any of a class (Agnatha) of fishes with a cartilaginous skeleton, an eel-like body, and a circular, sucking mouth lacking jaws, consisting of the lampreys and hagfishes
See jawless fish in American Heritage Dictionary 4
noun
Any of several eellike marine and freshwater fishes lacking a jaw and paired appendages that constitute the subphylum Agnatha. The only present-day jawless fish are the hagfish and lampreys.