- They have an elongated body bearing 6-15 pairs of gill slits for respiration.
- Cyclostomes have a sucking and circular mouth without jaws and body is devoid of scales and paired fins.
- Cranium and vertebral column are cartilaginous and show closed type of circulation.
- Cyclostomes are marine but migrate for spawning to fresh water, after spawning, within a few days, they die and their larvae, after metamorphosis, return to the ocean.
Examples: Petromyzon (Lamprey) and Myxine (Hagfish).