Echinoids have lived in the seas since the Late Ordovician, about 450 million years ago. The living representatives of echinoids are the familiar sea-urchins that inhabit many shallow coastal waters of the world.
Echinoids are marine animals belonging to the phylum Echinodermata and the class Echinoidea. They have a hard shell (test) covered with small knobs (tubercles) to which spines are attached in living echinoids. The test and spines are the parts normally found as fossils.
Spines, some poison-tipped, help protect echinoids from their predators, which include other echinoids, crustaceans, octopuses and fish. Some fossil echinoids made themselves less palatable as prey by having large solid spines. Echinoids also use their spines for moving around the sea-bed, and in some groups they are specially adapted for burrowing.
The Echinodermata take their name from the Greek words for spiny skin, a very conspicuous feature of many living echinoids. Since ancient times they have been revered as objects of religious or superstitious power.
In Denmark, echinoids were thought to be thunderbolts, and used to protect against lightning and witchcraft. An ancient Celtic tradition held that they were formed from froth thrown into the air by snakes, and retained magical powers if caught before falling to the ground.