Sea Urchins

Its round body has a radial arrangement divided into five sections or bands. It has tube feet  which are slender, extensible, and often sucker-tipped, and spines which are movable and the mouth is on the underside of the body.

When a sea urchin dies, all its spines fall off, leaving only its skeleton known as the test. If you look carefully at a test, you can see tiny bumps covering it where the spines were once connected. The base of the spines once fit over the bump like a snug-fitting cap. The spines can rotate extensively around this bump. In a live sea urchin, skin and muscle cover the test and can be pulled on to move the spines.