Links to other chordate pages in this series are indicated below, so you can use this classification as a kind of chordate table of contents. A simple classification of vertebrates to the species level would occupy hundreds of pages and leave the reader so mired in detail that it would be of little value, except to the specialist. It would be severely limited because most species are extinct and have never been found as fossils. It would be obsolete immediately as new data were discovered and opinions were changed. If we cannot really agree on what characters and which taxa constitute a class, how could we proceed to lower catagories with any pretense of accuracy.
Taxonomies show there is a hierarchical relationship by inference.
So, the subphyla exist and there is no ambiguity about their definition, as subphyla. Therefore, the position of urochordates, cephalochordates, and vertebrates in the list is stabile. This problem is discussed on the amphioxus webpage. Notice above that hagfish is not included with the vertebrates, although it is in the class Agnatha of Subphylum Vertebrata. The phylogenetic relationship between the three classes of jawed fishes remains fluid.