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symmetry, biological, similarity or balance between parts of an organism so that when a straight cut is made through a point or along a line, equal, mirror-image halves are formed. Symmetry in body shapes is related to the lifestyles of organisms. Asymmetry, or the absence of symmetry, most often occurs in sessile organisms or in slow-moving forms such as amebas. Most other organisms can generally be classified in three groups with respect to symmetry type.
In spherical, or point, symmetry, any straight cut through the central point of a sphere divides it into mirror-image halves. Point symmetry, often called universal symmetry by biologists, is seen in some floating animals with radiating parts, such as the single-celled protozoans of the order Radiolaria.
Radial, or line, symmetry, as exemplified by a cone or a disk that is symmetrical about a central axis, is especially suitable for sessile or floating animals. Most radially symmetrical animals are symmetrical about an axis extending from the center of the oral surface, which contains the mouth, to the center of the opposite, or aboral, end. Radial symmetry is seen in sessile organisms such as the sea anemone, floating organisms such as jellyfish, and slow-moving organisms such as sea stars, or starfish. Many jellyfish have four radial canals and are said to have tetramerous radial symmetry; sea stars, with five arms, have pentamerous radial symmetry. Many flowers, such as dandelions and daffodils, are radially symmetrical. Nonradial parts, such as the slit-shaped gullets of sea anemones, are often present in otherwise radial animals.
In plane, or bilateral, symmetry, one particular plane, termed the sagittal plane, divides the body into two equal halves, usually right and left halves that are mirror images of each other. Flowers such as orchids and sweet peas are bilaterally symmetrical. Bilateral symmetry is most suitable for actively moving organisms, as it permits streamlining, and is the most common symmetry among animals. In animals this symmetry type also favors the formation of main nerve centers and special sense organs and contributes to cephalization, or the evolutionary development of a head.
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