Bilateria
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Bilateria (Template:IPAc-en)<ref>Template:Cite Merriam-Webster</ref> is a large clade of animals characterised by bilateral symmetry during embryonic development. This means their body plans are laid around a longitudinal 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 is the echinoderms, which have pentaradial symmetry as adults, but bilateral symmetry as embryos. With few exceptions, bilaterian embryos are triploblastic, having three germ layers: endoderm, mesoderm and ectoderm, and have complete digestive tracts with a separate mouth and anus. Some bilaterians lack body cavities, while others have a primary body cavity derived from the blastocoel, or a secondary cavity, the coelom. Cephalization is a characteristic feature among most bilaterians, where the sense organs and central nerve ganglia become concentrated at the front end of the animal.
Bilaterians constitute one of the five main lineages of animals, the other four being Porifera (sponges), Cnidaria (jellyfish, hydrozoans, sea anemones and corals), Ctenophora (comb jellies) and Placozoa. They rapidly diversified in the late Ediacaran and the Cambrian, and are now by far the most successful animal lineage, with over 98% of known animal species. Bilaterians are traditionally classified as either deuterostomes or protostomes, based on whether the blastopore becomes the anus or mouth. The recently erected phylum Xenacoelomorpha, once thought to be flatworms, has provided an extra challenge to bilaterian taxonomy, as they likely do not belong to either group.
Body planEdit
Animals with a bilaterally symmetric body plan that mainly move in one direction have a head end (anterior) 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.<ref name="Minelli2009"/><ref name=Brusca2016>Template:Cite book</ref> 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.<ref name=Finnerty>Template:Cite journal</ref> Most bilaterians (nephrozoans) have a gut that extends through the body from mouth to anus (sometimes called a "through gut"<ref>Template:Cite journal</ref>), and sometimes a wormlike body plan with a hydrostatic skeleton. Xenacoelomorphs, on the other hand, 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.<ref name="Minelli2009">Template:Cite book</ref><ref name=Brusca2016/>
Some bilaterians have only weakly condensed nerve nets (similar to those in cnidarians), while others have either a ventral nerve cord, a dorsal nerve cord, or both (e.g. in Hemichordate).<ref>Template:Cite journal</ref>
EvolutionEdit
Common ancestorEdit
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The hypothetical most recent common ancestor of all Bilateria is termed the 'urbilaterian'. The nature of this 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 Template:Ill). This means that the urbilaterian had a solid body, and all body cavities therefore secondarily arose later in different groups. The other side poses that the urbilaterian had a coelom, meaning that the main acoelomate phyla (flatworms and gastrotrichs) have secondarily lost their body cavities.<ref>Template:Cite journal</ref><ref>Template:Cite journal</ref> This is the Archicoelomata hypothesis first proposed by A. T. Masterman in 1899.<ref name="Masterman 1899">Template:Cite journal</ref> Variations of the Archicoelomata hypothesis are the Gastraea by Ernst Haeckel in 1872<ref name="Levit 2022">Template:Cite journal</ref> or Adam Sedgwick, and more recently the Bilaterogastrea by Template:Interlanguage link,<ref name="Olsson 2007">Template:Cite journal</ref> and the Trochaea by Claus Nielsen.<ref name="Nielsen 1985">Template:Cite journal</ref>
One proposal, by Johanna Taylor Cannon and colleagues, is that the original bilaterian was a bottom dwelling worm with a single body opening, similar to Xenoturbella.<ref name=Cannon2016>Template:Cite journal</ref> An alternative proposal, by Jaume Baguñà and colleagues, is that it may have resembled the planula larvae of some cnidarians, which unlike the radially-symmetric adults have some bilateral symmetry.<ref>Template:Cite journal</ref> However, Lewis I. Held presents evidence that it was segmented, as the mechanism for creating segments is shared between vertebrates (deuterostomes) and arthropods (protostomes).<ref name="Held 2014">Template:Cite book</ref>
Bilaterians, presumably including the urbilaterian, share many more Hox genes controlling the development of their more complex bodies, including of their heads, than do the Cnidaria and the Acoelomorpha.<ref name="Hombria 2021">Template:Cite journal</ref>
Fossil recordEdit
