Vertebrates include fishes, reptiles, amphibians, birds, and mammals, all having 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):
- A skin or membrane of two divisions, including an outer epidermis and an inner dermis
- A skin or membrane often modified to produce hair, scales, feathers, glands, horn, etc.
- Replacement of notochord by a vertebral column more or less complete, depending on group
- A bony or cartilaginous endoskeleton consisting of cranium, visceral arches, limb girdles, and two pairs of appendages
- A 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
- Locomotion provided by muscles attached to endoskeleton
- A igestive system with large digestive glands, liver, and pancreas
- A ventral heart with 2-4 chambers
- Blood with red blood cells containing hemoglobin, and white blood cells
- A well developed body cavity (coelom) containing visceral systems
- Paired kidneys with ducts to drain waste to exterior
- Most vertebrates have two distinct sexes, each with paired gonads, although there are some exceptions
- A general body plan consisting of head, trunk, two pairs of appendages, and postanal tail (but these structures are highly modified in many vertebrates and sometimes absent).
Vertebrates are often contrasted with invertebrates, animals which do not have a backbone. Although most animals we tend to interact with in our daily lives are vertebrates, invertebrates are far more common, comprising more than nine phyla of life (while Vertebrata is only a single sub-phylum of the phylum Chordata).