No bones, no problem. Sea urchins look on the outside like spiny circles. Inside, they have no bones—just a shell called a “test.” You might spot a test at the beach. Look for a rounded shell (white, greenish, or purple) covered in little dots. When the urchin was alive, a little spine stuck out from each pearl-shaped mark. Can’t locate a test in the wild? Check a beach shop.

Here, there, and everywhere. Sea urchins work away busily in every ocean, in shallow water and deep, and at every temperature—even freezing! God made about 950 different kinds of these boneless spike balls. They eat mostly algae and kelp, though some prey on other little animals such as barnacles and mussels. You’ll find an urchin’s mouth on its underside. There, five bony plates clamp together—kind of like a beak closing. (This “beak” has a name: Aristotle’s lantern.) Some urchins have maws tough enough to drill into rock. These rock holes make good hiding spots.

(Almost) never fear. Urchins use their teeth, spiky spines, and suction-y spines (called tubefeet) to move around. Most urchin spines won’t harm humans. But some species sport spines loaded with toxic venom. You don’t want to step on any urchins . . . but especially not the venomous ones. Some urchin spines are tough enough to pierce divers’ wetsuits. If you ever step on an urchin, remove the spine from your foot right away. If it’s in too deep, a doctor can remove it for you.

Did you know? Sand dollars count as urchins too—but irregular ones. Unlike most urchins, sand dollars are flat instead of rounded.