The subphylum is one of the best known of all groups of animals. Its members include the classes Agnatha, Chondrichthyes, and Osteichthyes (all fishes); Amphibia (amphibians); Reptilia (reptiles); Aves (birds); and Mammalia (mammals).

Although the vertebral column is perhaps the most obvious vertebrate feature, it was not present in the first vertebrates, which probably had only a notochord. The vertebrate has a distinct head, with a differentiated tubular brain and three pairs of sense organs (nasal, optic, and otic). The body is divided into trunk and tail regions. The presence of pharyngeal slits with gills indicates a relatively high metabolic rate. A well-developed notochord enclosed in perichordal connective tissue, with a tubular spinal cord in a connective tissue canal above it, is flanked by a number of segmented muscle masses. A sensory ganglion develops on the dorsal root of the spinal nerve, and segmental autonomic ganglia grow below the notochord. The trunk region is filled with a large, bilateral body cavity (coelom) with contained viscera, and this coelom extends anteriorly into the visceral arches. A digestive system consists of an esophagus extending from the pharynx to the stomach and a gut from the stomach to the anus. A distinct heart, anteroventral to the liver, is enclosed in a pericardial sac. A basic pattern of closed circulatory vessels is largely preserved in most living forms. Unique, bilateral kidneys lie retroperitoneally (dorsal to the main body cavity) and serve blood maintenance and excretory functions. Reproductive organs are formed from tissue adjacent to the kidneys; this original close association is attested by the tubular connections seen in males of living forms. The ducts of the excretory organs open through the body wall into a cloacal chamber, as does the anus of the digestive tract. Reproductive cells are shed through nearby abdominal pores or through special ducts. A muscular tail continues the axial musculature of the trunk.

Approximately 45,000 living species constitute the vertebrates. Species of several classes are found from the high Arctic or Antarctic to the tropics around the Earth; they are missing only from interior Antarctica and Greenland and from the North Polar ice pack. In size, vertebrates range from minute fishes to elephants and whales (of up to 100 tons), the largest animals ever to have existed. Vertebrates are adapted to life underground, on the surface, and in the air. They feed upon plants, invertebrate animals, and one another. Vertebrate faunas are important to humans for food and recreation.

In order to give a broad and comparative view of their life histories, the vertebrates are subdivided here into major groups based on morphology: the cyclostomes (jawless fishes), the chondrichthyes (cartilaginous fishes), the teleostomes (bony fishes), and the tetrapods.

The living agnaths are predatory, the lampreys being well known for attacking salmonoid fishes. The lamprey attaches to its prey using its round, suctorial mouth, and it rasps a hole through the outer tissues using a tongue armed with keratinized teeth. It suctions off bits of tissue, blood, and body fluids. The hagfishes feed somewhat similarly, but on a variety of prey—invertebrates (worms and soft-bodied forms) and dead fishes.

The lampreys produce small eggs, which develop directly into larvae that burrow into the muddy bottom of the stream. With its mouth at the surface of the mud, the larva filter feeds until large enough to metamorphose and swim off as a small adult. In contrast, the hagfishes produce relatively large encapsulated, yolky eggs up to two centimetres in length. When laid, these eggs attach to any available object by terminal hooks. The encased egg develops more or less directly into a miniature adult.

A few sharks produce live young (viviparous) after internal fertilization. The posterior angle of the male's pelvic fins are modified into a clasper, which acts as an intromittent organ in copulating with the female. Most sharks lay large yolky, encapsulated eggs with hooks for attachment. The young develop directly and begin life as miniature adults. The young that develop in the mother's uterus obtain nutrients from the large yolk sac until they are born alive. In a few cases, the uterine wall secretes nutrients.

The teleostome, or osteichthyian, fishes (those having an internal bony skeleton) can be divided into two groups: the subclasses Actinopterygii (ray-finned fishes) and Sarcopterygii (lobe-finned fishes). The latter group includes the lungfishes (lungfish), which live in marshes, ponds, or streams, and are frequent air breathers. They lay fairly large eggs, with a limited amount of yolk, that are enclosed in jelly coats like those of an amphibian. The eggs develop into small fishes that feed on live prey. The larvae of the African lungfish have external gills to supplement oxygen intake.

Amphibians usually trap food using a tongue that can be shot out of the mouth, or they use the mouth itself to grasp and ingest food. There is great variation in foods; only the larvae of frogs and toads appear to be plant feeders, a specialization that is reflected in the highly modified jaws and guts of the tadpoles.

Amphibians have retained a simple egg cell with a gelatinous cover. The eggs are laid in ponds, streams, or even in damp places high in trees, usually in great numbers. Fertilized eggs develop into free-swimming larvae, which then metamorphose to adults, but in highly specialized forms.

