Sea lamprey

They have no jaws.

An excerpt from the article 15 facts about Sea lamprey
They have jawless suckers studded with teeth. A single sharp tongue is situated in the middle part of the mouth. It serves to penetrate the scales and skin of their prey.

During an attack, sea lampreys attach themselves to the skin of their prey. The teeth are used to grasp and hold on while they feed on body fluids.