The sea urchin is a spiny, hard-shelled animal that lives on the rocky seafloor, from shallow waters to great depths. These globular marine invertebrates move very slowly along the seabed. There are about 700 different species of sea urchins worldwide.
Many sea urchins have venomous spines.
Adult sea urchins have five-sided radial symmetry. Their skin has hard, chalky plates, and is called the test. Sea urchins have a globular body and long spines that radiate from the body. The spines are used for protection, for moving,and for trapping drifting algae to eat. Among the spines are five paired rows of tiny tube feet with suckers that help with locomotion, capturing food, and holding onto the seafloor.