Animal symmetry

Animal symmetry relates the organization of parts in animal bodies to the geometrical design that each type suggests. Spherical symmetry is exhibited by some protozoa, such as 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 coelenterates. 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, five in most echinoderms. Ctenophores and many sea anemones and corals possess biradial symmetry, 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, each a mirror image of the other.

Types of symmetry and the axes, planes, and regions in animal bodies. (After T. I. Storer et al., eds., General Zoology, 6th ed., McGraw-Hill, 1979)