CHORDATE

any member of the phylum Chordata, which includes the most highly evolved animals, the vertebrates, as well as two other subphyla. Some classifications also include the phylum Hemichordata with the chordates. All of the chordates possess certain features at some time in their life cycle, including a dorsal supporting rod called the notochord, pharyngeal apertures (gill slits), and a dorsal nerve cord. These features are present in the young of all chordates, but they are altered or absent in adults of some chordate groups. The chordates include the mammals, birds, reptiles, amphibians, fishes, and lampreys (subphylum Vertebrata); lancelets, or amphioxus (subphylum Cephalochordata); and the tunicates (subphylum Tunicata, or Urochordata). The tunicates and cephalochordates differ markedly from the vertebrates in lacking any kind of brain or skeleton, although tunicate larvae show some resemblance to vertebrate embryos. Cephalochordates superficially resemble fishes, but their heads are poorly differentiated from their bodies. Chordate bodies consist essentially of a body wall encasing a gut, with a space between called the coelom. The body is usually long, with the right and left sides similar to each other (bilateral symmetry). It is supported by the notochord, which contains a substance resembling mucus. Also running along the back is a nerve cord that is slightly enlarged at the anterior end but that only develops into a true brain in vertebrates. The mouth and sense organs are concentrated in the anterior end, and the anus is at the base of the tail. Movement is achieved by the contraction of muscle tissues (myotomes) flanking the notochord. All of these characteristics are present in adult cephalochordates, larval tunicates, and embryonic vertebrates. When hatched the tunicate larva possesses the basic chordate characteristics: long, bilaterally symmetrical body; notochord; neural cord; and myotomes. It also has a tail, a rudimentary eye, a mouth, gill slits, and a gut, but it does not have an anus, since it does not ingest food and pass waste. It retains this form for little more than a day before metamorphosing into either a sessile (attached) or a floating adult. The tail is resorbed, and the body develops a coat, or tunic (hence the name tunicate). Two apertures emerge from the tunic, one to admit water (and with it, food) into the body and another to pass water out. Suckers form on the anterior part of the body and are used to attach the animal to a surface. In the adult, the gill slits are the only remaining chordate features. Members of the subphylum Cephalochordata are commonly called lancelets, or amphioxi. They rarely are longer than 5 to 6 cm (about 2 inches) and usually live on the sea bottom. The adult animal is sedentary, passing its time for the most part half-buried in sand or gravel. Water enters its body through the mouth and yields oxygen to the blood as the water passes through the gill slits. Food is extracted from the water and passed to the gut. Vertebrates are able to attain large sizes owing to their internal skeleton, which is composed of bone or cartilage (or both) and a dorsal vertebral column that provides support and flexibility. Vertebrates also have a spinal cord. The vertebrate groups known as fishes retain gill slits, while the land-dwelling forms lose the gills during embryonic development and instead respire by lungs. Terrestrial vertebrates have developed much more complex hearts than the strictly marine forms, as well as more complex excretory systems. All vertebrates except the lampreys and hagfishes have movable jaws. Reproduction among chordates is usually a sexual process that involves the fertilization of eggs, although tunicates can reproduce by asexual means. Most vertebrate eggs (those of mammals being the exception) have a high yolk content, which affects the rate and degree of cleavage, or cell division, in the embryo. In vertebrate embryos the development of the head is especially notable, along with the brain and the organs of sight, smell, and hearing. Because of their soft bodies, the primitive chordates have left no fossils, making it difficult to determine their evolutionary history. It is believed, however, that the phyla Chordata, Hemichordata, Echinodermata, and Pogonophora have a common sessile ancestor. Among the chordates the tunicates revert to the sessile state at maturity, while the cephalochordates and vertebrates apparently are derived from forms that attained sexual maturity in the free-swimming state. The vertebrates were able to swim into the relative safety of freshwater, where their eggs and larvae could develop in size. The amphibians developed from the fishes and in turn gave rise to the wholly terrestrial reptiles. From the reptiles came the birds and mammals, animals capable of regulating their body temperature, a capacity that led to an intensified activity and greater brain size. The higher vertebrates include the classes Agnatha (lampreys and hagfishes), Chondrichthyes (sharks and rays), Osteichthyes (bony fishes), Placodermi (extinct fishes), Amphibia (newts, frogs, and toads), Reptilia (turtles, crocodiles, lizards, snakes, and many extinct forms, including dinosaurs), Aves (birds), and Mammalia (mammals). any member of the phylum Chordata, which includes the vertebrates, the most highly evolved animals, as well as two other subphyla-the tunicates and cephalochordates. Some classifications also include the phylum Hemichordata with the chordates. As the name implies, at some time in the life cycle a chordate possesses a stiff, dorsal supporting rod (the notochord). Also characteristic of the chordates are a tail that extends behind and above the anus, a hollow nerve cord above (or dorsal to) the gut, gill slits opening from the pharynx to the exterior, and an endostyle (a mucus-secreting structure) or its derivative between the gill slits. (A characteristic feature may be present only in the developing embryo and may disappear as the embryo matures into the adult form.) A somewhat similar body plan can be found in the closely related phylum Hemichordata. Additional reading E.J.W. Barrington, The Biology of Hemichordata and Protochordata (1965), is an account of the lower chordates and their evolution; N.J. Berrill, The Origin of Vertebrates (1955), argues the thesis that the urochordate larva represents the prototype from which cephalochordates and vertebrates are derived; A. Willey, Amphioxus and the Ancestry of the Vertebrates (1894), an early but good comprehensive account, presents the orthodox theory of chordate relationships; R.P.S. Jefferies, The Ancestry of the Vertebrates (1986), expounds an alternate theory of chordate origin; and Libbie H. Hyman, The Invertebrates, vol. 5, Smaller Coelomate Groups (1959), is a classic work treating the hemichordates in extensive detail. Later works include Charles K. Weichert and William Presch, Elements of Chordate Anatomy, 4th ed. (1975); and R. McNeill Alexander, The Chordates, 2nd ed. (1981); supplemented by Brian Bracegirdle and Patricia H. Miles, An Atlas of Chordate Structure (1978). N.J. Berrill, The Tunicata with an Account of the British Species (1950, reprinted 1968), a taxonomic survey with a useful section on tunicate biology; Pierre P. Grasse (ed.), Trait de zoologie: anatomie, systmatique, biologie, vol. 11, Echinodermes, stomocords, procords (1966), an advanced zoological treatise devoted to protochordates, with good illustrations; W.A. Herdman, "Tunicata (Ascidians and Their Allies)" and "Cephalochordata," in The Cambridge Natural History, vol. 7, pp. 35-138 (1904), an important general account; R.N. Millar, The Marine Fauna of New Zealand: Ascidiacea (1982), a morphological account of a single species of ascidian; Willard G. Van Name, "The North and South American Ascidians," Bulletin of the American Museum of Natural History, vol. 84 (1945). For a later treatment, see "Invertebrate Chordates: Tunicates and Lancelets," in Vicki Pearse et al., Living Invertebrates (1987). Michael T. Ghiselin