The chordates are animals which belong to the phylum Chordata. It includes the vertebrates (animals with spinal column) and animals that have notochord, such as tunicates and lancelets. This phylum gets its name from one of its three distinctive features or characteristics. The first one is the notochord; all chordates at some time during their lifetimes possess a flexible supporting rod along the back, called the notocord, which stiffens the animal's body. In most chordates, the notochord is lost in the adults, and is replaced by a series of jointed vertebrae, called the spinal column or backbone, which develops around the notochord. Thus, animals with backbones are called vertebrates; they comprise one of the three subphyla of the phylum Chordata.
The second important characteristic unique to chordates is the single tubelike nerve cord that develops along the back of the embryo. During development this nerve cord becomes the central nervous system, and in most members of the group, the front end enlarges greatly to form the brain. A third characteristic chordate feature is a series of openings in the wall of the pharynx called gill slits. In some aquatic forms they serve as strainer for filter feeding and respiration. In other forms they function mainly as respiration and provide the passageways through which water passes from the gills. In chordates adapted to land life, the gill slits and the part of the skeleton which supports them may appear only in the embryo and may later be altered to serve some purpose other than respiration. In reptiles, birds, and mammals, for instance, the embryonic gill slits disappear and some of their supporting tissues become part of the jaws, tongue, the larynx, facial muscles, and inner ear as well as the thyroid and parathyroid glands.