Echinoids are divided into regulars which are perfectly circular when viewed from the top and irregulars which are weakly to strongly heart-shaped in outline. Whilst regular echinoids show perfect radial symmetry of their water vascular structures and associated plates, irregular echinoids show bilateral symmetry secondarily superimposed on this radial arrangement. The mouth in regular echinoids is in the centre of the bottom surface, which in life is in contact with the sediment. Within the mouth cavity a complex array of plates supports the jaw apparatus, named the ' Aristotles Lantern '. The lantern contains five jaw plates, each bearing a tooth, and is used to rasp the surface of the sediment for algae and other food particles.
Movement is mainly through extension of tube feet on the lower surface.
Many regular sea-urchins show large spines borne by prominent bosses, termed tubercles , these serve to protect the animal from predators and strong currents. The also have specialised pincer-like pedicellariae on their outer surfaces responsible for cleaning and protection. In Diadema the spines are elongate and slender but bear barbs and chemical irritants to deter attackers. Spines are modified either to maintain circulation of water in the burrow, to excavate sediment or to locomote. The lantern is absent and tube feet are responsible for feeding, digging and respiration.