aquatic vertebrate - animal living wholly or chiefly in or on water

Agnatha, superclass Agnatha - superclass of eel-shaped chordates lacking jaws and pelvic fins: lampreys; hagfishes; some extinct forms

ostracoderm - extinct fishlike jawless vertebrate having a heavily armored body; of the Paleozoic

heterostracan - extinct jawless fish with the anterior part of the body covered with bony plates; of the Silurian and Devonian

cephalaspid, osteostracan - extinct jawless fish of the Devonian with armored head

anaspid - extinct small freshwater jawless fish usually having a heterocercal tail and an armored head; of the Silurian and Devonian

conodont - small (2 in) extinct eellike fish with a finned tail and a notochord and having cone-shaped teeth containing cellular bone; late Cambrian to late Triassic; possible predecessor of the cyclostomes

cyclostome - primitive aquatic vertebrate

lamper eel, lamprey, lamprey eel - primitive eellike freshwater or anadromous cyclostome having round sucking mouth with a rasping tongue

hagfish, slime eels, hag - eellike cyclostome having a tongue with horny teeth in a round mouth surrounded by eight tentacles; feeds on dead or trapped fishes by boring into their bodies