Bilaterians

Bilateria, or bilaterians, are a major clade of animals that includes most living animal phyla, such as chordates, arthropods, mollusks, annelids, and nematodes. They are characterized by bilateral symmetry as adults, a triploblastic body plan with three germ layers—ectoderm, mesoderm, and endoderm—and typically a through gut with a mouth and anus. Many also possess cephalization and a centralized nervous system. Body plans vary widely, and bilaterians can be acoelomates, pseudocoelomates, or coelomates.

Bilateria is divided into two major lineages: Protostomia and Deuterostomia. Protostomes include Ecdysozoa (such as arthropods and nematodes) and Lophotrochozoa (such as mollusks, annelids, and many worm groups). Deuterostomes include echinoderms and chordates. Developmental patterns differ between these groups, with protostomes typically showing determinate cleavage and mouth forming from the blastopore, while deuterostomes often show indeterminate development and the anus forming from the blastopore; the mouth forms later in development.

Origin and fossil record: The origin of Bilateria is estimated to lie in the Neoproterozoic era, with the earliest clear bilaterian fossils from the early Cambrian; the Cambrian explosion greatly expanded bilaterian diversity.

Ecology and diversity: Bilaterians occupy nearly all habitats, from deep oceans to terrestrial environments, and include organisms of major ecological importance across ecosystems. The group is one of the most studied in comparative anatomy and evolutionary developmental biology.