Vertebrates are characterized by the presence of a vertebra or backbone; a skull; skeletal bones that form an endoskeleton; a bilaterally symmetrical structure; and two pairs of appendages, such as fins, limbs or wings. Vertebrates' organ systems are more developed than those of invertebrates, and their nervous systems are more complex.
Vertebrates are categorized into five groups: mammals, birds, fishes, amphibians and reptiles. They comprise around three percent of more than 1 million animal species.
A vertebrate's central nervous system is composed of a well-developed brain, found in the skull, and a spinal cord encased by the vertebral column. Vertebrates have both axial and appendicular skeletons that combine to make a highly developed internal skeleton. They possess a closed circulatory system, meaning that their blood always flows in blood vessels, and their respiratory system is located close to the throat. Their bodies are separated into ventral and spinal areas, and their skins are covered with protective outgrowths, such as hairs, feathers or scales. Vertebrates' sense organs are also well developed.
Vertebrates thrive in various types of environments. Mammals live in forests, bodies of water and underground. Most birds are capable of flying and are found in forests, near the shore and in open fields. Amphibians primarily live on land but also visit the water for to lay eggs and reproduce. Because fish have gills, they cannot breathe air and can only survive in water.