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.

43 spp., best known one in North America is the Atlantic hagfish, Myxine glutinosa (Gr. myxa, slime) - secretes large amounts of mucosal slime to facilitate locomotion due to lack of paired fins; tentacles or barbels surround the mouth; eyes are degenerate and virtually non-functional, but they do have a very keen sense of smell and touch; the notochord persists in the adult; circulation and gas exchange facilitated by accessory hearts and atrial (buccal) chambers, especially in juvenile stage; development is direct (no larval stage); most spp. are hermaphroditic, but some spp. have only one functional set of sex organs and therefore function sexually as either distinctly male or distinctly female.

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.

The sea lamprey scientific name, Petromyzon marinus (Gr. petros, stone; myzon, sucking), refers to their habit of grasping stones with the mouth to hold position in currents; P. marinus is found in both American and European Atlantic coastal waters; may reach a length of 1 m.

Lamprey larvae are called ammocoetes; P. marinus ammocoetes live as filter feeders in low-current stream bed sediments. After a very long larval stage of 3 to 7 or more years, the ammocoetes metamorphose into parasitic adults that may remain in freshwater or may migrate to marine habitats.

When attached to the host fish, sea lampreys rasp away the flesh and such out body fluids, injecting an anticoagulant into the wound to prevent blood clotting. The adults that remain in freshwater live for one or two years before spawning and then they die; ones that return to marine habitats usually live a few years longer.

The brook lampreys (freshwater spp.) are nonparasitic; the ammocoete larvae filter feed, metamorphose into adults which do not feed, and then they spawn within a few months and die.