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Bilateria () is a large clade of characterised by bilateral symmetry during embryonic development. This means their 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 , which have pentaradial symmetry as adults, but bilateral symmetry as . With few exceptions, bilaterian embryos are triploblastic, having three : endoderm, mesoderm and ectoderm, and have complete with a separate mouth and anus. Some bilaterians lack body cavities, while others have a Pseudocoelomata derived from the blastocoel, or a secondary cavity, the Coelomata. 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, , and ), 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 or , 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.
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).
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 , 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 () and arthropods ().
Bilaterians, presumably including the urbilaterian, share many more controlling the development of their more complex bodies, including Cephalization, than do the Cnidaria and the Acoelomorpha.
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. 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 the Nephrozoa. Subsequently, the acoelomorphs were placed in phylum Xenacoelomorpha, together with the , and the sister relationship between Xenacoelomorpha and Nephrozoa supported in phylogenomic analyses.
A cladogram for Bilateria under the Nephrozoa hypothesis from a 2014 review by Casey Dunn and colleagues, is shown below. The cladogram indicates approximately when some clades radiated into newer clades, in millions of years ago (Mya).
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). 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". As of 2024, the issue of which hypothesis is correct remains unresolved.
Cladogram showing Xenambulacraria hypothesis with a Paraphyly Deuterostomia:
Cladogram showing hypothesis of Xenambulacraria within a monophyletic Deuterostomia:
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