A crustacean is a type of animal with a hard outer shell, jointed legs, and usually two pairs of antennae. Crabs, lobsters, shrimp, crayfish, and barnacles are all crustaceans. They belong to a large group of animals that mostly live in water, though a few (like pill bugs, those little rolly-pollies you find under rocks) live on land.

Crustaceans have a crusty, armor-like covering called an exoskeleton, which protects their soft bodies inside. Unlike your skeleton, which grows as you grow, a crustacean must molt, shedding its old shell and growing a new, larger one. For a short time after molting, the crustacean is vulnerable and soft until its new shell hardens.

Crustaceans range from tiny creatures barely visible to the eye to massive Japanese spider crabs with legs spanning twelve feet. Many crustaceans are scavengers, eating dead plants and animals on the ocean floor, which makes them nature's cleanup crew. Others are predators or filter feeders. When you crack open a lobster tail or watch hermit crabs at the beach, you're observing members of this ancient group of animals that have scuttled through Earth's oceans for over 500 million years.