Pentamerism in Echinoderms and the Calcite Skeleton

ONE of the many puzzling features of the Echinodermata is their pentameral symmetry. The theory put forward by Nichols¹ goes far towards accounting for the origin of pentamerism. Taking as his starting point a young echinoderm with its aboral surface covered only by an anal plate and a surrounding ring of plates, Nichols reasons that a strong outer covering (test) would be best developed if there were as few plates as possible in the circum-anal ring, with short sutures between each plate. An even number of plates would have a long suture of weakness cutting across the resultant test. With three plates, the sutures would be at 120°, again a weak arrangement. With five plates of uniform size, however, the sutures are few and short and each suture is opposite a solid plate.