Which of the following are animals belonging to the class $Cyclostomata$?

Skin without scales and presence of unicellular mucous glands are characteristics of which class of animals?

Petromyzon attaches itself to the host body with the help of:

Ventral muscular heart,kidneys for excretion and osmoregulation,and paired appendages are found in all of the following $EXCEPT$:

Which of the following pair of animals are jawless fishes?

Assertion: Cyclostomates are marine but migrate for spawning to fresh water.
Reason: After spawning,within a few days they die. Their larvae after metamorphosis return to the ocean.