The tunicates are marine, filter-feeding animals. The most prominent tunicates are the sea squirts (class Ascidiacea), which show affinities to other chordates only in the juvenile stage. Adult sea squirts are sessile (attached), globular or tubular animals, often with prominent incurrent and excurrent siphons; many kinds grow in colonies. Most of the body of the adult is occupied by a very large pharynx with numerous gill slits that act as a sieve for food. Water taken into the incurrent siphon enters the pharynx and passes out through the gill slits, leaving food particles trapped in the pharynx. A groove in the pharynx called the endostyle secretes mucus that traps the particles and conveys them into the digestive tract; the movement of the mucus is caused by the action of cilia. Water leaves the atrium, a sac surrounding the pharynx, by way of the excurrent siphon. Thus the gill slits in tunicates serve a feeding function, not a respiratory function.
The sea squirt larva is a free-swimming animal resembling a tadpole. The head, which will become the entire body of the adult, contains a rudimentary brain and sense organs, a small pharynx and digestive tract, and a ventral heart. Incurrent and excurrent openings are located at the top of the head. The tail is a muscular appendage that functions as a swimming organ. It contains a hollow nerve tube (connected to the brain), and a notochord that extends into the head and keeps the animal from telescoping when its muscles contract. When the larva is ready to undergo metamorphosis it attaches to an object head downward. The tail, notochord, and nerve cord degenerate, the pharynx enlarges, and the other organs shift in position; the incurrent and excurrent openings develop siphons.
There are two other classes of tunicates, both consisting of small planktonic animals. The salps (Thaliacea) metamorphose into barrel-shaped adults that swim by muscular contractions. The larvaceans (Larvacea) are neotenous, that is, they achieve sexual maturity and reproduce without losing the larval form. Many zoologists believe that tunicates of the sea squirt type were the first chordates and that the larval tail, with its notochord and nerve chord, was evolved as a means of dispersing their larvae. According to this theory, the later chordates, including the vertebrates, are descended from neotenous tunicates that, like the larvaceans, failed to assume the adult form.
This class includes the several species of lancelets, or amphioxi, small, fishlike, filter-feeding animals found in shallow water. A lancelet has a long body, pointed at both ends, with a large notochord that extends almost from tip to tip and is present throughout life. At one end is a mouth surrounded by prominent bristles and leading into a pharynx. The pharynx has gill slits, an endostyle similar to that of a sea squirt, and an atrium surrounding the pharynx. Water enters the mouth and leaves through the gill slits, and food is trapped in the pharynx. The dorsal, tubular nerve cord is slightly enlarged in the anterior region, forming a rudimentary brain. Nerves extend from the nerve chord to other parts of the body. The muscles, as in fishes, are a series of cone-shaped blocks that fit into each other like stacked paper cups. This is the most primitive occurrence of the segmental body wall structure characteristic of lower vertebrates. The colorless blood moves forward through a ventral vessel and back through a dorsal vessel, in the typical chordate pattern. There is no major heart, although many small enlargements of the vessel serve the function of hearts. There are no blood cells and no respiratory pigments. The excretory system, like that of many invertebrates, consists of segmentally arranged nephridia; there is no kidney. The gonads, unlike those of any other chordate, are numerous and segmentally arranged.
Vertebrates constitute the vast majority of living chordates, and they have evolved an enormous variety of forms. The backbone of vertebrates protects the nerve cord and serves as the axis of the internal skeleton. 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 (cyclostomes 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 is now believed to be a degenerate 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 eight vertebrate classes. Four are aquatic, and may be grouped together as the superclass Pisces, or fish; four are terrestrial or (in the case of amphibians) semiterrestrial, and may be 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.
The Agnatha, or jawless fishes, are the oldest known vertebrates. The only surviving members of this class are the hagfish and lampreys, known as cyclostomes. Cyclostomes have long, slender bodies with dorsal, ventral, and caudal (tail) fins, all in the median plane. Although in their lack of jaws or paired lateral appendages they represent a very primitive stage of vertebrate development, the modern cyclostomes are highly adapted for their particular ways of life. The hagfish is a specialized scavenger, and the lamprey is a parasite on other fishes. The lamprey has a round mouth without skeletal supports, a rasping tongue, and a single, dorsally located nostril. The gill passages are enlarged to form pouches and are lined with gill filaments that serve as a surface for the exchange of respiratory gases; in vertebrates the gill passages have acquired a respiratory function. In cyclostomes, as in all fishes, water is taken in through the mouth and expelled through the gill passages; as water passes over the thin-walled gill filaments, dissolved oxygen diffuses into the blood, and carbon dioxide diffuses out. The lamprey has a notochord extending from the head to near the tip of the tail. A few cartilaginous blocks around the notochord constitute the bare rudiments of a backbone; a cartilage framework supports the gill region, and there is a rudimentary cartilage braincase. The meagerness of the skeleton is considered a degenerate, not a primitive condition. The larva of the marine lamprey is a small animal, resembling a lancelet, that uses the pharynx and gill passages for filter-feeding. It metamorphoses into the adult form before migrating to the sea. The extinct relatives of the cyclostomes, called ostracoderms, were jawless fishes with bony armor and in some cases a well-developed bony skeleton.
