Agnatha

Class Agnatha consists of an ancient group of animals similar to fish but with some very noticeable differences. The agnathans lack jaws and paired fins. Instead of jaws, they have a cyclostomic (circular) toothed mouth with which they bore into the side of a fish and suck the blood of their victim. Agnathans lack an internal skeleton made of ture bone. As with sharks (another ancient group), the internal skeleton consists primarily of cartilage. Hagfish do not have a skeleton, except they do have a skull, which is made of cartilage. Because of this, many researchers think hagfish should not be in the subphylum Vertebrata. However, because of its fins and gills, they are called fish. The original 19th century classification groups hagfish and lampreys together as Agnatha. An alternative scheme proposed that jawed vertebrates are more closely related to lampreys than to hagfish, so vertebrates include lampreys but exclude hagfish. However, recent DNA evidence supports the original scheme placing both hagfish and lampreys as the oldest ancestors of the vertebrates.

There are two living groups of Agnatha - the Lampreys (class Hyperoartii) and the Hagfish (class Myxini)

Key Features of Agnatha

Jaws are absent.
Paired fins are generally absent.
Early species had heavy bony scales and plates in their skin, but these are not present in living species.
In most cases the skeleton is cartilaginous.
The embryonic notochord persists in the adult.
Seven or more paired gill pouches are present.
The digestive system lacks a stomach.

Lampreys

Lampreys are found in marine and freshwater environments. Some species are parasitic, attaching their sucker-like mouth to a fish, then using sharp teeth to rasp away at the animal's flesh. The lamprey has a larval stage. The ammocoete larva lives in fresh water, buried in mud (the adults can be either freshwater, brackish or marine depending on the species).The ammocoete larva has to undergo metamorphosis to change into an adult.

Hagfish

The modern hagfish are all marine, living on the bottom and usually in burrows. Rather worm-like in appearance, the hagfish lack paired fins, only having a slight tail fin. They have no eyes and no scales. They also have rows of horny teeth on the tongue which rasp at the food. Surrounding the mouth is a ring of tentacles. Hagfish produce a great deal of slime.

References

FISHES
http://www-biol.paisley.ac.uk/courses/Tatner/biomedia/units/fish1.htm

Agnatha - Jawless Fish
http://www.kheper.auz.com/gaia/biosphere/vertebrates/agnatha/