The Phylum Chordata is composed of animals known as Chordates. These are animals that sometime in their life cycle have four key anatomical characteristics: 1) a notochord which is a rod-shaped structure composed of cells and forms the axial skeleton of chordate embryos, 2) pharyngeal gill slits, 3) a postanal tail, and 4) a dorsal, hollow nerve tube. This Phylum is divided into three different Subphyla: Urochordata, Cephalochordata, and Vertebrata. Animals include tunicats and lancelets.