Any of a variety of marine animals about 2.5 cm/1 in long. They have no skull, brain, eyes, heart, vertebral column, centralized brain, or paired limbs, but there is a notochord (a supportive rod) which runs from end to end of the body, a tail, and a number of gill slits. Found in all seas, lancelets burrow in the sand but when disturbed swim freely. (Genus Amphioxus, phylum Chordata, subphylum Cephalocordata.)

Taxonomically they are significant since the notochord may be regarded as the precursor of the backbone (spinal column).