Cyclostomata

Cyclostomata (literally, 'round mouths'), a class of vertebrates often included along with fishes, but separated from them by some fundamental divergences. Hag-fish (Myxine) and Lamprey (Petromyzon) are the two most familiar examples, and are types of the two orders into which the Cyclostomata (or Marsipobranchii) are divided.