All chordates are vertebrates, they differ from tunicates and lancelets in two ways .
Backbone Notochord becomes surrounded, and then replaced during the course of the embryo's development, by a bony vertebral column. In all vertebrates but the earliest fishes, there is a distinc and well-differentiated head, with a skull a brain within it. Embryonic development During the course of evolution, or of development, the flexible notochord is surrounded and eventually replaced by a cartilaginous or bony covering, the centrum. The neural tube is protected by an arch above the centrum, and the vertebra may also have a hemal arch, which protects the major blood vessels below the centrum. The vertebral column functions as a strong, flexible rod against which the muscles pull when the animal swims or moves.