini-Plawen [ nl ] ), while the other poses that the first bilaterian was a coelomate organism and the main acoelomate phyla ( flatworms and gastrotrichs ) have lost body cavities secondarily (the Archicoelomata hypothesis and its variations such as
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#1732854790037360-753: Is that the Ambulacraria are sister to Xenacoelomorpha together forming the Xenambulacraria . The Xenambulacraria may be sister to the Chordata or to the Centroneuralia (corresponding to Nephrozoa without Ambulacraria, or to Chordata + Protostomia). The phylogenetic tree shown below depicts the latter proposal. Also, the validity of Deuterostomia (without Protostomia emerging from it) is under discussion. The cladogram indicates approximately when some clades radiated into newer clades, in millions of years ago (Mya). While
390-555: Is the echinoderms , which extend to pentaradial symmetry as adults, but are only 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. Bilaterians constitute one of the five main metazoan lineages, the other four being Porifera ( sponges ), Cnidaria ( jellyfish , hydrae , sea anemones and corals ), Ctenophora ( comb jellies ) and Placozoa (tiny "flat animals"). For
420-416: The blastocoel , as pseudocoeloms ) or secondary cavities (that appear de novo , for example the coelom ). Some of the earliest bilaterians were wormlike, and a bilaterian body can be conceptualized as a cylinder with a gut running between two openings, the mouth and the anus. Around the gut it has an internal body cavity, a coelom or pseudocoelom. Animals with this bilaterally symmetric body plan have
450-450: The echinoderms , hemichordates , chordates , and the extinct Vetulicolia . The protostomes include most of the rest, such as arthropods , annelids , mollusks , flatworms , and so forth. There are several differences, most notably in how the embryo develops. In particular, the first opening of the embryo becomes the mouth in protostomes, and the anus in deuterostomes. Many taxonomists now recognize at least two more superphyla among
480-531: The sediments . There are species that are variously free-living, parasitic , and symbiotic . They can be found at depths of almost 4 km (2.5 mi) and near hydrothermal vents . The phylum is hermaphroditic (all individuals have both male and female sex organs) and reproduces sexually with direct development , meaning they skip a potentially vulnerable larval stage. Xenoturbella have external fertilization, and Acoelomorpha has internal fertilization. All xenacoelomorphs are bilateral, meaning they have
510-451: The DNA of the animals within the clade). Prior to molecular studies, xenacoelomorphs were considered to be flatworms based on their superficial similarities. Like flatworms, they do not have a coelom and are dorsoventrally flattened. With the advent of phylogenetics , Xenoturbella and Acoelomorpha were found to be sister groups and only distantly related to flatworms. Initially this phylum
540-573: The Gastrea by Haeckel or Sedgwick , the Bilaterosgastrea by Gösta Jägersten [ sv ] , or the Trochaea by Nielsen). One hypothesis is that the original bilaterian was a bottom dwelling worm with a single body opening, similar to Xenoturbella . Alternatively, it may have resembled the planula larvae of some cnidaria, which have some bilateral symmetry. However, there is evidence that it
570-1536: The Nephrozoa. Subsequently the acoelomorphs were placed in phylum Xenacoelomorpha , together with the xenoturbellids , and the sister relationship between Xenacoelomorpha and Nephrozoa confirmed in phylogenomic analyses. A modern consensus phylogenetic tree for Bilateria is shown below, although the positions of certain clades are still controversial (dashed lines) and the tree has changed considerably since 2000. Cnidaria [REDACTED] Placozoa [REDACTED] Proarticulata ? † [REDACTED] Xenoturbellida [REDACTED] Nemertodermatida Acoela [REDACTED] Echinodermata [REDACTED] Hemichordata [REDACTED] † Cambroernida [REDACTED] Cephalochordata [REDACTED] Tunicata [REDACTED] Craniata / Vertebrata [REDACTED] † Saccorhytus coronarius † Vetulocystids [REDACTED] † Vetulicolians [REDACTED] Nematoda [REDACTED] Nematomorpha [REDACTED] Loricifera [REDACTED] Onychophora [REDACTED] Tardigrada [REDACTED] Arthropoda [REDACTED] Priapulida [REDACTED] Kinorhyncha [REDACTED] Rotifera and allies [REDACTED] Chaetognatha [REDACTED] Platyhelminthes and allies [REDACTED] Mollusca [REDACTED] Annelida and allies [REDACTED] ¿† Kimberella ? [REDACTED] Kimberella ? † [REDACTED] A different hypothesis
600-522: The below tree depicts Chordata as a sister group to Protostomia according to analyses by Philippe et al., the authors nonetheless caution that "the support values are very low, meaning there is no solid evidence to refute the traditional protostome and deuterostome dichotomy". Cnidaria [REDACTED] Placozoa [REDACTED] † Proarticulata Xenoturbellida [REDACTED] Nemertodermatida Acoela [REDACTED] Echinodermata [REDACTED] Ascopariidae Ascopariidae
