A subphylum of the phylum Chordata comprising the lancelets, including Branchiostoma (amphioxus). They are also known as the Leptocardii. Lancelets are small fishlike animals, not exceeding 3.2 in. (80 mm) in length. They burrow in sand on the ocean bed or in estuaries in tropical and temperate regions throughout the world. Only two genera are recognized, Branchiostoma with 23 species and Asymmetron with 6 species.