Echinoidea contains animals known as sea urchins and sand dollars that have
compact bodies enclosed in an endoskeletal test made of closely fitted plates.
Sea urchins have a globular shape with moveable spines. Although the spines of
most species are fairly blunt, some have long, sharp spines that can break off
easily in the skin and cause great pain! Often these spines are attached to
venom glands, making an encounter with them even more unpleasant.