Symmetry is the body arrangement in which parts lie on opposite side of the axis are identical. If the animals lack a definite body plane or irregular shaped and any plane passing through the center of the body does not divide them into two equal halves, these are known as asymmetrical, e.g., sponges, adult gastropods.
When any plane passing through the central axis of the body divides an organism into two equal parts, it is known as radial symmetry. They have a top and bottom side, e.g., cnidarians. Echinoderms have five planes of symmetry and show pentamerous radial symmetry. Animals which have two pairs of symmetrical sides are biradially symmetrical. Animal which have two similar halves on either side of the control plane show bilateral symmetry.
