Chordata
. Chordate, common name for animals of the phylum Chordata, which includes vertebrates as well as some invertebrates that possess, at least for some time in their lives, a stiff rod called a notochord lying above the gut and beneath a single, hollow dorsal 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...