- A taxonomic class inside the superclass Cryptosyringida — the ocean urchins.
- The course Echinodermata which include the ocean urchins. Obtained a calcareous layer, frequently almost spheroidal or disk-shaped, composed of numerous united plates, and covered with movable spines. See spatangoid, clypeastroid.
- A class of phylum or subkingdom Echinodermata; the sea-urchins or sea-eggs.
- water urchins and sand dollars