The echinoids are encased by a rounded skeleton called a test. Their bodies have an enormous number of sharp spines protruding in all directions, providing protection from potential predators. The spines attach to the body with a ball-and-socket like attachment, which can also aid in movement.
Echinoids have five paired rows of podia (structures somewhat like a foot) which, when extended, are long enough to reach beyond their spines. Their mouth (containing five teeth that are pointed towards the centre) is located on the underside of the animal.
Herbivorous urchins use their podia to pull themselves against the ocean floor and gnaw at algae with their mouth.
Echinoids are also a model animal in developmental biology and were the first organism used to prove that sperm fertilised the ovum.