- Kingdom: Animalia
- Phylum: Chordata
- Norwegian: ryggstrengdyr
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.
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).