The class Reptilia retains many of the structural characteristics of the ancestral amphibian. While most reptiles (reptile) are carnivorous, feeding on other organisms, a few are herbivorous (e.g., tortoises). As cold-blooded animals, reptiles tend to be limited to temperate and tropical areas, but, where found, they are relatively common, although not as large or conspicuous as birds or mammals. Most reptiles are terrestrial, but a few are aquatic. As basic tetrapods, reptiles move about by creeping or swimming in a fashion similar to amphibians. Some reptiles, however, can lift the body from the ground and run rapidly either in a quadrupedal or bipedal fashion. Reptiles lay relatively large, shelled eggs. In a few instances, the eggs and young are cared for by the female; in others, the young are born alive (ovovivipary).

Birds (bird) are warm-blooded, and, although most are capable of flight, others are sedentary and some are flightless. Like their relatives the reptiles, birds lay shelled eggs that differ largely in the amount of calcification (hardening) of the shell. The young are usually cared for in a nest until they are capable of flight and self-feeding, but some birds hatch in a well-developed state that allows them to begin feeding immediately or even take flight. The megapods lay their eggs in mounds of rotting vegetation, which supplies the heat for incubation. (Nesting activities similar to those of some birds are seen in the crocodilians.)

The mammals (mammal) range in size from tiny shrews or small bats weighing only a few grams to the largest known animals, the whales. Most mammals are terrestrial, feeding on both animal and vegetable matter, but a few are partially aquatic or entirely so, as in the case of the whales or porpoises. Mammals move about in a great variety of ways: burrowing, bipedal or tetrapedal running, flying, or swimming. Reproduction in mammals is usually viviparous, the young developing in the uterus, where nutritive materials are made available through an allantoic placenta or, in a few cases, a yolk sac. The fertilized egg develops directly into the adult. The monotremes (monotreme) (platypus and echidna) differ from other mammals in that they lay eggs which hatch, and the relatively undeveloped young are carried in a pouch or kept in a nest; the growing young lap up a milk nutrient fluid exuded from the belly of the mother.

The evolution of the notochord, dorsal nerve tube, and pharyngeal slits in chordate structure suggests improved swimming capability and probably greater ability to capture prey. Specialization in the vertebrate for the active capture of larger prey is evident both in the structure of the mouth and in the relatively simple structure of the pharynx, with its strong gill development. Specialization for feeding is again seen in the two basic groups of vertebrates, the agnathans and gnathostomes. Swimming adaptations are also numerous and involve variations both in body form and in medial fins and the two pairs of lateral fins.

Support and protection are provided by the exoskeletal and endoskeletal divisions of the skeletal system. The exoskeleton, when present, is basically protective but functions in tooth support in the mouth region. The endoskeleton protects the brain and spinal cord and assists primarily with locomotion in the trunk and tail regions. The endoskeleton begins as cartilage and may remain so or may develop into bone. The cartilaginous endoskeleton, found in the shark or chimaerid, is usually calcified so as to be stiffer and stronger. Bone is distinctive but highly variable; some types of bone contain cells, others do not, or the bone may be laminar, spongy, or arranged in sheathing layers around blood channels.

Tissue development in the vertebrate is unique in its complexity; tissues in the strict sense (defined as a mass or sheet of similar cells with a similar function), however, do not exist. The simplest situation is seen in the epidermis, but even here there is a layered system in which different cell types provide different functions (such as protection and secretion). The stratified epithelium of the vertebrate is highly characteristic of that group (a similar one is seen in only one invertebrate group, the class Chaetognatha).

Other tissues of the vertebrate are more complex than the epithelium. For example, skeletal muscle consists not only of striated muscle fibres but also of connective tissue, which binds it together and attaches it by way of tendons. This contractible tissue includes nerves and blood vessels and their contained blood. Skeletal muscles thus appear as simple organs, just as do the smooth muscles in the wall of the gut or the iris muscles of the eye. Such unique histological complexity runs through the entire body of the vertebrate.

The dorsal position, tubular structure, and epidermal origin of the central nervous system are definitive of the chordates, although some may see similarities with the hemichordates. The sensory structures are distinctive of the chordates and include the paired nasal, optic, and otic organs (along with the strongly differentiated head).

The nasal vesicle is variously open to the environment, and its sensory cells, as chemical receptors, are not unlike those in the taste buds of the mouth. The eye is the most complex organ of the head and is a lateral outpocketing of the anterior end of the brain tube. Later it acquires a lens of epidermal origin. The act of focusing the eye (accommodation) shows extensive adaptive variation among the different groups of vertebrates.

The otic vesicle starts from a simple sac formed by the invagination of an ectodermal placode. These developmental changes also include the changes of innervation. Whereas the original structure was basically an equilibrium adaptation, other functions, such as an awareness of movement or the sensation of the proximity of prey, developed.

