A subphylum of chordates intermediate between the invertebrates and the true vertebrates. It includes the LANCELETS. Its members are characterized by a NOTOCHORD which extends into the adult stage head.
A portion of the animal phylum Chordata comprised of the subphyla CEPHALOCHORDATA; UROCHORDATA, and HYPEROTRETI, but not including the Vertebrata (VERTEBRATES). It includes nonvertebrate animals having a NOTOCHORD during some developmental stage.