Diadema antillarum
The Diadema antillarum is the most commonly known sea urchin. The skeletal structure of the Diadema antillarum consists of radial symmetrical plates that form a disk shape. The test which is the outer shell of the body, is made up of calcium carbonate and is covered in a thin epithelium layer. The test is a rigid structure meant to protect the inner body but is also fragile and is almost never found whole on the shore or seabed after death. The size of this species varies based on the conditions and population density but typically the height measures up to 50% if the width. Diadema antillarum are on average, 10 cm in diameter as adults. Their spines are hollow, crystalline, bone-like structures made primarily of magnesium and calcium.