Vertebrates are the animals with backbones. All vertebrates are chordates. A chordate is an animal that at some time in its life has a large nerve code running down its back. In vertebrates, a backbone surrounds the nerve cord.

Vertebrates have an endoskeleton, which is an internal skeleton that supports the body. Most vertebrates have two sets of paired limbs, such as fins, arms, or legs.

The vertebrates are grouped into seven classes.

• Jawless fish include hagfish and lampreys. They are similar to the first vertebrates that appeared about 540 million years ago. Lampreys use their mouths to grip other fish and feed on their blood. Hagfish eat bits of dead fish or sea worms.

• Cartilaginous fish include sharks, rays, and skates. Their skeletons are made of cartilage. Cartilage is softer than bone. It is the same type of tissue in the tip of your nose and the flaps of your ears.

• Bony fish are the largest class of fish. Their skeletons are made of bone, which is much harder than cartilage. Bony fish also use a swim bladder to rise and sink. A swim bladder is an expandable air sac. The air changes the fish’s density.

• Amphibians include frogs, toads, and salamanders. Most live in water when young and on land as adults. The early amphibians were the first tetrapods, meaning they had four feet.

• Reptiles of today include snakes, turtles, lizards, and crocodiles and alligators. But during the “age of reptiles”—which began 300 million years ago—dinosaurs and other reptiles were the most common kind of vertebrate on Earth. Reptiles have lungs and breathe air. They have hard scales. Most lay soft, leathery eggs. All reptiles are cold-blooded, meaning they cannot automatically keep steady body temperatures.

• Birds have feathers, wings, lightweight bones, and beaks without teeth. All birds are warm-blooded, meaning they always keep a warm body temperature. Scientists use evidence from fossils to argue that birds are close relatives of dinosaurs.

• Mammals feed their young with milk made by the mother. The milk is made in organs called mammary glands. Mammals are covered in hair or fur to keep them warm. Sweat glands help them cool off.

Over Earth’s long history, most species that developed have gone extinct. Today, human actions often threaten the life of a species. Overhunting, cutting down forests, draining marshes, and pollution can all kill off species. Building parks, passing laws, and stopping pollution can save species.