Chordata

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Chordata

(kôrdā`tə,–dä`–), phylum of animals having a notochord, or dorsal stiffening rod, as the chief internal skeletal support at some stage of their development. Most chordates are vertebrates (animals with backbones), but the phylum also includes some small marine invertebrate animals. The three features unique to chordates and found in all of them at least during early development are: the notochord, composed of gelatinous tissue and bound by a tough membrane; a tubular nerve cord (or spinal cord), located above the notochord; and gill slits leading into the pharynx, or anterior part of the digestive tract (the throat, in higher vertebrates). In addition, all have blood contained in vessels, and the tunicates and vertebrates have a ventrally located heart. All have a postanal tail, that is, an extension beyond the anus of the notochord or backbone and of the body-wall musculature, containing no internal organs. In vertebrates—animals of the subphylum Vertebrata—a backbone of bone or cartilage segments called vertebrae develops around the notochord; its upward projections partially surround the nerve cord. In many fishes and in early fossil amphibians and reptiles the notochord persists in the adult and is enclosed by the vertebrae; in higher vertebrates, however, it disappears during embryonic development. There are two invertebrate subphyla: the Urochordata, or tunicatestunicate
, marine animal of the phylum Chordata, which also includes the vertebrates. The adult form of most tunicates (also called urochordates) shows no resemblance to vertebrate animals, but such a resemblance is evident in the larva.
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, and the Cephalochordata, or lancelets. A third invertebrate group, comprising the acorn worms and their relatives, shows affinities with chordates and has sometimes been considered a chordate subphylum, but is now often classified in a phylum of its own, the HemichordataHemichordata
, small phylum of marine invertebrates closely related to both the echinoderms (phylum Echinodermata) and chordates (phylum Chordata). Acorn worms, class Enteropneusta, are the most common hemichordates.
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.

Subphylum Urochordata

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.

Subphylum Cephalochordata

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.

Subphylum Vertebrata

Vertebrates constitute the vast majority of living chordates, and they have evolved an enormous variety of forms. The backbone, or vertebral column, of most vertebrates protects the nerve cord and serves as the axis of the internal skeleton; only the hagfish lacks a vertebral column. 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 (jawless fishes 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 may be an evolved 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 a number of vertebrate classes. Those that are aquatic have traditionally been grouped together as the superclass Pisces, or fishfish,
limbless aquatic vertebrate animal with fins and internal gills. Traditionally the living fish have been divided into three class: the primitive jawless fishes, or Agnatha; the cartilaginous (sharklike) fishes, or Chondrichthyes; and the bony fishes, or Osteichthyes.
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; those that are terrestrial or (in the case of amphibians) semiterrestrial, and have been 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.

Classes Myxini and Hyperoartia

The jawless fishes, traditionally considered a single class, Agnatha, are the oldest known vertebrates. The only surviving members of this group are the hagfishhagfish,
primitive, jawless marine fish of the family Myxinidae, of worldwide distribution in cold and temperate waters. Its rudimentary skeleton, of cartilage rather than bone, has a braincase, but no jaw. The circular sucking mouth has rows of horny teeth.
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 (class Myxini) and lampreyslamprey,
name for several primitive marine and freshwater jawless fishes of the order Petromyzontiformes. As in the other jawless fish, the hagfish, the adult lamprey retains the notochord, the supporting structure that in higher vertebrates is found only in the embryo.
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 (class Hyperoartia). These two classes may be grouped together in the superclass Cyclostomata. 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 hagfish has a rudimentary skeleton, of cartilage rather than bone, with a braincase, but no jaw. Its circular sucking mouth has rows of horny teeth. There is a single median nostril and the eyes are poorly developed. The lamprey has a round mouth without skeletal supports, a rasping tongue, and a single, dorsally located nostril. The lamprey has a few cartilaginous blocks around the notochord that constitute the bare rudiments of a backbone; a cartilage framework supports the gill region, and there is a rudimentary cartilage braincase. 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. Throughout life, hagfish and lamprey retain the notochordnotochord
, in biology, supporting rod running most of the length of animals of the phylum Chordata and present at varying times in the life cycle. Composed of large cells packed within a firm connective tissue sheath, the notochord lies between the neural tube (spinal cord) and
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, a supporting structure found in other vertebrates only in the embryo; they lack sympathetic nervous systems, spleens, and scales. The extinct relatives of the cyclostomes, called ostracoderms, were jawless fishes with bony armor and in some cases a well-developed bony skeleton.

