WebSea urchins or urchins ( / ˈɜːrtʃɪnz /) are typically spiny, globular animals, echinoderms in the class Echinoidea. About 950 species live on the seabed, inhabiting all oceans and depth zones from the intertidal to 5,000 metres (16,000 ft; 2,700 fathoms). [1] Their tests (hard shells) are round and spiny, typically from 3 to 10 cm (1 to 4 ...