Animals that walk on water pressure and chew with five teeth.

Sea stars and sea urchins are echinoderms, a group of marine animals with a remarkable water-vascular system.

Instead of a conventional blood-and-heart circulation system, sea stars use seawater-filled canals and chambers to operate hundreds of tiny tube feet. The feet extend, grip, and release in coordinated waves, allowing the animal to crawl across rock and hold onto prey.

A sea star can maintain a surprisingly powerful grip. Its tube feet can pull repeatedly on a mussel until the mussel's closing muscles become exhausted.

Sea stars have an extraordinary way of feeding. When a sea star manages to pull a mussel's shells slightly apart, it can push part of its stomach out through its mouth and into the opening.

Digestive enzymes break down the mussel's soft tissues outside the sea star's body. Once feeding is finished, the stomach is pulled back inside.

Green sea urchins use a completely different tool: Aristotle's lantern, a complex five-toothed jaw on the underside of the animal. They scrape algae from rock and also feed on kelp and other seaweeds.

Heavy urchin grazing can dramatically reduce kelp in some coastal ecosystems.

A sea star that loses an arm can often regenerate it over time. But not every detached arm can become a whole new sea star.

In some species, a detached arm containing enough of the central disc can regenerate into a complete animal. Other species can reproduce by splitting their bodies and regenerating the missing parts.

So a missing arm isn't necessarily the end — for some sea stars, it can be the beginning of something new.