Cephalochordata (from Greek: κεφαλή kephalé, "head" and χορδή khordé, "chord") is a chordatesubphylum defined by the presence of a notochord that persists throughout life. It is represented in the modern oceans by the Amphioxiformes (lancelets, also known as amphioxus). Along with its sister phylum Urochordata, Cephalochordata can be classified as belonging to the taxon Protochordata.
The characteristics of Cephalochordata are that they are marine animals, segmented, and that they possess elongated bodies with a notochord that extends the length of the body and cirri surrounding the mouth for obtaining food. In cephalochordata the notochord extends from head to tail and it persists throughout their life. The members of this subphylum are very small and have no hard parts, making their fossils difficult to find. Fossilized species have been found in very old rocks predating vertebrates. There is famous fossil shale from the Middle CambrianBurgess Shale of British Columbia, which has yielded Pikaia fossils. Recently a different cephalochordate fossil (Yunnanozoon) has been found in south China. It dates to the early Cambrian period and is the earliest known fossil of the cephalochordate lineage. They have numerous gill slits, and have separate sexes.
phylogeny is based on a combination of studies of extinct  and extant  species.