Chordata: Subphylum Vertebrata

Subphylum Vertebrata

Vertebrates constitute the vast majority of living chordates, and they have evolved an enormous variety of forms. The backbone, or vertebral column, of most vertebrates protects the nerve cord and serves as the axis of the internal skeleton; only the hagfish lacks a vertebral column. 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. It was formerly thought that vertebrates with cartilage skeletons (jawless fishes and sharklike fishes) were descended from early vertebrates that had not yet developed bone. However, very primitive fishes with bone skeletons are known from the fossil record, so lack of bone may be an evolved 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 a number of vertebrate classes. Those that are aquatic have traditionally been grouped together as the superclass Pisces, or fish; those that are terrestrial or (in the case of amphibians) semiterrestrial, and have been grouped as the superclass Tetrapoda, or four-footed animals. Fishes breathe water by means of gills located in internal passages, although they may also have lungs as supplementary air-breathing organs. Most move through the water by weaving movements of the trunk and tail. All have fins, and most have two sets of paired fins (pelvic and pectoral). Tetrapods breath air, usually by means of lungs, and never have gills as adults, although the amphibians go through a gilled, water-breathing stage. Except where the appendages have been lost, as in snakes, all have two pairs of limbs, generally used for locomotion; these are homologous to the pelvic and pectoral fins of fish.

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