1) The name comes from the greek (Agnatha = “negation”) + (Gnathos = “mandible”), and it’s commonly called jawless fish.
2) It comprises little more than 100 species and a large number of fossil forms.
3) Agnathans have few characteristics in common; the only universal character of all agnathans (fossils and currents) is the absence of jaws, a primitive trait in ancestral species.
4) The Palaeozoic forms were of very different aspect to the current Agnathan species; many had strong bony shields on their heads that would protect them from predators and were covered with scales of various kinds.
5) The current Agnathan species have an eel-like appearance, lack scales and have a slippery mucous skin.
6) At present only the Lampreys (Petromyzontidae) and the Hagfishes (Myxini) survive.
7) Lampreys and Hagfishes would be the latest current representatives closest to those primitive animals of the Paleozoic Era, which emerged some 510 million years ago, and which were very abundant until their disappearance about 370 million years ago.
8) The current agnathans feed in a very particular way that is not given in the rest of vertebrates, due to the absence of jaws.
9) They are hematophagous in the case of the lampreys, and ghouls in the case of the Hagfishes.
10) They inhabit the temperate waters of both hemispheres, and in the colder tropical waters.