When the sea urchin is alive its round, rigid case is covered with spines (sometimes red-tipped) resembling the prickly back of a hedgehog. They are not good to tread on with a bare foot! The living, breathing sea urchin has a mouth and anus but no trailing arms or legs.
When sea urchins die they are washed ashore, and in the process are rolled around by waves, and crashed onto rocky reefs and sand bars until all their spines are either broken or worn away. Usually the rounded case is broken too.
Every now and then though, a perfect sea egg washes up onto the sand, lying as treasure in a tangle of flotsam. Whenever I find and pick up one of these exquisitively patterned eggs, I think of it as a priceless gift from the sea.