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 of bone. As with sharks (another ancient group), the internal skeleton consists primarily of cartilage. There are two living groups of Agnatha - the Lampreys and the Hagfish
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 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.
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.
Agnatha - Jawless Fish