Sea urchins are echinoderms in the class Echinoidea, that possess a hard calcareous shell (test) armed with spines, which may be long and pointed, or short and pointed, or dull. The spines are used for locomotion (along with tube feet), protection, and for trapping drifting food particulates, such as algae. Most sea urchins are algal grazers but some feed on sponges, ectoprocts and ascidians, and others on detritus. Sea urchins help to keep corals free of overgrowing algae.