biradial

Biradial symmetry is a form of body organization that sits between radial and bilateral symmetry. In biradial organisms, two planes of reflection symmetry extend through a common longitudinal axis, yielding a pattern that is symmetric along two directions rather than along all possible radial planes or along a single left-right plane.

In practice, biradial symmetry is described as two planes of symmetry that share the main axis of the body. This contrasts with radial symmetry, where any plane through the axis provides a mirror image, and with bilateral symmetry, where there is a single plane dividing the body into left and right halves. Biradial patterns can result in an organism that appears to have a defined front region while retaining a radial organization around the axis.

Biradial symmetry is relatively uncommon in mature, free-living animals and is most often discussed in the contexts of developmental biology, morphology, and paleontology. It has been noted in certain cnidarians and other colonial or loosely organized groups, particularly in transitional developmental stages or in fossils where the arrangement of body parts shows two primary planes of symmetry along the main axis. Because definitions and usage can vary among disciplines, biradial symmetry is sometimes described as an intermediate or transitional form between radial and bilateral symmetry.

See also: bilateral symmetry, radial symmetry, pentaradial symmetry.