Subphylum Cephalachordata consists of very simple animals that exhibit the hallmarks of the phylum Chordata. The lancelets, as animals in this subphylum are known, have a mouth in the head end of the body, gill slits in the pharynx region, a dorsal nerve cord contained inside a notochord, and a tail that extends past the anus. Unlike tunicates, the lancelets exhibit these features as adults and as larvae. Lancelets also show bilateral symmetry, and are much longer than they are wide. They have bladelike structures on the outside of their body and have segmented muscles that can bend, allowing the animal to slowly swim. However, lancelets live in marine sands and feed with the majority of their body submerged in the sand but with their head sticking out. The gill apparatus traps food particles in the water, just like the process that occurs in tunicates. It is believed that lancelets are the closest living relatives of vertebrates, and they shared a common ancestor that was stationary and probably resembled an adult tunicate. This subphylum is very small, consisting of only about 20 species.

This subphylum is unique because it contains the only chordates that show the four chordate hallmarks in the adult stage yet lack a brain and a vertebral column. The tunicates lack these two features of vertebrates as well, but they do not show the four chordate hallmarks (a nerve cord, a notocord, gill slits, and a post anal tail) in maturity. This makes subphylum Cephalachordata unique.