Chordates - Chordata

Characteristics:

All members of the phylum Chordata have a notochord, a hollow dorsal nerve cord, at some point. The spinal cord of tunicates degenerate at a certain phase in their life span. Humans and other vertebrates replace the notochord with a more complex skeleton structure during the embryonic stage.

Chordates have a pair of gills or similar organs to extract oxygen from a liquid environment. In humans, lungs replace rudimentary gill slits after the embryonic stage. As you know, most fish keep their gills for the rest of their lives.

Taxonomy:

Marine vertebrates (animals with an internal skeleton) like cartilaginous fish, ray-finned fish, hagfish and mammals are organized in the subphylum Vertebrata. Two other subphyla include species that show only remains of a chord string. These are the sea squirts (Tunicata) and the lancelets (Cephalochordata).