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
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
- 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
- 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
- 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.