630-739: The body, making it longer, and an opposing set of longitudinal muscles, that shorten the body; these enable soft-bodied animals with a hydrostatic skeleton to move by peristalsis . Most bilaterians (nephrozoans) have a gut that extends through the body from mouth to anus, while xenacoelomorphs have a bag gut with one opening. Many bilaterian phyla have primary larvae which swim with cilia and have an apical organ containing sensory cells. However, there are exceptions to each of these characteristics; for example, adult echinoderms are radially symmetric (unlike their larvae), and certain parasitic worms have extremely plesiomorphic body structures. The hypothetical most recent common ancestor of all bilateria
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#1732854790037660-414: The coast of Japan. Their phylogenetic analysis confirmed the first team's hypothesis and placed X. japonica within the shallow clade. The other two groups, Nemertodermatida and Acoela , have less clear relationships as species-level phylogenies have not been conducted. Nemertodermatida only has two families and six total genera. Ascopariidae contains two of these genera, while Nemertodermatidae has
690-482: The development for such a system. In acoelomorphs, which has gone through rapid evolutionary rates and chromosomic rearrangements, about 60% of the genes shared between protostomes and deuterostomes are missing. How many of these genes which are present or absent in Xenoturbella will require a whole genome sequencing. While other animals that are diploblastic (only have two germ layers: ectoderm and endoderm) also lack
720-689: The earliest known bilaterian, but may also represent an infilled bubble. Fossil embryos are known from around the time of Vernanimalcula ( 580 million years ago ), but none of these have bilaterian affinities. Burrows believed to have been created by bilaterian life forms have been found in the Tacuarí Formation of Uruguay, and were believed to be at least 585 million years old. However, more recent evidence shows these fossils are actually late Paleozoic instead of Ediacaran. The Bilateria has traditionally been divided into two main lineages or superphyla . The deuterostomes traditionally include
750-441: The most part, bilateral embryos are triploblastic , having three germ layers : endoderm , mesoderm and ectoderm . Except for a few phyla (i.e. flatworms and gnathostomulids ), bilaterians have complete digestive tracts with a separate mouth and anus . Some bilaterians lack body cavities ( acoelomates , i.e. Platyhelminthes , Gastrotricha and Gnathostomulida ), while others display primary body cavities (deriving from
780-459: The other four. A 2016 study analyzed three of the four Nemertodermatid genera and found that Sterreria and Meara are closer to each other than to Nemertoderma , while Nemertinoides was left unplaced. Acoela phylogeny is even less certain as it is by far the most diverse part of the phylum and is very understudied. A 2011 study attempted to solve this problem and recovered numerous traditional families as polyphyletic . They also recovered
810-516: The protostomes, Ecdysozoa (molting animals) and Spiralia . The arrow worms ( Chaetognatha ) have proven difficult to classify; recent studies place them in the Gnathifera . The traditional division of Bilateria into Deuterostomia and Protostomia was challenged when new morphological and molecular evidence found support for a sister relationship between the acoelomate taxa, Acoela and Nemertodermatida (together called Acoelomorpha ), and
840-443: The remaining bilaterians. The latter clade was called Nephrozoa by Jondelius et al. (2002) and Eubilateria by Baguña and Riutort (2004). The acoelomorph taxa had previously been considered flatworms with secondarily lost characteristics, but the new relationship suggested that the simple acoelomate worm form was the original bilaterian body plan and that the coelom, the digestive tract, excretory organs, and nerve cords developed in
870-688: Was considered to be a member of the deuterostomes , but because of recent transcriptome analyses, it was concluded that phylum Xenacoelomorpha is the sister group to the Nephrozoa , which includes both the protostomes and the deuterostomes, which makes the phylum the basalmost bilaterian clade. This would mean they are neither deuterostomes nor protostomes. Xenacoelomorpha [REDACTED] Ambulacraria [REDACTED] Chordata [REDACTED] Ecdysozoa [REDACTED] Spiralia [REDACTED] Their larvae show similarities with cnidarian planula larvae and poriferan parenchyma larvae, but it
900-465: Was segmented, as the mechanism for creating segments is shared between vertebrates (deuterostomes) and arthropods (protostomes). The first evidence of bilateria in the fossil record comes from trace fossils in Ediacaran sediments, and the first bona fide bilaterian fossil is Kimberella , dating to 555 million years ago . Earlier fossils are controversial; the fossil Vernanimalcula may be
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