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Bilateral symmetry

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noun, plural: bilateral symmetries

A form of symmetry in which the opposite sides along a midline is a duplicate of the other in terms of body parts or appearance


In biology, symmetry is a characteristic of certain organisms in which there is regularity in parts on a plane or around an axis. An organism that is symmetrical (showing symmetry) would have a balanced distribution of duplicate parts on each side of the axis. It may not necessarily be an exact duplicate but an approximate repetition. An example of symmetry in organisms is bilateral symmetry.

Bilateral symmetry is a form of symmetry in which the opposite sides are similar. The external appearance is the same on the left and right sides in a sagittal plane (such as the body plan of most animals, including humans). When an organism shows bilateral symmetry, the body plan is divisible into equal mirror halves in a sagittal plane. The internal body parts such as organs may not necessarily be symmetric. A taxonomic clade, Bilateria, includes animals with bilateral symmetry. These animals (also referred to as bilaterians) have left and right sides to distinguish them from those with different form of symmetry (e.g. radial symmetry) as well as those lacking symmetry (asymmetry).

Bilateral symmetry in the plant kingdom is exemplified by the orchid and pea families.


  • plane symmetry


  • radial symmetry

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