Why are the tunicates and lanceletes classified as invertebrate chordates?

Phylum Chordata simply consists of all animals that, at least in their embryonic phase, have a structure called the notochord. In the vertebrates, the notochord develops into the vertebrae, i.e. the backbone. Some primitive chordates, though, including the tunicates, lancelets, and hagfish, do not ever develop vertebrae, although they have/have had a notochord. Hence, Subphyla Urochordata and Cephalochordata, and Class Myxini are invertebrate chordates.