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Chordate

                        CHORDATE, common name for animals of the phylum Chordata, which includes
                        vertebrates as well as some invertebrates that possess, at least some time in their
                        lives, a stiff rod called a notochord lying above the gut and beneath a single, hollow
                        nerve cord. About 43,700 living species are known, making the chordates the third
                        largest animal phylum. Three subphyla exist: Cephalochordata, the fishlike
                        lancelets, with 25 species; Tunicata, the highly modified tunicates, with about 2000
                        species; and Vertebrata, animals with backbones made up of vertebrae (including
                        fishes, amphibians, reptiles, birds, and mammals), with about 41,700 species. The
                        closest relatives of the chordates, the acorn worms of the phylum Hemichordata,
                        are sometimes treated as a chordate group but show only a trace of a dorsal
                        nerve cord and a structure vaguely resembling a notochord. The best indication
                        of their relationship to chordates is the presence of holes in the pharynx. In both
                        hemichordates and lower chordates the pharynx with its gill slits forms a complex
                        structure that strains food particles from water.

                        Lancelets

                        These animals, which look rather like fish, have a well-developed notochord that
                        provides support for muscles used in swimming. Lancelets live in sand and feed with
                        their gill apparatus. Although they are much simpler than fish?no heart or paired
                        fins exist, and scarcely a trace of a brain?the arrangement of parts is similar in
                        these animals.

                        Tunicates

                        Only the very young tunicates reveal their relationship to other chordates. The
                        tadpolelike larva has a globular body and a tail used in swimming. When it attaches
                        itself to the seafloor, however, it loses the tail, notochord, and dorsal nerve cord. The
                        adult is covered with a protective covering called a tunic and feeds with its gill
                        apparatus. 

                        Vertebrates

                        Many features found in lancelets and young tunicates can be detected in modified
                        form in vertebrates, especially in embryos and in such primitive animals as jawless
                        fishes. The pharyngeal holes, for example, are retained in fishes and in the embryos
                        of more advanced animals, but in the latter the feeding and then the respiratory
                        functions of the gill slits become lost. The notochord of a vertebrate becomes
                        reinforced with bone and is surrounded by the spinal column; the dorsal nerve cord
                        grows more complex, with a brain and a protective skull. Many vertebrate features
                        can be traced to precursors in lancelets.