Sea urchins and sand dollars are called echinoids. They look like a pincushion. The type here in the Kaibab had very thick, bumpy spines. Shown on the right is a partially concealed spine. The swollen base of the spine is to the right where there is a hole. The left photo shows a single plate of the urchin’s body with concentric circles called the axial boss—this is where the spine attached. Dozens of such plates make up the urchin’s pincushion-shaped body—like crinoids, they fall apart after death.
Sea urchins are related to starfish and crinoids. Most such animals display a five-rayed or pentamerous symmetry in some parts of their bodies, and most are spiny. “Echino-“ means spine.