Sea urchins are invertebrate animals belonging to the class Echinoidea in the phylum Echinodermata. The animal is encased in a calcareous test which is globular, oval or discoidal in shape. The test is covered with spines of various length, which fall off after the animal’s death. Sea urchins have adapted to a wide range of marine habitats from shallow to deep sea. The number of known species is around 160 in Japan and 900 in the world. Species living on rocks graze on algae, and those burrowing into soft sediments ingest particulate organic matter.