The placoderms, an entirely extinct group of armored fishes, were the first jawed vertebrates. Jaws enabled vertebrates to become predators, an important factor in the later development of active, complex forms. The placoderms were also the first vertebrates to have the two pairs of lateral appendages (supported by pelvic and pectoral girdles) that characterized all later vertebrate groups. These primitive paired fins gave rise to the pelvic and pectoral fins of modern fishes and to the limbs of four-footed animals. The ostracoderms are thought to have given rise to both the sharklike and the bony fishes.
The almost exclusively marine sharks, rays, and chimaeras of the class Chondrichthyes have skeletons made of cartilage. The mouth, equipped in most sharks with numerous sharp teeth, is located on the underside of the head. Passages called gill arches lead from the pharynx to the exterior and are lined with gill filaments. The gill arches are supported by gill bars. Except in chimaeras, the external gill slits are not covered and are conspicuous on the surface of the body. The jaw consists of two distinct pieces; the upper part is not fused to the braincase as in higher vertebrates. The tail is asymmetrical, curving upward in a shape found in early fossil fishes and thought to be primitive. There is no lung or swim bladder. The skin is studded with toothlike structures called denticles. Sharks have typical vertebrate kidneys that excrete a very dilute urine consisting mostly of water; presumably the earliest vertebrates (ancestral to sharks) evolved in freshwater, where this function is necessary to maintain the correct concentration of the physiologically important salts in the tissues against the tendency for them to be diluted by the inward diffusion of water. In marine species, on the other hand, it is necessary to prevent the concentration of those salts from increasing. Although the kidneys of sharks pump out water, their body fluids contain ammonia in concentrations high enough to make the osmotic pressure equal to that of seawater; this prevents the inward diffusion of salts. Sharks have internal fertilization and lay large eggs, well supplied with yolk and protected by leathery shells. In a few species the eggs are hatched within the body.
The bony fish of the class Osteichthyes are the predominant class of living fishes. In this group the bony skeleton has been retained and lungs and swim bladders have evolved. Early bony fishes evolved in freshwater under conditions of periodic drought and stagnation and developed an internal, moisture-retaining organ, the lung, for gas exchange. Those fishes gave rise to two lines of descendants.
Members of one line, the fleshy-finned fish, had thick fins with supporting bones, used for crawling. The only survivors of that group are the coelacanth, or lobefin, which has a vestigial lung and crawls on the seafloor, and the freshwater lungfishes of drought-ridden areas, which can crawl over land in search of water and even live out of water for several years. Early fleshy-finned fish gave rise to the first land vertebrates, the amphibians.
The second line, the ray-finned fish, constitutes the predominant modern group. Ray-finned fish are highly specialized for aquatic life; they have developed thin, lightweight fins supported by slender rays, and used only for balance and steering. The lung, a ventral outpocketing of the pharynx, was no longer necessary as these fish invaded freshwaters and oceans throughout the world; it shifted to a dorsal position and evolved into a hydrostatic organ called the swim bladder, or air float. The swim bladder, along with the strong, lightweight skeletal construction, makes ray-finned fishes much lighter-bodied than sharks. The gill passages of ray-finned fishes resemble those of sharks, but have a bony covering, called the operculum, over the external gill slits. Ray-fins have a typical vertebrate kidney which, in freshwater forms, maintains the proper salt concentration in the tissues by excreting excess water. In the marine forms the activity of the kidney is offset by the activity of salt-secreting glands; in addition, the kidney may be modified so as to produce a more concentrated urine. The heart, like that of sharks, has two chambers, and there is no separation of oxygenated and deoxygenated blood in the circulatory system. A few primitive ray-fins (the sturgeon, the paddle fish, and the bowfin) have asymmetrical tails and thick scales regarded as primitive in construction.
The higher ray-fins, or teleosts, have more or less symmetrical tail fins extending above and below the vertebral column, and typical fish scales made of very thin layers of bone. Most marine teleosts produce enormous numbers of small eggs that are externally fertilized and float in plankton; only a few of these survive. In many species there is a larval stage that is quite dissimilar to the adult. Teleosts have evolved a tremendous variety of forms and occupy very diverse ecological niches, both freshwater and marine.
