What is the most primitive fish

The sea lamprey is in many ways the most primitive of living animals with backbones. It attaches itself to a fish rasps a hole in the creature, and sucks out the blood and body fluids. The inset shows the sucking mouth of the lamprey, which has a large number of sharp teeth, and contains a tongue also provided with teeth.

Sea lampreys, however, can remain in fresh water all their lives, as is the case with the descendants of those that found their way through the Welland canal into the Great Lakes. Here the sea lamprey is considered a serious pest because it is believed to destroy large numbers of lake trout. In the ocean, too, this species attacks many commercially valuable fish. Although eaten today only in restricted localities, sea lampreys were considered a delicacy during the Middle Ages, and were used for food in the northeastern United States until about one hundred years ago. Not all lampreys prey on other fish; the brook lampreys do not feed at all as adults, but simply reproduce and die.