Chordates derive their name from one of their synapomorphies, or derived features indicating their common ancestry. This is the notochord , a semi-flexible rod running along the length of the animal. In those chordates which lack bone, muscles work against the notochord to move the animal. All chordates have a notochord at some stage in their lives, but in some (such as tunicates) the notochord is lost in the adult, whereas in others (such as the vertebrates) the notochord is present in the embryo, but in later stages is largely replaced and surrounded by the vertebrae, or backbones.
The notochord runs beneath the dorsal nerve cord, which is another chordate feature. This is in contrast to organisms such as annelids and arthropods, in which the main nerve cord is ventral. The chordate nerve cord is hollow, with pairs of nerves branching from it at intervals and running to the muscles. The anterior (forward) end of the nerve cord is often enlarged into a brain.
Pharyngeal slits are a third chordate feature; these are openings between the pharynx, or throat, and the outside. They have been modified extensively in the course of evolution. In primitive chordates, these slits are used to filter food particles from the water. In fishes and some amphibians, the slits bear gills and are used for gas exchange. In most land- living chordates, the "gill slits" are present only in embryonic stages; you had pharyngeal slits at one time. The slits are supported by gill arches, which have also been highly modified in various groups of vertebrates.
Lastly, all chordates have a post-anal tail, or extension of the notochord and nerve cord past the anus. This feature is also lost in the adult stages of many chordates, such as frogs and people.
Chordates also have a closed circulatory system, and most, but not all, chordates have a heart. The blood of most chordates contains the oxygen-carrying molecule hemoglobin. The muscles of the body are segmented into blocks called myotomes. Like their relatives the echinoderms, chordates are deuterostomes: in early embryonic development, the anus forms before the mouth.
Buchsbaum, R., Buchsbaum, M., Pearse, J. and Pearse, V. 1987. Animals Without Backbones, 3rd ed. University of Chicago Press, Chicago.