A mollusk is a soft-bodied animal that usually has a hard shell to protect it. Snails, clams, oysters, and octopuses are all mollusks, which makes this one of the most diverse groups in the animal kingdom.

Most mollusks live in the ocean, though you'll also find them in freshwater lakes and rivers, and even on land (like the garden snails that munch on vegetables). What makes an animal a mollusk? The soft body is key. Many mollusks protect that vulnerable body with a calcium shell they build themselves, adding new layers as they grow. Clams have two shells that hinge together. Snails carry a single spiraling shell on their backs. But some mollusks, like octopuses and squid, have no external shell at all, making them especially flexible and quick.

Mollusks have been around for over 500 million years, and they've found success in almost every watery environment on Earth. Some, like giant clams, stay in one spot their whole lives. Others, like the nautilus, jet through the ocean. The variety is remarkable: from tiny periwinkles clinging to rocks at the beach to giant squids longer than a school bus, prowling the deep sea.