The lateral-line system of canals and sensory organs is a unique vertebrate feature. The elements of this system are found on the head as well as the body. This system is related to the ear and presumably at its origin served a similar function. This system is lost in terrestrial vertebrate forms.

The digestive system of the vertebrate is distinctive in its structure but not in its function. The mouth and pharynx can be considered as parts of this system; the latter as an expanded cavity in the head is unmatched in any other group. The stomach and gut have been discussed above.

Presumably the original condition of the digestive glands was that of a ventral diverticulum which may have received the food mass into its cavity. This diverticulum, matched by the diverticulum seen in the amphioxus or the “intestine” of the tunicate, produced the secretions (bilelike and enzymes) of both liver and pancreas. Through time, the liver gradually differentiated from the pancreas. The size and separation of the liver from the gut suggest its separate blood and metabolic activities. The most obvious by-product of the liver, bile, necessitated the formation of a gall bladder and a duct connection with the gut. The pancreas, in contrast, continued to produce digestive enzymes, but its secretory cells were no longer in direct contact with the food mass. Because the pancreas was only a partial source of intestinal digestive enzymes, it was sometimes reduced in size and enclosed in the gut wall itself (agnaths) or dispersed as tiny bits of tissue in the mesentary supporting the gut (actinopterygians).

The excretory system is unique in its nephrons, which filter the blood in the glomeruli and remove a variety of wastes from the body through selective secretion and reabsorption. In the shark or the coelacanth Latimeria, urea is used to raise the osmotic pressure of the blood to that of the marine habitat, thus saving these organisms considerable metabolic energy. The large intestine (sometimes centred in a rectal gland) acts as an auxiliary excretory organ, as do also the gills of fishes or the sweat glands of mammals.

Respiration, like excretion, involves specialized body structures, such as lungs or gills, but also can involve other areas, such as the skin itself. Respiration involves exchange of gases both between the body of the organism and the environment and between the blood system and the body tissues. It also involves cellular respiration where oxygen is used and carbon dioxide is produced. There is nothing characteristic of the vertebrate in this functional area; even the hemoglobin of the blood is suggested in the respiratory pigments of other animals.

The circulatory system of vertebrates is closed in that fluids course through vessels, but there is free movement of cells in and out of blood. Some leukocyte (white blood cell) movement out of the capillaries and fluid leakage are observed in all tissues. Blood tissues are distinctive in the range of specialized cells, although these vary in detail among animals. The immune function of the blood is best developed in the vertebrate.

The endocrine system is characterized by its separate organs. The occurrence of a pituitary or a thyroid gland is suggestive of the evolutionary change and specialization that took place within this group. The relatively unspecialized nature of some parts of this system is seen in certain scattered cells in the gut wall or even the clumps of islet cells of the pancreas.

The knowledge of vertebrates as revealed by fossils has grown rapidly during the past few decades, but there is much still to be discovered. The ancestral vertebrate (protovertebrate) has been sought for more than 100 years, and the likelihood of finding it today is not much greater than in the past. It can be assumed that the protovertebrate was small and soft-bodied, two factors that suggest the improbability of finding a fossilized form in a recognizable condition. There are Cambrian fossils that have been suggested to be fossil cephalochordates and there are scales of agnath fishes, but the first type of fossil is too simple and the second already too complex to explain the transition.

The classification of animals is presently in a state of flux. The classification presented here is traditional and conservative. Because traditional theories of taxonomy tend to be nonquantitative, various interpretations of relationships or patterns can be presented and defended.

The alternative cladistic style of taxonomy is an attempt to force taxonomy into a testable, highly objective operation. One tentative classification based in cladistics separates the vertebrates into two superclasses (Agnatha and Gnathostomata). Agnathans are jawless, while the gnathostomates encompass the remainder of the jawed vertebrates. Living agnathans are placed in the class Cyclostomata. Gnathostomates can be further divided into the epiclasses Elasmobranchiomorphi (sharks and rays) and Teleostomi (bony fishes and tetrapods). The former group are identified primarily by a cartilaginous skeleton, while the latter group have developed a bony skeleton. Two subepiclasses of the teleostomes are Ichthyopterygii (or Osteichthyes; bony fishes) and Cheiropterygii (tetrapods), the latter being further divided into the classes Amphibia, Reptilia, Aves, and Mammalia.

Although this classification includes and uses traditional taxonomic categories, their position in the hierarchy may be changed. Separation of agnath and gnathostome is opposed by those cladists who chart the origin of gnathostomes from the agnath, believing that the differences in mouth and tooth structure are a result of modification. The Gnathostomata is subdivided into the Elasmobranchiomorphi and the Teleostomi largely on the basis of mouth and tooth structure. The creation of epiclasses and subepiclasses in the alternative classification is not important in itself; the creation of a dichotomy between Ichthyopterygii and Cheiropterygii, however, is important, although from the evolutionary view it is evident that the one evolved from the other.