Class Cyclostomata

Cyclostomata (Greek cyklos, circle; stomata, mouth) are the most primitive living vertebrates, and every member of the class is placed in division Agnatha because none of them possess jaws. They are primitive, poikilothermic (cold-blooded), jawless aquatic vertebrates, and all are ectoparasites on other fishes.

The body is slender and eel-like, and bears six to fifteen pairs of gill slits along the sides for respiration. The mouth is circular, jawless, and suctorial (adapted for sucking/attaching to a host) — the feature that gives the class its name. The heart is two-chambered and circulation is closed. Cyclostomes lack paired appendages entirely, and their cranium and vertebral column remain cartilaginous rather than bony (consistent with their notochord persisting into adulthood — the defining feature of division Agnatha). …