Class Placodermi

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.

Class Chondrichthyes

The almost exclusively marine sharksshark,
member of a group of almost exclusively marine and predaceous fishes. There are about 250 species of sharks, ranging from the 2-ft (60-cm) pygmy shark to 50-ft (15-m) giants. They are found in all seas, but are most abundant in warm waters.
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, raysray,
extremely flat-bodied cartilaginous marine fish, related to the shark. The pectoral fins of most rays are developed into broad, flat, winglike appendages, attached all along the sides of the head; the animal swims by rippling movements of these wings.
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, and chimaeraschimaera
, cartilaginous marine fish, related to the sharks. Also called ratfishes, chimaeras are found in temperate oceans throughout the world, mostly in deep water. They have large heads, long, thin, ratlike tails, and large, fanlike pectoral fins.
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 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.

Class Actinopterygii

The bony fish (superclass Osteichthyes), a group that has retained the bony skeleton and evolved lungs and swim bladders, are subdivided into two classes. Actinopterygii, the ray-finned fishes, constitutes the predominant modern group of both fish and vertebrates. Ray-finned fishes 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 not necessary as these fish invaded freshwaters and oceans throughout the world; in most ray-finned fishes, it has 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.

Class Sacropterygii

The second class of bony fish, the fleshy-finned fish, have thick fins with supporting bones. The only survivors of this group are the coelacanths, or lobefinslobefin,
common name for any of a group of lunged, fleshy-finned, bony fishes, also called crossopterygians, that were dominant in the Devonian period and may have given rise to amphibians.
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, which have a vestigial lung, and the freshwater lungfisheslungfish,
common name for any of a group of fish belonging to the families Ceratodontidae, Lepidosirenidae, and Protopteridae, found in the rivers of Australia, South America, and Africa, respectively.
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 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.

Class Amphibia

The amphibiansamphibian,
in zoology, cold-blooded vertebrate animal of the class Amphibia. There are three living orders of amphibians: the frogs and toads (order Anura, or Salientia), the salamanders and newts (order Urodela, or Caudata), and the caecilians, or limbless amphibians (order
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, 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 salamanderssalamander,
an amphibian of the order Urodela, or Caudata. Salamanders have tails and small, weak limbs; superficially they resemble the unrelated lizards (which are reptiles), but they are easily distinguished by their lack of scales and claws, and by their moist, usually
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 and newtsnewt,
name for members of a large salamander family, widely distributed in the Northern Hemisphere and including the common European salamanders. Newts are lizardlike in shape and are usually under 6 in. (15 cm) long including the slender tail.
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 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 frogsfrog,
common name for an amphibian of the order Anura. Frogs are found all over the world, except in Antarctica. They require moisture and usually live in quiet freshwater or in the woods. Some frogs are highly aquatic, while others are better adapted to terrestrial habitats.
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 and toadstoad,
name applied to certain members of the amphibian order Anura, which also includes the frog. Although there is no clear-cut distinction between toads and frogs, the name toad
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 are specialized for jumping, with long, muscular hind legs, while the tropical caecilianscaecilian
, any of the legless, tailless tropical amphibians belonging to the order Gymnophiona (or Apoda). Most adult caecilians resemble earthworms superficially but have vertebrate characteristics such as jaws and teeth.

Caecilians range in size from 7 in. to 4.
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 are burrowing forms that have lost all but vestigial traces of their limbs.

