Key: Step 5. Hagfishes/lampreys

5a. Snout with prominent barbels. Nasal opening at tip of mouth. Mouth not formed as funnel or disc. A single pair of external gill openings remote from head. Dorsal fin reduced, not separate from caudal fin – Family Myxinidae.

Step 6. Family Myxinidae

5b. Snout without barbels. Nasal opening on upper side of head. Mouth formed as funnel or sucking disc. Seven pairs of external gill openings close to head. Dorsal fins well developed, separate from caudal fin – Family Petromyzonidae.

Step 7. Family Petromyzonidae
Decision path