Left: An internal flint mould of an echinoid (Echinocorys) from Peacehaven. Right: A small echinoid (Cidaris) from Woodeaton Quarry. Despite their alien appearance, echinoids, or sea-urchins as they are better known, are very common in the seas and oceans of today and are common fossils too.
Sea urchins or urchins (/ ˈ ɜːr tʃ ɪ n z /) are typically spiny, globular animals, echinoderms in the class Echinoidea. About 950 species live on the seabed, inhabiting all oceans and depth zones from the intertidal to 5,000 metres (16,000 ft; 2,700 fathoms).
The sand dollar is a type of sea urchin that appears quite different from it's rounder, spinier cousin, but sea urchins and living sand dollars have more in common than you might realize. Echinoderms Echinoderms are a group of sea-dwelling invertebrate species characterized by tube-like feet, spiny skin, and radially symmetrical bodies.
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.