Sea urchins or urchins are usually spiny, spherical animals, echinoderms in the class Echinoidea. About 950 species live on the sea floor, inhabiting all oceans and depth zones from the intertidal zone to 5,000 meters (16,000 ft; 2,700 fathoms). Their research (hard shells) are round and spiny, typically 3–10 cm (1–4 in) across. Sea urchins move slowly, crawl with their tube feet, and sometimes push themselves with their spines. They mainly feed on algae, but also eat slow-moving or sessile animals. Their predators include sea otters, starfish, wolf eels and triggerfish. Like other echinoderms, hedgehogs have quintuple symmetry in adults, but their pluteus larvae have bilateral (mirror) symmetry, indicating that they belong to the Bilateria, the large group of animal phyla that also includes chordates, arthropods, annelids, and molluscs. They are widely distributed across all oceans, all climates from tropical to polar and inhabit marine benthic (seabed) habitats from rocky coasts to hadal zone depths. Sea urchins have a rich fossil record dating back to the Ordovician, about 450 million years ago. Their closest echinoderm relatives are the sea cucumbers (Holothuroidea); both are deuterostomes, a clade that includes chordates. The animals have been studied since the 19th century as model organisms in developmental biology because their embryos were easy to observe; This has continued with studies of their genomes because of their unusual fivefold symmetry and relationship to chordates. Species such as the pencil urchin are popular in aquariums, where they are useful for controlling algae. Fossil hedgehogs were used as protective amulets.