Hagfish, of the class Myxini and order Myxiniformes, are eel-shaped jawless fish. Hagfish are the only known living animals that have a skull but no vertebral column, although they do have rudimentary vertebrae. Hagfish are marine predators and scavengers who can defend themselves against other larger predators by releasing copious amounts of slime from mucous glands in their skin.

Agnatha is an infraphylum of jawless fish in the phylum Chordata, subphylum Vertebrata, consisting of both living (cyclostomes) and extinct species. Among recent animals, cyclostomes are sister to all vertebrates with jaws, known as gnathostomes.