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 suck 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;
• Those 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.