Sea Urchins

Sea urchins (Echinoidea) belong to the echinoderms. Like other Echinoderms, they have a five-ray symmetry. The body consists of fused calcareous plates. The spines were mobile and were attached to the calcareous plates.  Regular sea urchins are completely five-rayed symmetrical and have their anus on the top and their mouth on the bottom. Irregular sea urchins often have their mouth and anus both on the bottom and are irregular in shape.