Any of numerous animals belonging to the phylum Chordata, having at some stage of development a dorsal nerve cord, a notochord, and gill slits and including all vertebrates and certain marine animals, such as the lancelets.
There are a few creatures which belong in the phylum Chordata, but which are not vertebrates (no backbones). These have a "notochord" for nerve transmission.
nova net anwser: back.