Short answers, pulled from the story.
Agnatha, or jawless fish, is a paraphyletic infraphylum of vertebrates characterized by the lack of jaws. The group includes living cyclostomes such as hagfishes and lampreys, along with extinct clades like conodonts and cephalaspidomorphs.
Living jawless fish comprise about 120 species in total. They are a minor element of modern marine fauna, though agnathans were prominent among the early fish of the early Paleozoic.
Agnathan teeth cannot move up and down, which limits their possible food types. Hagfish scavenge mostly dead animals using a row of sharp teeth, while lampreys feed on carrion and other fish and inject anticoagulant fluids into a host to make it yield more blood.
The skin of hagfish has copious slime glands, and the slime is their defense mechanism. The slime can clog up the gills of enemy fishes, causing them to die.
Lampreys reproduce in freshwater riverbeds, working in pairs to build a nest and burying their eggs about an inch beneath the sediment. Fertilization is external, lampreys can only reproduce once, and the hatchlings go through four years of larval development before becoming adults.
Conodonts lived from the Cambrian until the beginning of the Jurassic and went extinct during the lower Jurassic period, with some of the last surviving populations in Japan. It is now thought they were out-competed by newer Mesozoic taxa rather than wiped out by the end-Triassic extinction.
Molecular data from rRNA and mtDNA, together with embryological data, strongly supports that hagfishes and lampreys are more closely related to each other than to jawed fish, forming the superclass Cyclostomi. Recent DNA evidence supports grouping them together as cyclostomes.