Fossil sea urchins from Erfoud Morocco.

Sea urchins and sea apples (sea burdock, heart urchin) are spiny, invertebrate aquatic animals. They form the class of Echinoidea which belongs to the phylum of Echinodermata (echinoderms). Sea urchins are usually more or less spherical, heart-shaped or disc-shaped and have an inferior mouth. They have a hard shell that is usually covered with spines. The shell consists of small plates on which there are knobs on which the spines are movably attached by means of a ball and socket joint. The spines serve for locomotion in many species. Some species bury themselves with the help of the spines. The spines also have a protective function. Some species can even emit poison. Between the spines are short-stalked, diverse pedicellariae. These also serve for defense and for keeping the skin clean. See also: sand dollar. From the mouth (underside) to the anus (on the back, at the top = aboral) there are 10 pairs of placed rows of sucking feet in the regular sea urchins (Regularia such as Psammechinus, Echinus). The plates through which these sucking feet come out are called ambulacral plates. The other plates are the adambulacral plates. All Regularia have a built-in chewing apparatus: the Aristotelian lantern. In the irregular sea urchins (Irregularia, such as Echinocyamus, Echinocardium) the sucking feet are located on the upper side of the shell, where they form a flower or star-shaped figure. In this group the anus is located on the side and the mouth on the underside. They also do not have an Aristotelian lantern. The sea urchins usually live on vegetable food or detritus. Some species also take animal food. They usually bring their food to the mouth with tube feet and spines.