The chordates are either asymmetrical or bilaterally symmetrical. Chordates all share, at some time in their life, the characteristic of a notochord. The notochord is a cartilagelike rod that provides longitudinal support. Chordates are also characterized by a hollow, dorsal nerve cord. At least in the embryo, all chordates have pharyngeal slits, which differentiate into other structures in most chordates. The chordates range from the simple juglike tunicates to a broad range of vertebrates. The vertebrates evolved an internal skeleton, which houses and protects the nerve cord. There is also a broad range of diversity in the class Vertebrata. The vertebrates include the jawless fishes, Agnatha, that retain the notochord in the adult. Chondrichthyes are fishes that have cartilage instead of bones. The Osteichthyes are vertebrates that have bones. Leaving the fishes behind and invading the land are the amphibians. Better adapted to a life on land are the reptiles. The birds live on land, in the air, and on, but not in, the water. The mastery of all three environments is left to the mammals.
Each group of vertebrates can lay claim to evolutionary advances. The fish were the first to have a skeleton for support and a cranium for protection. Fish also evolved the single-loop circulatory system and a true heart. Each successive group of vertebrates improved on the heart design. Legs and lungs were major advances made by the amphibians. Reptiles evolved the allantoic egg, dry skin, and a thorax. Obviously, birds are associated with flight. To achieve efficient flight, birds had to evolve feathers, a flight skeleton, efficient respiration, and efficient circulation. While most of the birds' feathers were improvements on preexisting systems, they were the first to have endothermy, an internal system for generating body heat. Mammals evolved hair, an efficient adaptation for enhancing body heat regulation. Mammals fine-tuned many preexisting systems, but their major innovations had to do with reproduction. Mammals developed the placenta and milk glands.