Phylum Chordata

Subphylum Vertebrata

Superclass Agnatha

Classes Myxini (hagfish) and Cephalaspidomorphi or Petromyzontes (lampreys)

Class Myxini (hagfish) - entirely marine, feed mostly on dead fish, worms, molluscs and crustaceans, therefore are mostly scavengers rather than parasites like lampreys.

Class Cephalaspidomorphi (lampreys) - both marine and freshwater spp.; all spp. breed in freshwater; most of the marine forms spend nearly all of their adult lives parasitizing marine, bony fish.