Five-fold radial symmetry shown by the shell of a sea-urchin.
Radial symmetry is symmetry about a single polar axis. It is a condition rare in nature though not infrequent among protozoans. Less than perfect radial symmetry is more common, five-fold symmetry being characteristic of echinoderms, including sea-urchins and starfish. This five-fold symmetry is shared by no other animals.