The amphibians, the first vertebrates to have limbs, evolved during the Devonian period. They are only partially terrestrial: Their externally fertilized eggs are laid in freshwater, and they go through a gilled, aquatic larval stage (the tadpole stage) before metamorphosing into land-living adults. The skin of the adult is water-permeable, and the animal must live in a moist environment to prevent desiccation. The adult usually breathes by means of lungs, although some breathe directly through the skin. The heart is a three-chambered structure that creates a partial separation between oxygenated blood, destined for the body tissues, and depleted blood, destined for the lungs; this provides better oxygenation than does a system in which the two kinds of blood mix. There are only three groups of amphibians living today. The salamanders are closest to the basic amphibian stock in form and in method of locomotion. Although supported by limbs, they move with a wriggling motion similar to that of a fish. The frogs and toads are specialized for jumping, with long, muscular hind legs, while the tropical caecilians are burrowing forms that have lost all but vestigial traces of their limbs.
The reptiles, which evolved from amphibians during the Carboniferous period, were the first vertebrate group to become entirely independent of water. This was made possible by the development of a scaly, water-resistant skin and of the terrestrial, or amniote, type of egg found in all higher land vertebrates. The amniote egg has an elaborate series of internal membranes (one of which is called the amnion) surrounding a pool of liquid in which the embryo develops; the membranes prevent desiccation and allow inward diffusion of oxygen. Reptilian eggs have porous shells and large amounts of yolk. Fertilization is internal. In most cases the eggs are laid unhatched; in a few species they are retained and hatched in the body. Reptiles, including such forms as turtles and sea snakes that have returned to an aquatic life, are air-breathing at all stages, and nearly all lay their eggs on land. Gill passages appear, as in birds and mammals, only in the embryo.
During the Mesozoic era, reptiles were exceedingly diverse and numerous. The reptilian dinosaurs included the largest terrestrial animals that have ever lived, as well as many smaller forms. There were also flying and aquatic reptiles. With the rise of the early mammals the decline of the reptiles began. The only large and successful modern group of reptiles is the order of lizards and snakes. Snakes are descended from lizards, but have lost their limbs. Reptiles, like fish and amphibians, are cold-blooded, that is, they have little ability to regulate their body temperature, which approaches that of the environment. The reptiles gave rise to the two warm-blooded vertebrate groups, the birds and the mammals.
The birds evolved from reptiles in the Jurassic period. Their front limbs are modified into wings, and the breastbone is greatly enlarged to support flight muscles. They have an insulating covering of feathers, which has been an important factor in their ability to regulate body temperature. The other advance that enabled birds to become warm-blooded was the evolution of a four-chambered heart, making the circulatory system a complete double circuit: oxygenated blood is pumped from the lungs to the tissues, and deoxygenated blood is pumped from the tissues to the lungs. The only major group besides insects to invade the air, birds are much less restricted by external temperature requirements than cold-blooded animals, and they have spread throughout every part of the world. They live in many kinds of habitat and have evolved a diversity of forms. Some have become flightless terrestrial animals, while others are aquatic, using their wings for swimming instead of or in addition to flying. Fertilization is internal. The eggs of birds are similar to those of reptiles, but parental care of the eggs and young is highly developed.
The mammals also arose from reptiles in the Jurassic period and are now the dominant form of terrestrial vertebrate life. Like the birds, they have a four-chambered heart and a double-circuit circulatory system and are able to regulate body temperature. In the case of mammals the insulating covering is provided by hair, a feature unique to the class, although in a few forms (particularly in marine species) nearly all the hair is lost, and insulation is provided by fat. A second distinguishing characteristic of mammals is the production of milk by the females for the nourishment of the young. All mammals have internal fertilization, and all but the most primitive (the egg-laying monotremes of Australia) bear live young. The mammalian egg contains little yolk. In the marsupials the young are born at an extremely undeveloped stage and continue to develop in a milk-supplied pouch. In the vastly more numerous placental mammals nourishment is passed from the circulatory system of the mother to that of the embryo by means of a placenta, and the young are born well-developed. Most mammals have highly evolved sense organs and larger brains than other vertebrates. As a group they display great adaptability to a variety of conditions and have spread to all regions of the world.
The earliest placental mammals were small animals of the insectivore type, but adaptive radiation has resulted in great diversity of forms and ways of life. Some mammals are predators; others are herbivores with specialized digestive systems. Some have taken up an aquatic existence and a few marine forms (whales and sirenians) even give birth at sea. Members of one group, the bats, have developed membranous wings supported by elongated fingers and lead an aerial existence. The primates, the group that includes humans, are fairly close to the original mammalian type in general structure (for example, they have five fingers and toes and walk flat on the sole of the foot), but they have undergone great evolutionary advances in the development of the brain, vision, and manual dexterity.
See C. K. Weichert, Anatomy of the Chordates (4th ed. 1970); R. M. Alexander, The Chordates (2d ed. 1981); H. Eugene Lehman, Chordate Development (2d ed. 1983).