Fish are the oldest known vertebrates. They 
live in water and are often covered in scales. Most fish have a bony skeleton. However, sharks and rays have a skeleton made of cartilage, a strong and flexible tissue.
Animals that live part of their life on land and part of their life in water are called amphibians. Frogs and salamanders are examples of amphibians. Most young amphibians live in water and breathe with gills. As they grow they form lungs and legs and move onto land.
The first vertebrates to live completely on land were reptiles—for example, lizards, snakes, and crocodiles. Reptiles have dry, scaly skin. Some reptiles, such as turtles, spend a lot of time in water, but they breathe air.
Vertebrates that have wings and feathers are called birds. Most, but not all, birds can fly.
Vertebrates that feed their babies with milk from the mother are called mammals. Mammals include mice, deer, seals, whales, monkeys, and humans. Mammals live on land in all parts of the world. A few types, such as whales, live in the ocean but still must breathe air.
