Lampreys and Hagfishes
Adult lampreys have relatively large dorsal and ventral fins on the latter half of their bodies, and have a well-developed visual sense. These animals have a circular mouth that can be used to attach sucker-like to the body of a fish. Parasitic species of lamprey then rasp a hole in the body wall of their victim, using rows of keratinized, epidermal structures that function like teeth, and a tongue that can protrude beyond the mouth. The lamprey then feeds on the ground-up tissues and bloody discharges of its prey. If the victim is a large fish, it will generally survive the lamprey attack, and perhaps several attacks during its lifetime. The victim is seriously weakened, however, and may fail to reproduce, or may eventually succumb to environmental stresses a more vigorous animal could tolerate. Because lampreys do not usually kill their victims directly, they are generally considered to be a parasite, rather than a predator.
Lampreys also use their disk-mouths to hold onto rocks to stabilize themselves in moving water, and to move pebbles while digging their nests in a stream. The name of the common genus of lampreys, Petromyzon, translates as "stone sucker" from the Greek.
Lampreys can pump water directly into and out of their seven gill cavities through separate gill slits. This ability allows lampreys to ventilate water over their gills, even though their mouth may be actively used for feeding or sucking on rocks.
Most species of lampreys are anadromous, spending their adult life at sea or in a large lake, and swimming upstream in rivers to their breeding sites in gravelly substrate. The larvae of lampreys, known as an ammocoete, look very unlike the adult and were once believed to be a different species. The ammocoete larvae live in muddy sediment, and are a filter-feeder on suspended aquatic debris and algae. The larval stages can last for more than four years, finally transforming into the adult stage, when they reenter the oceans on lakes.