Urchins (Class Echinoidea) are a type of echinoderm, related to sea stars and sea cucumbers. They are mostly spherical and spiny, moving by way of tube feet that are interspersed in their spines. When they die, urchins leave a round shell or "test", which often washes up on beaches and is commonly collected. Species can usually be identified as a live animal, or just from the test.
