Lancelet. Branchiostoma lanceolatum, or Amphioxus lan-ceolatus) , a chordate animal of subphylum Acrania (Cephalo-chordata). The body of the lancelet is transparent, up to 8 cm long, and pointed at both ends.The notochord extends into the anterior end of the body.The caudal fin is lancet-shaped.ranchiostoma lanceolatum, or Amphioxus lan-ceolatus) , a chordate animal of subphylum Acrania (Cephalo-chordata). The body of the lancelet is transparent, up to 8 cm long, and pointed at both ends. The notochord extends into the anterior end of the body. The caudal fin is lancet-shaped.