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.

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.

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.

[6][7] 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.

[11] Variations of the Archicoelomata hypothesis are the Gastraea by Ernst Haeckel in 1872[12] or Adam Sedgwick, and more recently the Bilaterogastrea by Gösta Jägersten [sv],[13] and the Trochaea by Claus Nielsen.

[14] 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.

[15] 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.

[31][32][33] 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.

[27] 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.

[39] 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".