Dictionary > Symmetry


noun, plural: symmetries
Correspondence of parts (in terms of form, size, distribution, or arrangement) on opposite sides of a dividing plane or on sides around an axis
In biology, symmetry pertains to an attribute of organisms showing 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. In most instances, the parts are not an exact but a near repetition. This also accounts for the patterns in nature. The absence of symmetry is termed asymmetry.
Examples of symmetry in organisms are bilateral symmetry, radial symmetry, and biradial 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).
Radial symmetry is a symmetry in which the sides exhibit correspondence or regularity of parts around a central axis. It is lacking left and right sides (e.g. sea stars, sea anemones, jellyfish, sea urchins, and many flowers). Biradial symmetry is a combination of radial and bilateral symmetries. An example is that of ctenophores (comb jellies).
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