The chordates are either asymmetrical or bilaterally symmetrical. Chordates all share, at some time in their life, the characteristic of a notochord. The notochord is a cartilage-like rod that provides longitudinal support. The chordates range from the simple jug-like tunicates to a broad range of vertebrates. The vertebrates evolved an internal skeleton and a hollow dorsal nerve chord. Vertebrates also have pharyngeal gill clefts at least in the embryo. Various structures are derived from those gill clefts and the tissue between them. There is also a broad range of diversity in the class Vertebrata. The vertebrates include the jawless fishes, Agnatha, that retain the notochord in the adult. Chondrichthyes are fishes that have cartilage instead of bones. The Osteichthyes are vertebrates that have bones. Leaving the fishes behind and invading the land are the amphibians. Better adapted to a life on land are the reptiles. The birds live on land, in the air, and on, but not in, the water. The mastery of all three environments is left to the mammals. The vertebrates will not be elaborated on extensively in this chapter.