All vertebrates have an inner skeleton, including a skull, backbone, and ribs. These complex animals vary in size from less than 0.4 in (1 cm) to over 100 ft (30 m) long.
Fish are the largest vertebrate group, with more species than all the other groups put together.
The skeleton is a strong frame that supports the body and anchors the muscles. Bones such as ribs protect the heart and other delicate parts, and the skull shields the brain. Most vertebrates have bony skeletons, but shark skeletons are rubbery cartilage.
Most vertebrates apart from fish have four limbs. In birds and bats, the front limbs evolved (developed) into wings for flying. A fish’s limbs are its fins, which vary in number between species. Snakes evolved from four-legged ancestors millions of years ago.