The Bilateria /baɪləˈtɪəriə/ 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 and a back (ventral-dorsal axis).[2] Nearly all are bilaterally symmetrical as adults as well; the most notable exception is the echinoderms, which achieve secondary pentaradial symmetry as adults, but are bilaterally symmetrical during embryonic development.
Most animals are bilaterians, excluding sponges, ctenophores, placozoans and cnidarians. For 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 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.[a] Animals with this bilaterally symmetric body plan have 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.[4][2]
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.[5] The body stretches back from the head, and many bilaterians have a combination of circular muscles that constrict the body, making it longer, and an opposing set of longitudinal muscles, that shorten the body;[2] these enable soft-bodied animals with a hydrostatic skeleton to move by peristalsis.[6] 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.[4][2]
The hypothetical most recent common ancestor of all bilateria is termed the "Urbilaterian".[8][9] 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), 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 the Gastrea by Haeckel or Sedgwick, the Bilaterosgastrea by Gösta Jägersten, 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.[3] It may have resembled the planula larvae of some cnidaria, which have some bilateral symmetry.[10]