The group Agnatha consists of the jawless fishes, the most primitive group of extant vertebrates. While most agnathan species are now extinct, fossil evidence indicates that the group was once highly successful and extremely varied. Two lineages of agnathans have survived to the present, the lampreys and the hagfish.
Characteristics of Agnathans
As the most primitive members of the vertebrates, agnathans differ from all others in several important respects. First, they lack hinged upper and lower jaws and instead have unhinged circular mouths. They also lack the paired appendages (fins or limbs) that are found in other vertebrates. In addition, the internal skeleton of agnathans is not bony but cartilaginous . However, many extinct agnathans had extensively developed bony plates directly under the skin. These were most often found in the region of the skull and served as a protective armor. Bony plates are not present in extant agnathan species.
Major Groups of Living Agnathans
There are two major groups of living agnathans, the lampreys and the hag-fish. Both appear fishlike or eel-like.
Lampreys are parasitic species that use their suckerlike mouths to attach to a fish host. They use the many teeth in their mouths and on their tongues to rub at the flesh of their prey. Adult lampreys inhabit a saltwater marine environment but swim up rivers to reach freshwater breeding grounds. Lampreys breed only once in their lifetime, in a single tremendous reproductive bout, and die soon after. Lampreys pass through an immature larval stage before metamorphosing into adults. The larval lamprey is always in freshwater. It grows and matures for several years before undergoing metamorphosis and migrating to saltwater habitats . Before it was known that the larva was a larval lamprey, it was thought to be a separate species. The larva is of particular interest to biologists who study vertebrate evolution because it shares many features with the cephalochordate Branchiostoma (formerly called Amphioxus), which is the group believed to be most closely related to the vertebrates. The resemblance between Branchiostoma and the larval form of a very primitive vertebrate is striking, and supports the closeness of the relationship between the two groups.
The second group of living agnathans is the hagfish. Hagfish are scavenger species that feed off dead and wounded organisms in the ocean. They are also well-known for their defense mechanism; when threatened, hagfish ooze out great amounts of foul slime.
Evidence from the fossil record suggests that agnathans reached their peak of diversity between about 500 million and 340 million years ago. During this period, they were plentiful both in the seas and in freshwater habitats. More than 200 fossil species are known. The majority of these species were fairly small, perhaps a few inches long. The species that have survived to the present are but the remains of a group that was once considerably more diverse.
see also Phylogenetic Relationships of Major Groups.
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Hickman, Cleveland P., Larry S. Roberts, and Allan Larson. Animal Diversity. Dubuque, IA: Wm. C. Brown, 1994.
Hildebrand, Milton. Analysis of Vertebrate Structure. New York: John Wiley, 1994.