Echinoidea (Sea Urchins)

Sea urchins or urchins (/ˈɜːrtʃɪnz/) are spiny, globular animals, echinoderms in the class Echinoidea. About 950 species inhabit all oceans, and zones from the intertidal to 5,000 metres (16,000 ft; 2,700 fathoms) deep. Their tests (hard shells) are round and spiny, typically from 3 to 10 cm (1.2 to 3.9 in) across. Sea urchins move slowly, crawling with their tube feet, and defended by their sharp spines, which are sometimes toxic. They feed primarily on algae but also eat slow-moving or sessile animals. Their predators include sea otters, starfish, wolf eels, and triggerfish. The name "urchin" is an old word for hedgehog, which sea urchins resemble; they have archaically been called sea hedgehogs. The name is derived from Old French herichun, from Latin ericius, hedgehog.
Like other echinoderms, urchins have fivefold symmetry as adults, but their pluteus larvae have bilateral (mirror) symmetry, indicating that they belong to the Bilateria, the large group of animal phyla that includes chordates, arthropods, annelids and molluscs. They are widely distributed across all the oceans, all climates from tropical to polar, and inhabit most marine habitats from rocky shores to hadal zone depths. Echinoids have a rich fossil record dating back to the Ordovician, some 450 million years ago. Their closest relatives are the sea cucumbers (Holothuroidea) among the echinoderms, which are deuterostomes, along with the chordates.
Humans often notice sea urchins when swimmers are injured by the animals' sharp and sometimes poisonous spines. The animals have been studied since the 19th century as model organisms in developmental biology, as 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 slate pencil urchin are popular in aquariums, where they are useful for controlling algae. Fossil urchins have been used as protective amulets.
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== Orders ==
* Camarodonta
* Diadematoida