Radial and Biradial-Metazoa

Radial symmetry is the symmetry in which the parts are so arranged around a central axis or shaft, like the spokes of a wheel, that any vertical cut through the axis would divide the whole animal into two identical halves. The common jelly fish and hydra (cnidaria) - exhibit radial symmetry. The starfish and their relatives have a modified form of radial symmetry. They can be divided along 5 planes, each giving two distinct halves. This is known as pentermerous symmetry.

One side of the body has the mouth and is known as the oral surface; the opposite side is aboral. Cuts made along the oral-aboral axis will result in identical halves. Biradial symmetry is a variant of this and it is found in sea anemones and ctenophores. Though the animal appears to be radially symmetrical, it can be divided only into two equal halves along two per-radial positions - along the tenticular plane and along the sagittal plane at right angles to it. Radial and biradial animals are usually sessile, floating freely or weak swimmers. These animals are called the Radiata.