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 Template:Ma.<ref name=Fedonkin1997>Template:Cite journal</ref> Earlier fossils are controversial; the fossil Vernanimalcula may be the earliest known bilaterian, but may also represent an infilled bubble.<ref name=Bengtson2004>Template:Cite journal</ref><ref name=VernAcritarch>Template:Cite journal</ref> Fossil embryos are known from around the time of Vernanimalcula (Template:Ma), but none of these have bilaterian affinities.<ref name=Hagadorn2006>Template:Cite journal</ref> 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.<ref name="sci">Template:Cite journal</ref> However, more recent evidence shows these fossils are actually late Paleozoic, not Ediacaran.<ref>Template:Cite journal</ref>
PhylogenyEdit
The Bilateria are now by far the most successful animal lineage, with over 98% of known animal species.<ref name="Cat of Life">{{#invoke:citation/CS1|citation |CitationClass=web }}</ref> The group has traditionally been divided into two main lineages or superphyla.<ref name=Nielsen-2008>Template:Cite journal</ref> The deuterostomes traditionally include the echinoderms, hemichordates, chordates, and the extinct Vetulicolia. The protostomes include most of the rest, such as arthropods, annelids, molluscs, and flatworms. 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 recognise at least two more superphyla among the protostomes, Ecdysozoa<ref name="Halanych 1641–1643">Template:Cite journal</ref> and Spiralia.<ref name="Halanych 1641–1643"/><ref>Template:Cite journal</ref><ref>Template:Cite journal</ref> The arrow worms (Chaetognatha) have proven difficult to classify; recent studies place them in the Gnathifera.<ref>Template:Cite journal</ref><ref>Template:Cite journal</ref><ref name="Fröbius">Template:Cite journal</ref>
The traditional division of Bilateria into Deuterostomia and Protostomia was challenged when new morphological and molecular evidence supported a sister relationship between the acoelomate taxa, Acoela and Nemertodermatida (together called Acoelomorpha), and the remaining bilaterians.<ref name=Nielsen-2008/><ref name="Hejnol 2009">Template:Cite book</ref><ref>Template:Cite journal</ref> The latter clade was called Nephrozoa by Jondelius et al. (2002) and Eubilateria by Baguña and Riutort (2004).<ref name=Nielsen-2008/> 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 the Nephrozoa.<ref name=Nielsen-2008/><ref name=Cannon-2016>Template:Cite journal</ref> Subsequently, the acoelomorphs were placed in phylum Xenacoelomorpha, together with the xenoturbellids, and the sister relationship between Xenacoelomorpha and Nephrozoa supported in phylogenomic analyses.<ref name=Cannon-2016/>
A cladogram for Bilateria under the Nephrozoa hypothesis from a 2014 review by Casey Dunn and colleagues, is shown below.<ref name="Dunn Giribet Edgecombe Hejnol 2014">Template:Cite journal</ref> The cladogram indicates approximately when some clades radiated into newer clades, in millions of years ago (Mya).<ref>Template:Cite journal</ref>
A different hypothesis is that Ambulacraria is sister to Xenacoelomorpha, together forming Xenambulacraria. Xenambulacraria may be sister to Chordata or to Centroneuralia (corresponding to Nephrozoa without Ambulacraria, or, as shown here, to Chordata + Protostomia).<ref>Template:Cite journal</ref> A 2019 study by Hervé Philippe and colleagues presents the tree, cautioning that "the support values are very low, meaning there is no solid evidence to refute the traditional protostome and deuterostome dichotomy".<ref name="PhilippePoustka2019">Template:Cite journal</ref> As of 2024, the issue of which hypothesis is correct remains unresolved.<ref>Template:Cite journal</ref><ref>Template:Cite journal</ref>
Cladogram showing Xenambulacraria hypothesis with a paraphyletic Deuterostomia:<ref name="Mulhair McCarthy 2022">Template:Cite journal</ref>
Cladogram showing hypothesis of Xenambulacraria within a monophyletic Deuterostomia:<ref name="Mulhair McCarthy 2022"/>
Taxonomic historyEdit
The Bilateria were named by the Austrian embryologist Berthold Hatschek in 1888. In his classification, the group included the Zygoneura, Ambulacraria, and Chordonii (the Chordata).<ref name="Nielsen 2009">Template:Cite journal</ref><ref>Template:Cite book</ref> In 1910, the Austrian zoologist Karl Grobben renamed the Zygoneura to Protostomia, and created the Deuterostomia to encompass the Ambulacraria and Chordonii.<ref name="Nielsen 2009"/><ref>Template:Cite book</ref>
See alsoEdit
NotesEdit
ReferencesEdit
External linksEdit
- Tree of Life web project — Bilateria Template:Webarchive
- University of California Museum of Paleontology — Systematics of the Metazoa