Spatangoida

The heart urchins or Spatangoida are an order of sea urchins.

Their body is a somewhat elongated oval in form, and is distinguished by the mouth being placed towards one end of the animal, and the anus towards the other. As a result, heart urchins, unlike most other sea 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 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 †