Bilateral symmetry

Bilateral symmetry is a body plan in which a single plane divides an organism into approximate mirror-image right and left halves. It is associated with distinct front and rear ends and directional movement.

Symmetry (biology): A correspondence of parts in an organism’s body under a transformation such as reflection or rotation. Bilateral symmetry is one of the main forms of symmetry used to describe animal body plans.
Left-right asymmetry: A consistent difference between the left and right sides of an organism’s body or internal organs. Bilateral symmetry is approximate; organ placement and other traits often break left-right equivalence.
Bilateria: The animal clade whose members develop bilateral organization at some stage of their life cycle. This major animal group is named for the body plan at the center of this concept.
Cephalization: The evolutionary concentration of sensory organs and nervous tissue at an animal’s anterior end. Directional movement in bilaterally organized animals favors gathering sensory and neural structures at the front.
Radial symmetry: A body organization in which parts are arranged around a central axis. Radial organization contrasts with a body divided into distinct right and left halves.
Plane of symmetry: A plane that divides a figure or body into two parts related by reflection. The defining plane separates the approximate left and right halves of a bilaterally symmetric body.
Nodal signaling pathway: A signaling system involving Nodal proteins that helps establish left-right patterning during vertebrate development. Nodal signaling assigns left-right identity rather than making the two sides developmentally identical.
Urbilaterian: The hypothetical last common ancestor of living bilaterian animals. Its reconstructed traits help explain how bilateral organization may have arisen early in animal evolution.
Locomotion: The movement of an organism from one place to another. Bilateral organization is often associated with coordinated, forward-directed movement.
Biradial symmetry: A body arrangement with two planes that can divide it into similar halves. It combines aspects of radial organization with a limited number of mirror-like divisions.

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