Sea urchins are a group of marine invertebrates that today can be found in almost every major marine habitat from the poles to the equator and from the intertidal zone to depths of more than 5,000 metres. There are around 800 extant species and the group has a long and detailed fossil record stretching back about 450 million years ago to the Late Ordovician Period.
They belong to the Echinoidea, one of the five classes of the phylum Echinodermata, the others being holothurians, crinoids, starfish and brittlestars. Like all echinoderms, echinoids have a skeleton composed of calcitic plates embedded in their skin (their skeleton is internal, like ours). This skeleton has a very particular structure, termed stereom. In almost all groups of echinoid the plates are firmly bound together to form a solid skeleton, called the test. Plates in the test are arranged into columns radiating from an aboral apical zone to the mouth. In regular echinoids the test is globular and perfectly pentaradiate, but in irregular echinoids there is a secondary bilateral symmetry superimposed upon the primary pentaradiate pattern.
Sea urchins have a very characteristic larval development, passing first through a bilateral planktonic larval stage. After several weeks this larva undergoes a metamorphosis which gives rise to the adult body form, and the juvenile settles to the sea floor. The planktonic larva is morphologically complex and the various taxonomic groups all have their own distinctive larval forms.
All echinoderms have tube-feet and in echinoids these play a very important role in feeding and respiration. Echinoids move by means of spines and climb and cling on to hard substrata by means of their tube-feet. The spines also offer the primary means of defence. Sea urchins feed in a variety of ways. Regular echinoids have a powerful internal jaw and graze on algae or sedentary organisms. Sand dollars and cassiduloids bulk-process sands to feed on the tiny organic particles trapped in the sediment, while heart urchins are selective deposit feeders, using specialised tube-feet to pick up organic-rich detritus from the sediment.
Echinoids differ from other echinoderm classes in that the majority of their skeleton is built up of oral elements (ambulacral and interambulacral plating systems). The aboral plating system is reduced to a tiny periapical region.