Class Reptilia

The reptilesreptile,
name for the dry-skinned, usually scaly, cold-blooded vertebrates (see Chordata) of the order Reptilia. Reptiles are found in a variety of habitats throughout the warm and temperate regions (except on some islands), with the greatest variety in the tropics.
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, 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 dinosaursdinosaur
[Gr., = terrible lizard], extinct land reptile of the Mesozoic era. The dinosaurs, which were egg-laying animals, ranged in length from 2 1-2 ft (91 cm) to about 127 ft (39 m).
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 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 lizardslizard,
a reptile of the order Squamata, which also includes the snake. Lizards form the suborder Sauria, and there are over 3,000 lizard species distributed throughout the world (except for the polar regions), with the greatest number found in warm climates.
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 and snakessnake,
common name for an elongated, limbless reptile of the order Squamata, which also includes the xlizards. Most snakes live on the ground, but some are burrowers, arboreal, or aquatic; one group is exclusively marine. In temperate climates they hibernate.
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. 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.

Class Aves

The birdsbird,
warm-blooded, egg-laying, vertebrate animal having its body covered with feathers and its forelimbs modified into wings, which are used by most birds for flight. Birds compose the class Aves (see Chordata). There are an estimated 9,000 living species.
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 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.

Class Mammalia

The mammalsmammal,
an animal of the highest class of vertebrates, the Mammalia. The female has mammary glands, which secrete milk for the nourishment of the young after birth. In the majority of mammals the body is partially or wholly covered with hair; the heart has four chambers, and
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 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 monotremesmonotreme
, name for members of the primitive mammalian order Monotremata, found in Australia, Tasmania, and New Guinea. The only members of this order are the platypus, or duckbilled platypus, and the several species of echidna, or spiny anteater.
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 of Australia) bear live young. The mammalian egg contains little yolk. In the marsupialsmarsupial
, member of the order Marsupialia, or pouched mammals. With the exception of the New World opossums and an obscure S American family (Caenolestidae), marsupials are now found only in Australia, Tasmania, New Guinea, and a few adjacent islands.
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 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.

Adaptive radiation has resulted in great diversity of placental mammalian 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 primatesprimate,
member of the mammalian order Primates, which includes humans, apes, monkeys, and prosimians, or lower primates. The group can be traced to the late Cretaceous period, where members were forest dwellers.
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, 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.

Bibliography

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).

Chordata

The highest phylum in the animal kingdom, which includes the lancelets or amphioxi (Cephalochordata), the tunicates (Urochordata), the acorn worms and pterobranchs (Hemichordata), and the vertebrates (Craniata) comprising the lampreys, sharks and rays, bony fish, amphibians, reptiles, birds, and mammals. Members of the first three groups, the lower chordates, are small and strictly marine. The vertebrates are free-living; the aquatic ones are primitively fresh-water types with marine groups being advanced; and the members include animals of small and medium size, as well as the largest of all animals. See Vertebrata

The typical chordate characteristics are the notochord, the dorsal hollow nerve cord, the pharyngeal slits, and a postanal tail. The notochord appears in the embryo as a slender, flexible rod filled with gelatinous cells and surrounded by a tough fibrous sheath, and contains, at least in some forms, transverse striated muscle fibers; it lies above the primitive gut. In lower chordates and the early groups of vertebrates, the notochord persists as the axial support for the body throughout life, but it is surrounded and gradually replaced by segmental vertebrae in the higher fish.

The dorsal hollow nerve cord grows from a specialized band of ectoderm along the middorsal surface of the embryo by a folding together of two parallel ridges. The anterior end enlarges slightly in larval tunicates and somewhat more in lancelets, but enlarges greatly in the vertebrates to form the brain. Vertebral evolution is characterized by continual enlargement of the brain. See Nervous system (vertebrate)

