What are echinoids?
The Class Echinoidea are marine animals closely related to starfish, sea cucumbers, featherstars, sea lilies and brittle-stars. Together these animals constitute the Phylum Echinodermata, meaning spiny skinned. In echinoids numerous plates make up the skeleton, called the 'test'. This forms a protective casing around the gut and other internal organs.
The plates are actually embedded in the 'skin' of the sea urchin. On the outside of the test are tube feet, which help in feeding, movement and respiration, and also articulated spines which act as a defence. Some species of echinoids inhabit rocks on the sea-bed but others burrow down beneath the surface into the sand or mud.
Fossil echinoids can be found in rocks back to the Ordovician, up to 450 million years old. However, Palaeozoic species generally had tests with plates that did not interlock and tended to fall apart during fossilization. Younger fossil sea urchins, in contrast, often have intact tests, sometimes even with the associated spines.
An estimated 800 species of echinoids are alive at the present day.