- Norwegian: sjøpiggsvin
They have a globoid shape without any arms are features the sea urchins share with the sea cucumbers. (The tentacles surrounding the mouth of sea cucumbers have evolved from tube feet.) Like other echinoderms, they have a fivefold symmetry and move around on a large number of hydraulically powered tube feet. Sea urchins have a outer shell equipped with protecting spines.
Together with the sea cucumbers, the sea urchins constitute the subphylum Echinozoa. Most sea urchins in the Norwegian fauna are members of one of two orders:
- There are seven species of regular (globular and symmetrical) sea urchins (order Camarodonta), many of them abundant along the entire Norwegian coast.
- There also seven species of irregular (asymmetrical) sea urchins (order Spatangoida), but they are frequently encountered by scuba divers, as they live buried in sandy or muddy substrate.