Vertebrates, which include fishes, reptiles, amphibians, birds, and mammals, all share a vertebral column, or a chain of bony elements (vertebrae) that run along the dorsal surface from head to tail and form the main skeletal axis of the body. The vertebral column surrounds and more or less replaces the notochord as the chief "stiffener" of the body in locomotion. Some characteristics shared by most or all vertebrates (in addition to those traits shared among all chordates) include the following (after Hickman, 1994):
- integument of two divisions, including an outer epidermis and an inner dermis; integument often modified to produce hair, scales, feathers, glands, horn, etc.
- replacement of notochord by vertebral column more or less complete, depending on group
- bony or cartilaginous endoskeleton consisting of cranium, visceral arches, limb girdles, and 2 pairs of appendages
- muscular, perforated pharynx; this structure is the site of gills in fishes but is much reduced in adult land-dwelling forms (although it is extremely important in embryonic development of all vertebrates)
- movements provided by muscles attached to endoskeleton
- digestive system with large digestive glands, liver, and pancreas
- ventral heart with 2-4 chambers
- blood with red blood corpuscles containing hemoglobin, and in addition, white corpuscles
- well developed body cavity (coelom) containing visceral systems
- paired kidneys with ducts to drain waste to exterior
- most vertebrates with two sexes, each with paired gonads (there are some exceptions)
- general body plan consisting of head, trunk, 2 pairs of appendages, and postanal tail (but these structures are highly modified in many vertebrates and sometimes absent).
Phil Myers (author), Museum of Zoology, University of Michigan-Ann Arbor.