All living members of class Cyclostomata are ectoparasites on some fishes. They have an elongated body bearing 6 to 15 pairs of gill slits for respiration. Cyclostomes have a sucking and circular mouth without jaws, so they belong to Agnatha. Their body is devoid of scales and paired fins. The cranium and vertebral column are cartilaginous. Circulation is of closed type. Cyclostomes are marine but migrate for spawning to fresh water. After spawning, within a few days, they die. Their larvae, after metamorphosis, return to the ocean. Examples include Petromyzon (lamprey) and Myxine (hagfish). The class is regarded as the most primitive group of chordates in the summary of the chapter. Questions often focus on the lack of jaws, the absence of scales and paired fins, the presence of 6 to 15 pairs of gill slits and the migration of these marine animals to fresh water for spawning, followed by the death of the adults after spawning and the return of metamorphosed larvae to the ocean. Because they are jawless, they are the only living members of the division Agnatha, in contrast to all other vertebrate classes, which are jawed and are therefore placed in Gnathostomata, a contrast that is often used in matching and true-false questions.