Subphylum Urochordata (sea squirts)| Subphylum Cephalochordata
(lancelets) Subphylum Myxini (hagfish) Subphylum Vertebrata
(vertebrates) Placodermi - extinct Chondrichthyes (cartilaginous
fish) Acanthodii - extinct Actinopterygii (ray-finned fish)
Chordates (phylum Chordata) include the vertebrates, together
with several closely related invertebrates. They are united by
having, at some stage in their life, a hollow dorsal nerve cord
(the notochord), pharyngeal slits, a tail extending past the
anus, and bands of muscles that go around the body. The
traditional classification of vertebrates contains a wide
variety of paraphyletic groups, which in newer systems may
either be abandoned or greatly extended. No particular standard
system has developed yet, and the groups given at right should
be considered tentative. Other groups that have been used (in
alphabetical order):