Agnatha is a superclass of jawless fishes. They are the most primitive class of vertebrates because they do not have some of the other vertebrates characteristics: they do not have jaws (so their mouth is constantly open and round), they do not have vertebrae, they do not have paired limbs... The oldest fossil agnathans appeared in the Cambrian. Two groups of these ancient fishes still survive today: the lampreys and the hagfish, comprising about 120 species in total.
