Introduction
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).
General features
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.
Natural history
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 cyclostomes]]
The cyclostomes include two classes of living, jawless fishes (agnathous)—Petromyzontiformes (lamprey]] eels) and Myxiniformes (hagfishes]]). The hagfishes are totally marine, often living in deep waters associated with muddy bottoms. The lampreys may be marine as adults but spawn in fresh waters, where the larvae spend some time before metamorphosing to the adult. Some lampreys live entirely in fresh water and may change only slightly in habit as a result of metamorphosis. Without lateral fins, lampreys swim by undulations of the body and can control direction only for short distances.
The chondrichthyes]]
The sharks]], rays, and chimaerids are usually marine, but some sharks have entered fresh waters (the Amazon) or even live there permanently (Lake Nicaragua). In size, sharks range from the whale shark, nearly 10 metres in length, to rather small species, three centimetres in length. They usually weigh 25 to 200 kilograms (55 to 440 pounds). Sharks are predatory animals. Some large shark species (basking and whale sharks) filter feed on small crustaceans. Herbivorous sharks are unknown. Sharks swim by undulations of the tail, but rays “fly” through the water by undulations of the pectoral fins. Most species occur in near-shore waters, but some range widely throughout the oceans. A few are found in deep water.
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]], 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.
The teleostomes]]
The tetrapods
The tetrapods live primarily on land and are rather similar in habit. Members include the amphibians]], reptiles, birds, and mammals. Amphibians are widespread in the warmer parts of the continents, being absent only in the far north and in the Antarctic. Three orders are recognized: Candata (the salamanders), the frogs and toads (Anura, or Salientia), and the Apoda or Gymnophiona (caecilians). Modification takes many forms, from the moist glandular skin (some scale remnants persist in apodans) to the loss of many of the bones of the skull. Like their ancestors, amphibians are cold-blooded and tend to be aquatic or limited to moist surroundings. Salamanders]] are seemingly the least modified in body form. They do not actively pursue prey and at best are only marginal swimmers. In swimming or crawling, the salamander’s body and tail undulate. Frogs and toads hop using hind-limb propulsion and the forelimbs as body props. This dominance of the hind limb in locomotion is best seen in swimming when the forelimbs are drawn back against the body. In contrast to the salamanders and frogs, the burrowing, wormlike apodans are without limbs.
The class Reptilia retains many of the structural characteristics of the ancestral amphibian. While most reptiles]] 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).
The mammals]] 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]] (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.
Form and function
External features
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.
Internal features
The skeletal system]]
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.
Tissues]] and muscles
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.
Nervous system]] and organs of sensation
The digestive system
The excretory system
Respiration and gas exchange
The circulatory system
The endocrine system
Evolution and paleontology
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.
Malcolm T. Jollie
Classification
Annotated classification
- Subphylum Vertebrata (or Craniata)
- Bilaterally symmetrical; internal skeletal support with skull enclosing a highly developed brain and a vertebral column and nerve cord; paired, jointed appendages; skin; advanced organ systems; sense organs concentrated in head.
- Class Agnatha]] (hagfishes, lampreys)
- Primitive; jawless; paired fins are poorly developed or lacking; rasping tongue; notochord without bone; skin is soft, glandular, and slimy; true gill arches absent; marine habitat.
- Class Placodermi]] (placoderms)
- †Extinct; fishlike; jaws supported by both cranium and hyoid arch (amphistylic); partly ossified cranium; primitive; head and trunk have armour that is jointed at the neck; pelvic fins present or absent; pectoral fins or finlike structures often present; gill arches.
- Class Chondrichthyes]] (sharks, rays, and skates)
- Cartilaginous fishes; jaws; paired fins; no swim bladder; pelvic fins in males often modified to form claspers; gill arches internal to gills; reduced notochord; lateral-line system; paired nostrils; internal nares absent; separate sexes; internal fertilization and direct development; oviparous, ovoviviparous, or viviparous.
- Subclass Elasmobranchii (sharks and rays)
- Numerous teeth derived of placoid scales; 5 to 7 gill clefts; operculum absent; cloaca; upper jaw not fused with braincase; dorsal fin nonerectile; with spiracles; worldwide distribution.
- Subclass Holocephali]] (chimeras)
- Teeth fused to bony plates; no scales; 4 gill pairs under 1 gill opening on each side; no cloaca; no spiracles; operculum present; upper jaw fused to braincase; dorsal fin erectile; whiplike tail; claspers present in males; temperate marine freshwater.
