Spatangoida

The heart urchins are members of the order Spatangoida of sea urchins. Their body are somewhat elongated ovals in form, and are distinguished by the mouth being placed towards one end of the animal, and the anus towards the other. As a result, unlike most other sea urchins, heart urchins are bilaterally symmetrical, and have a distinct anterior surface. The presence and position of the mouth and anus typically give members of this group the distinct "heart" shape from which they get their common name. Heart urchins have no feeding lantern, and often have petaloids sunk into grooves. They are a relatively diverse order, with a number of varying species.

==Taxonomy== According to World Register of Marine Species : suborder Brissidina Stockley, Smith, Littlewood, Lessios & MacKenzie-Dodds, 2005 family Asterostomatidae Pictet, 1857 family Brissidae Gray, 1855 family Palaeotropidae Lambert, 1896 superfamily Spatangoidea Fischer, 1966 family Eupatagidae Lambert, 1905 family Eurypatagidae Kroh, 2007 family Loveniidae Lambert, 1905 family Macropneustidae Lambert, 1905 family Maretiidae Lambert, 1905 family Megapneustidae Fourtau, 1905 † family Spatangidae Gray, 1825 family Hemiasteridae H. L. Clark, 1917 suborder Micrasterina Fischer, 1966 family Aeropsidae Lambert, 1896 family Micrasteridae Lambert, 1920a family Palaeostomatidae Lovén, 1868 suborder Paleopneustina Markov & Solovjev, 2001 superfamily Paleopneustoidea A. Agassiz, 1904 family Paleopneustidae A. Agassiz, 1904 family Pericosmidae Lambert, 1905 family Prenasteridae Lambert, 1905 family Schizasteridae Lambert, 1905 family Somaliasteridae Wagner & Durham, 1966a † family Toxasteridae Lambert, 1920a †