MODERN JAWLESS FISH: HAGFISH & LAMPREYS
There are two groups of living jawless, elongate eel-like animals lacking paired fins: hagfish and lampreys. These animals are the only living representatives of ancient creatures that gave rise to the fish and eventually humans. Although superficially alike, hagfish and lampreys are very different: while lampreys are clearly vertebrates, hagfish should not be considered as vertebrates at all.
Sub-Phylum Vertebrata, Class Cephalaspidomorphi lampreys (40+ living species)
Lampreys possess a skull, vertebrae and are assigned to the class Cephalaspidomorphi within the subphylum Vertebrata. Adults of most species of lampreys are parasitic with a funnel-like mouth that attaches to the host fish by suction. The oral disc and tongue is covered by tooth-like plates of keratin that are used to grasp the prey and rasp a hole through which the hosts tissues are sucked. Lamprey eggs develop into sedentary filter-feeding larvae ammoecetes. The ammocetes burrow into mud and feed on small organisms with only their oral hoods sticking out above the surface. This larval stage may last for 3 7 years. Once the ammoecetes have metamorphosed into adults, lampreys migrate to lakes and seas to feed; marine species return to fresh water to spawn.
1. Petromyzon marinus, sea lamprey: juvenile.
2. Petromyzon marinus, sea lamprey: ammocoete showing oral hood.
3. Petromyzon marinus, sea lamprey: oral disc of adult.
4. Petromyzon marinus, sea lamprey: adult.