About 43,700 living species are known, making the chordates the third largest animal phylum. 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.
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.