Sea urchins belong to the Echinoidea class, and the Echinodermata phylum. Other animals in the Echinodermata phylum include sea stars, brittle stars, and others. Sea urchins are round in shape and are called ‘regular echinoids.’ Two subclasses from this group are the Euechinoidea and Cidaroidea. The Euechinoidea includes most all modern sea urchins and Cidaroidea includes most primitive sea urchins. ‘Irregular echinoids’ are those with irregular and unsymmetrical shapes such as sand dollars and sea biscuits.