Amphioxus, plural amphioxi, or amphioxuses, also called lancelet, any of certain
members of the invertebrate subphylum Cephalochordata of the phylum Chordata.
Amphioxi are small marine animals found widely in the coastal waters of the warmer
parts of the world and less commonly in temperate waters. Both morphological and
molecular evidence show them to be close relatives of the vertebrates. Amphioxi are
seldom more than 8 cm (3 inches) long and resemble small, slender fishes without eyes
or definite heads. They are grouped in two genera—Branchiostoma (also
called Amphioxus) and Epigonichthyes (also called Asymmetron)—with about two dozen
species. The chordate features—the notochord (or stiffening rod), gill slits, and dorsal
nerve cord—appear in the larvae and persist into adulthood.
Amphioxi spend much of their time buried in gravel or mud on the ocean bottom,
although they are able to swim. When feeding, they let the anterior part of the body
project from the surface of the gravel so that they can filter food particles from water
passing through their gill slits. At night they often swim near the bottom. They burrow
into sand using rapid movements of the body, which is tapered at both ends and is
covered by a sheath (the cuticle).
The animals swim by contracting the muscle blocks, or myotomes, that run from end to
end on each side of the body. The blocks on each side are staggered, producing a
side-to-side movement of the body when swimming. Amphioxi are not buoyant, and they
sink quickly when they stop swimming. A dorsal fin runs along the entire back, becomes
a caudal fin around the tip of the tail, and then continues as a ventral fin; there are no
The notochord runs through the body from tip to tip, providing a central support. A slight
bulge distinguishes the anterior end of the nerve cord. Although there is no brain or
cranium, growing evidence suggests that the vertebrate brain evolved from a portion of
the nerve cord in a lancetlike ancestor. Blood flows forward along the ventral side and
backward along the dorsal side, but there is no distinct heart.