Bilaterian

Bilateria is a major clade of animals defined by bilateral symmetry and triploblastic development, meaning they arise from three germ layers: ectoderm, mesoderm, and endoderm. Members typically have a distinct anterior (head) and posterior end, a dorsal and ventral surface, and a concentration of sensory and neural structures at the front, a feature known as cephalization. The clade includes a vast diversity of forms, from worms and snails to insects, fish, and mammals, though not all show perfect bilateral symmetry at all life stages (for example, echinoderms are bilateral in the larval stage but display pentaradial symmetry as adults).

During development, bilaterians are commonly divided into Protostomia and Deuterostomia. Protostomes often exhibit spiral, determinate cleavage and mouth formation before the anus, while deuterostomes typically show radial, indeterminate cleavage with the anus forming before the mouth. Bilaterians may be coelomate, pseudocoelomate, or in some groups acoelomate, reflecting a variety of body cavity architectures.

Phylogenetically, Bilateria encompasses major groups such as Protostomia (which includes Lophotrochozoa and Ecdysozoa) and Deuterostomia (which includes Ambulacraria—echinoderms and hemichordates—and Chordata). Echinoderms are deuterostomes despite their adults’ radial symmetry. The fossil record places the earliest bilaterians in the late Neoproterozoic, with rapid diversification during the Cambrian explosion. Molecular and developmental studies highlight conserved gene families, such as Hox genes, that underlie their distinctive body plans.