Sea Urchins and Sand Dollars: Echinoidea

Sea urchins and sand dollars are spine-covered sea animals with five arms and prickly skin. Most sea urchins are almost spherical and have five-way symmetry. Sand dollars usually are flat and have two-way symmetry. Sea urchins and sand dollars have rows of tube feet that run from the anus (AY-nuhs), which is on the top of the animal, to the mouth, which is on the bottom of the animal. At the top of the shell is a small plate with holes that allow seawater to pass into a system of tubes running through the body to the tube feet. Muscles contract to draw seawater into the tubes and send it to the tube feet, which then extend under the force of the water pressure.

The shell of sea urchins and sand dollars is made up of tightly packed plates. This design keeps cracks from spreading if the shell is damaged. The shell of the smallest urchin is 0.2 inches (5 millimeters) across, and that of the largest is about 15 inches (38 centimeters) across.

All sea urchins are covered with moveable spines, but the structure of the spines varies among species. Some spines are thick and blunt; others are long, pointed, and venomous. Spines are used for movement, for fighting predators, for camouflage from predators, and to make shade for protection from direct sunlight.