Bilateralisia

Bilateralisia, also called bilaterians, is a major clade of animals defined by bilateral symmetry and triploblastic development. In these organisms, the body can be divided into two mirror-image halves along a single plane, and there is typically a distinct anterior and posterior end, with dorsal and ventral surfaces. Embryologically, bilaterians form three germ layers—ectoderm, mesoderm, and endoderm—and many develop a coelom, a body cavity that provides space for organ development.

The bilateral body plan is commonly associated with cephalization, the concentration of sensory organs and nervous tissue at the anterior end, and with a centralized nervous system in many lineages. Bilaterians establish several body axes (anterior-posterior, dorsal-ventral, left-right) that guide development and movement. Major subgroups differ in early development: protostomes, in which the mouth forms from the first embryonic opening, and deuterostomes, where the anus forms first.

Bilateralia comprises most animal phyla, including arthropods, mollusks, annelids, and chordates. Echinodermata are deuterostomes and bilaterally symmetric as larvae; many adults exhibit pentaradial symmetry, yet their embryology and genetics place them within Bilateria. The bilateral plan has facilitated diversification through cephalization, segmentation, and varied modes of locomotion, contributing to rapid ecological and morphological diversification during the Cambrian and later periods.