The chordates are the most advanced of all animals and are characterized
by the presence of a well-developed nervous system and a body supported
by a bony or cartilaginous _notochord_ and/or _spinal column_. In the
higher chordates (the vertebrates) the notochord is normally replaced by
bone, but in the lower chordates (for example, the graptolites) it
remains in a cartilaginous condition.
The phylum Chordata contains only two subphyla of paleontological
significance. These are the subphylum Hemichordata, composed of
primitive chordates (including the graptolites which are important
fossils), and the Vertebrata, which includes all animals with backbones.