The word Agnatha means no jaws, a simple description that hides a complex evolutionary story. Scientists classify this group as a paraphyletic infraphylum within the subphylum Vertebrata of the phylum Chordata.
The oldest fossil agnathans appeared in the Cambrian period. Conodonts arose in the early Cambrian and remained common enough until their extinction in the Triassic period.
Living jawless fish comprise about 120 species in total today. These animals include hagfishes and lampreys which form the superclass Cyclostomi alongside extinct clades such as conodonts and cephalaspidomorphs.
Lampreys reproduce in freshwater riverbeds working in pairs to build a nest and burying their eggs about an inch beneath the sediment. The resulting hatchlings go through four years of larval development before finally becoming adults ready for reproduction themselves.
The skin of hagfish has copious slime glands that constitute their primary defense mechanism against predators. This slime can sometimes clog up enemy fishes' gills causing them to die from suffocation.