- Class Osteichthyes (bony fishes]])
- Jaws; partly or fully ossified skeleton; usually a swim bladder; paired fins; gills covered by a bony operculum; scales; paired nostrils with or without internal nares; lateral-line system; mostly oviparous with external fertilization; some ovoviviparous or viviparous.
- Subclass Actinopterygii (ray-finned fishes)
- Generally lack choanae; no fleshy base to paired fins; no internal nares; air sacs usually function as swim bladder; skeleton usually well ossified.
- Subclass Sarcopterygii (lobe-finned fishes)
- Usually possess a choana; paired fins with a fleshy base over a bony skeleton; persisting notochord; 2 dorsal fins; nares are internal.
- Class Amphibia]]
- Cold-blooded; respire by lungs, gills, skin, or mouth lining; larval stage in water or in egg; skin is usually moist with mucous glands and without scales; tetrapods; freshwater and terrestrial; paired appendages are legs; 10 pairs of cranial nerves; separate sexes; external fertilization with development into tadpole larvae; some have internal development, ovoviviparous or viviparous.
- Order Aponda (or Gymnophiona; caecilians]])
- Wormlike; no limbs or girdles; compact skull; lidless, minute eyes; persistent notochord; tail; scales present in some species.
- Order Anura]] (or Salientia; frogs and toads)
- Tailless; elongated hind limbs modified for jumping; larvae lack true teeth and external gills.
- Order Caudata (or Urodela; salamanders]])
- Tail; limbs normal; many skeletal elements cartilaginous; larvae with true teeth and external gills.
- Class Reptilia]]
- Cold-blooded; no larval stage; breathing by lungs; well-ossified skull; dry skin; scales; no glands; 5-toed limbs; claws; 3- or 4-chambered heart with incomplete ventricle separation; 12 pairs of cranial nerves; internal fertilization, direct development; oviparous and ovoviviparous.
- Subclass Anapsida (turtles, tortoises, terrapins)
- No temporal skull openings; body encased in bony shell; no teeth in living members; oviparous.
- Subclass Lepidosauria
- No bipedal specializations; 2 complete temporal openings; complete palate; oviparous; male is without penis.
- Subclass Archosauria]] (ruling reptiles)
- Some ancient forms had bipedal locomotion; longer hind legs; semiaquatic; webbed feet; teeth in sockets; single penis; oviparous; includes extinct dinosaurs.
- Subclass Ichthyopterygia
- †Extinct; temporal openings high up on skull; fishlike; spindle-shaped body; high tail fin; triangular dorsal fin; paddlelike legs; marine.
- Class Aves]]
- Warm-blooded; skull has only 1 condyle; front limbs primarily modified for flight; hind limbs are legs with 4 or fewer toes; body covered with feathers; scales on feet; 4-chambered heart; no teeth; horny beak; lungs with extended air sacs; 12 pairs of cranial nerves; internal fertilization; oviparous.
- Subclass Archaeornithes
- †Extinct; teeth in both jaws; long, feathered tail; less specialized for flight; body elongated and reptilelike; forelimb had 3 clawed digits; small brain and eyes; nonpneumatic bones.
- Subclass Neornithes (true birds)
- Well-developed sternum; tail is not long; no teeth; forelimbs modified to wings; teeth replaced by horny rhamphoteca over bill.
- Class Mammalia]]
- Warm-blooded; mammary glands; lower jaw is composed of 1 bone; hair; advanced brain; skin with different glands and hair; ears with 3 middle-ear bones; 12 pairs of cranial nerves; 4-chambered heart; young nourished by milk from mammary gland; internal fertilization; mostly viviparous, some oviparous.
Critical appraisal
Additional Reading
Charles G. Crispens, Jr., The Vertebrates, Their Forms and Functions (1978); J.Z. Young, The Life of Vertebrates, 3rd ed. (1981); Libbie H. Hyman, Hyman’s Comparative Vertebrate Anatomy, 3rd ed., edited by Marvalee H. Wake (1979); Edwin H. Colbert, Evolution of the Vertebrates: A History of the Backboned Animals Through Time, 3rd ed. (1980); Alfred Sherwood Romer and Thomas S. Parsons, The Vertebrate Body, 6th ed. (1986); Leonard B. Radinsky, The Evolution of Vertebrate Design (1987); Robert L. Carroll, Vertebrate Paleontology and Evolution (1988); and F. Harvey Pough, John B. Heiser, and William N. McFarland, Vertebrate Life, 3rd ed. (1989).
Malcolm T. Jollie