Sea urchins or urchins are small, spiny, globular animals which, with their close kin, such as sand dollars, constitute the class Echinoidea of the echinoderm phylum. There are c. 950 species of echinoids inhabiting all oceans from the intertidal to 5000 meters deep. Their shell, or “test”, is round and spiny, typically from 3 to 10 cm (to in) across. Common colors include black and dull shades of green, olive, brown, purple, and red. They move slowly, feeding mostly on algae.