Bilateria

Bilateria (/ˌbaɪləˈtɪəriə/) 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 phylum Xenacoelomorpha, once thought to be flatworms, was erected in 2011, and has provided an extra challenge to bilaterian taxonomy, as they likely do not belong to either group.

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. 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. Most bilaterians (nephrozoans) have a gut that extends through the body from mouth to anus (sometimes called a "through gut"), 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.

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).

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 Luitfried von Salvini-Plawen [nl]). 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. This is the Archicoelomata hypothesis first proposed by A. T. Masterman in 1899. Variations of the Archicoelomata hypothesis are the Gastraea by Ernst Haeckel in 1872 or Adam Sedgwick, and more recently the Bilaterogastrea by Gösta Jägersten [sv], and the Trochaea by Claus Nielsen.

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. 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. However, Lewis I. Held presents evidence that it was segmented, as the mechanism for creating segments is shared between vertebrates (deuterostomes) and arthropods (protostomes).

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.

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 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, not Ediacaran.