Cross references: Chordates Hemichordates Protochordates
chordate subphylum defined by the presence of a notochord that persists throughout life. It is represented in the modern oceans by the 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 Cambrian Burgess 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."