Paired slits develop as outpocketings of the posterior end of the mouth on the sides of the embryonic pharynx, a part of the digestive system, and are retained in all aquatic chordates. Pharyngeal slits originated as adaptations for filter feeding but soon became the primary respiratory organ, as blood vessels line the fine filaments on the margins of each slit. Water passing over the gills serves for gas exchange in addition to the original filter-feeding function, which was soon lost in the vertebrates. Internal gills were lost with the origin of tetrapods; larval and some adult amphibians possess external gills which are different structures. The pharyngeal slits in embryonic tetrapods close early in life, with the pharyngeal pouches becoming the site for development of glands, for example, the thyroid and the tonsils. See Respiratory system

The chordate tail is part of the skeletal support, muscles, and nervous system which continues posteriad to the anus or posterior opening of the digestive system. It is a feature not found in any other animal group and serves to increase the force available to the animal for locomotion.

Much controversy still exists about the limits, origin, and affinities of the chordates. For example, opinions differ considerably as to whether the Hemichordata and the Pogonophora are related to the Chordata, although there is no question that the Hemichordata are closely related and part of the pharyngeal-slit filter-feeding radiation; the Hemichordata are here considered as members of the phylum Chordata, not as a separate phylum. Almost all workers agree that the Echinodermata are the closest relatives of the Chordata because of evidence ranging from embryonic development to biochemical resemblances, but there is dispute over which group is the more primitive. See Echinodermata, Pogonophora

The Chordata apparently arose from a group of elongated, segmented worms with three sets of body musculature (longitudinal, circular, and transverse) and transverse septa. The first change was the evolution of a segmented coelom, associated with improved locomotion; these animals possessed a hydrostatic skeleton and moved with a sinusoidal or peristaltic locomotion. The first chordate feature to appear was the notochord, which provided a stronger skeleton and permitted the reduction of the transverse and circular muscles. A notochord resulted in a fixed body length and the loss of peristaltic locomotion. The dorsal longitudinal muscles enlarged, and with this modification came the evolution of the dorsal hollow nerve cord. Having a notochord for support rather than a hydrostatic skeleton permitted the appearance of pharyngeal slits through the lateral walls of the anterior parts of the body, which served for increased filter feeding and subsequently for respiration. The presence of the notochord also permitted the appearance of a postanal tail and increased force for locomotion.

The earliest chordate with all of the typical features of the phylum probably looked much like the present-day lancelet or amphioxus (Cephalochordata), which burrows in shifting sands and needs considerable force to move through the heavy sand. Presumably all other chordates developed from this ancestral type, with their differing characteristics evolving because of conditions of their differing habitats.

Chordata

 

the highest phylum of coelomates. The phylum includes the subphyla Tunicata, Acrania, and Vertebrata. The chordates all have, during some stage of their development, a notochord, a dorsal nerve cord, and gill slits. In almost all individuals the dorsal nerve cord persists for life. In vertebrates the brain is formed on the anterior tip of the nerve cord, while in adult ascidians the nerve cord is replaced by a ganglion. In aquatic forms the gill slits persist for life, while in terrestrial vertebrates, which breathe by means of pulmonary respiration, they are found only in larvae and in early stages of development. The bodies of chordates are segmented; for example, metamerism is observed in acranians and vertebrates during their embryonic development.

Chordata

[kȯr′däd·ə]
(zoology)
The highest phylum in the animal kingdom, characterized by a notochord, nerve cord, and gill slits; includes the urochordates, lancelets, and vertebrates.
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Although the two phyla appear quite different today, chordates are often linked with the hemichordates and another dissimilar phylum, the echinoderms, which include starfish.
By pushing the record of chordates so close to this pivotal event, the new discovery "accentuates the speed of the diversification," says Conway Morris.
If Chen and his colleagues prevail, Yunnanozoon will indicate that the chordates began with all the other phyla during the Cambrian explosion.
The Cambrian marked the birth of most complex animal phyla on Earth today, including the arthropods, mollusks, echinoderms, and our own group, the chordates.
In particular the deep sea species show how a primitive animal with some chordate characteristics but no tail or limbs can drag itself across the sea floor using a muscular collar to gather food and then float above the sea floor to new feeding sites.