Sea urchins are normally spherical, spiny animals with a protective test formed from a set of fused ossicles. They possess a system of bones and teeth which form an efficient rasping set of jaws – Aristotle’s lantern – used for scraping algae from rocks. The irregular urchins have reduced pentaradial symmetry as an adaptation to a burrowing existence. Of these, the heart urchin lacks Aristotle’s lantern. Echinoids use their movable spines as well as their podia to roam across the sea bed.
Heart urchins burrow in sand with special broad spines. They gather food-laden sand with their long tube feet and stuff it into their mouths with stout buccal tube feet.