A sea urchin is an echinoderm, which is a marine creature belonging to the class Echinoidea. Echinoderms are characterized by their radial symmetry, spiny exterior, and a hard, calcium-rich test (shell) that protects their internal organs. Sea urchins primarily inhabit ocean floors and can be found in various marine environments, from shallow coastal waters to deep-sea areas. They play a vital role in marine ecosystems, particularly in grazing on algae and serving as prey for various marine species. Their unique anatomy includes tube feet and a specialized feeding structure called Aristotle’s lantern, which allows them to scrape food from surfaces.