What is Sea urchin?
{{Taxobox | name = Sea urchin | image = SeaUrchin.jpg | image_width = 230px | image_caption = | regnum = [[Animal]]ia | phylum = [[Echinodermata]] | classis = '''Echinoidea''' <!-- | subdivision_ranks = Subclasses | subdivision = *[[Euechinoidea]] **Superorder [[Atelostomata]] ***Order [[Cassiduloida]] ***Order [[Spatangoida]] (heart urchins) **Superorder [[Diadematacea]] ***Order [[Diadematoida]] ***Order [[Echinothurioida]] ***Order [[Pedinoida]] **Superorder [[Echinacea (animal)|Echinacea]] ***Order [[Arbacioida]] ***Order [[Echinoida]] ***Order [[Phymosomatoida]] ***Order [[Salenioida]] ***Order [[Temnopleuroida]] **Superorder [[Gnathostomata (echinoid)|Gnathostomata]] ***Order [[Clypeasteroida]] (sand dollars) ***Order [[Holectypoida]] *[[Perischoechinoidea]] **Order [[Cidaroida]] (pencil urchins) -->| subphylum = [[Echinozoa]] }} '''Sea urchins''' are spiky members of the [[Class (biology)|class]] '''Echinoidea''' of the [[phylum]] [[Echinodermata]]. Like the rest of the echinoderms, they are entirely [[Ocean|marine]]. They are globe-shaped, and protected by [[calcareous]] plates and spines. ''Urchin'' is an old word for [[hedgehog]], and in many foreign languages these animals are called sea hedgehogs. Like other echinoderms they have five-fold [[symmetry]] (called pentamerism) and move by means of hundreds of tiny, transparent, adhesive 'tube feet'. The symmetry is not obvious in the living animal, but is easily visible in the dried test. Sea urchins mostly feed on [[algae]] and small animals. They have a special chewing apparatus called ''[[Aristotle's lantern]]'', after the [[Ancient Greece|Greek]] [[philosopher]] [[Aristotle]] who was fascinated by sea urchins. With this apparatus they can scrape organisms stuck to the surface over which the urchin is moving. == Spines== The spines are long and sharp in some species,<ref>Rubber slippers will protect feet</ref> and protect the urchin from [[predator]]s. The spines inflict a painful wound when they penetrate human skin, but are usually not dangero