Subphylum Vertebrata Chordata Subphylum Vertebrata Vertebrates constitute the vast majority of living chordates, and they have evolved an enormous variety of forms. The backbone of vertebrates protects the nerve cord and serves as the axis of the internal skeleton. The skeleton provides strength and rigidity to the body and is an attachment site for muscles. The vertebrae in the middle region of the trunk give rise to pairs of ribs, which surround and protect the internal organs. A cartilaginous or bony case encloses the brain.
Bone is a substance unique to vertebrates.
However, very primitive fishes with bone skeletons are known from the fossil record, so lack of bone is now believed to be a degenerate rather than a primitive feature. All but the most primitive vertebrates, known as jawless fishes, have jaws and paired appendages. The fishes and, to a lesser extent, the amphibians and reptiles show a segmental arrangement of the muscles of the body wall and of the nerves leading to them. There are eight vertebrate classes. Fishes breathe water by means of gills located in internal passages, although they may also have lungs as supplementary air-breathing organs.