Depending on the type of classification system used, the chordates may be placed into three subphyla, the first two of which are sometimes called the "protochordates" because they share a number of primitive features in common, including the lack of an internal skeleton.  The third subphylum (the vertebrates) contains the majority of chordate species, most of which have a spinal column made of interlocking vertebrae. 

  Subphylum Tunicata (Urochordata):  sea squirts, salps, larvaceans

     Subphylum Cephalochordata: lancelets

     Subphylum Vertebrata: vertebrates

           Superclass Agnatha

                  Class Myxini: hagfishes

                  Class Petromyzontida: lampreys

           Superclass Gnathostomata

                  Class Chondrichthyes: cartilaginous fishes (sharks, skates, rays)

                  Class Actinopterygii: ray-finned fishes (gars, sturgeons, teleosts, etc.)

                  Class Sarcopterygii: lobe-finned fishes (coelacanths, lung fishes)

                  Class Amphibia: amphibians (frogs, toads, salamanders, caecilians)

                  Class Reptilia: reptiles (turtles, crocodilians, snakes, lizards)

                  Class Aves: birds

                  Class Mammalia: mammals