The Chordata phase is divided into three subphases. namely Urochordata, Cephalochordata and Vertebrata. Urochordata and Cephalochordata together are called Protochordata. Chordates have three unique characteristics. These are – elastic notochord, dorsal hollow nerve cord and pharyngeal gill opening. All these characteristics are present in protochordata at any stage of life or for life. But all these characteristics of vertebrates are present in embryo but not in later life. In vertebrates, the notochord transforms into the spinal cord, the hollow nerve cords form the brain and spinal cord, and the gills close to form the lungs. From all this it can be understood that all the characteristics of chordates are present in vertebrates, but not all the characteristics of vertebrates are present in chordates. So it can be said that all vertebrates are chordates, but not all chordates are vertebrates.