- any of several small, lancet-shaped burrowing marine animals of the subphylum Cephalochordata, having a notochord and bearing structural similarities to both vertebrates and invertebrates.
- any of several marine animals of the genus Branchiostoma (formerly Amphioxus), esp B. lanceolatus, that are closely related to the vertebrates: subphylum Cephalochordata (cephalochordates)Also called: amphioxus
- Any of various small, transparent, fishlike marine organisms of the subphylum Cephalochordata that are related to vertebrates but have a notochord instead of a true backbone. Unlike other primitive chordates, lancelets have a body divided into serially repeated muscular segments. Also called amphioxus