Phylum Chordata  
   Brief introduction to the chordates, defined as organisms that possess a structure called a notochord, at least during some part of their development. Includes links to information about subgroups and representative animals.
   http://animaldiversity.ummz.umich.edu/chordata.html

Gnathostomata  
   Philippe Janvier's article about the jawed vertebrates in the Tree of Life website.
   http://ag.arizona.edu/ENTO/tree/eukaryotes/animals/chordata/gnathostomata.html

Subphylum Vertebrata  
   What makes a vertebrate a vertebrate? They have more in common than a backbone. Find out more in this site from the Animal Diversity Web.
   http://animaldiversity.ummz.umich.edu/vertebrata.html

Vertebrata  
   Introduction to the vertebrates and their classification, including the Hyperoartia, or lampreys, and the Gnathostomata, or jawed vertebrates.
   http://tolweb.org/tree?group=Vertebrata&contgroup=Craniata

Chordata  
   Overview of the taxonomy of Phylum Chordata, which includes the well-known vertebrates (fishes, amphibians, reptiles, birds, mammals), hagfishes, tunicates, and lancelets.
   http://tolweb.org/tree?group=Chordata&contgroup=Animals