All true fish belong to the phylum Chordata. Animals of this phylum possess a hollow backbone, and at various stages of development, all have grooves and slits in the area of the throat behind the mouth and a tail that extends past the anus.
The phylum has three major subphylums: Tunicata, Cephalochordata and Vertebrata. The third subphylum includes jawed and jawless infraphylums. Jawed fish contain three superclasses: four-limbed animals called tetrapods, cartilaginous fish such as sharks, and bony fish that comprise the majority of fish species. Other species such as hagfish, lancets and lampreys exist outside the superclass of bony fish, but they are not considered true fish.