These belong to the bottom-living marine animals from the echinoderms (Echinodermata) strain. There are about 950 species of sea urchins. Their bodies are almost completely enclosed by a skeleton of limestone slabs and carry movable spines. Sea urchins are found in all the world's oceans, but especially on the coasts of Europe, Asia, Central America, Australia and the Pacific islands. The limestone plates of the skeleton are pierced and equipped with suction feet (ambulatory sacral feet). These are used to hold on to the prey, to move around, but also for breathing and sensory perception.
The echinoderms prefer to live on sandy or rocky ground. They often hide in small cavities or between stones, so that bathers easily overlook them and step on them.