Storer, Tracy I. Formerly, Department of Zoology, University of California, Davis, California.
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The symmetrical disposition of organs and other constituent parts of the bodies of living animals with respect to imaginary axes. Animal symmetry relates the organization of parts in animal bodies to the geometrical design that each type suggests. The term asymmetrical applies to most sponges and some protozoa because the body lacks a definite form or geometry, and it cannot be subdivided into like portions by one or more planes. Spherical symmetry is exhibited by some protozoa, including the Heliozoia and Radiolaria. The body is spherical, with its parts concentrically around, or radiating from, a central point. Radial symmetry is exemplified by the echinoderms and most cnidarians. The body is structurally a cylinder, tall or short, having a central axis named the longitudinal, anteroposterior, or oral-aboral axis (see illustration). Any plane through this axis divides the animal into like halves. Often several planes, from the axis outward, can divide the body into a number of like portions, or antimeres, with five being most common in echinoderms. Ctenophores and many sea anemones and corals possess biradial symmetry, which is basically radial but with some parts arranged on one plane through the central axis. Most animals have bilateral, or two-sided, symmetry, in which a median or sagittal plane divides the body into equivalent right and left halves, with each being a mirror image of the other. See also: Animal; Bilateria
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