Sea urchins are the Class Echinoidea of the Phylum Echinodermata.Like the rest of the Echinoderms, literally “spike-skinned”, they are entirely marine.They are usually globe-shaped, and protected by calcareous plates and spines. p65 Urchin is an old word for hedgehog, and in many foreign languages these animals are called “sea hedgehogs”.. Like other echinoderms they have five-fold symmetry …

: https://simple.wikipedia.org/wiki/Echinoidea