Bilateria

Bilateria refers to a large clade of animals characterized by bilateral symmetry during at least one life stage and by triploblastic development, with three germ layers. They typically exhibit an anterior-posterior axis and cephalization, an organized nervous system, and a complete digestive tract with a mouth and an anus. Most bilaterians have regionalized organ systems and, in many groups, body segmentation.

The clade includes most living animal phyla, such as chordates (including vertebrates), echinoderms, arthropods, molluscs, annelids, nematodes, and flatworms. Adult echinoderms are radially symmetric, but they are bilaterians because they develop from bilaterally symmetric embryos and have bilateral larval stages; thus, their placement in Bilateria is based on developmental origin rather than adult form.

Taxonomy within Bilateria centers on two major clades: Protostomia and Deuterostomia. Protostomia comprises Ecdysozoa (for example, arthropods and nematodes) and Lophotrochozoa (for example, molluscs and annelids). Deuterostomia includes hemichordates, echinoderms, and chordates. These divisions are supported by molecular data and by certain embryological features such as patterns of early cell cleavages and gastrulation, including the development of the mouth and anus.

The origin of Bilateria is placed in the Neoproterozoic to early Paleozoic, with the fossil record showing bilaterian-like forms in the late Precambrian and a major diversification during the Cambrian Explosion. Molecular estimates suggest an earlier origin, but precise timing remains debated.