Which of the following animals are ectoparasites on fish in their adult stage?

The larva of $Petromyzon$ is known as

Describe reproduction in Cyclostomata.

Which of the following is a jawless class?

Which jawless fish lays eggs in freshwater,and after metamorphosis,its $Ammocoetes$ larvae return to the ocean?

Which of the following statements are correct for Cyclostomata?
$I$ - They have a sucking and circular mouth without jaws.
$II$ - Their body is devoid of scales and paired fins.
$III$ - Their cranium and vertebral column are cartilaginous.
$IV$ - They have a closed type of circulatory system.
$V$ - Cyclostomes live in freshwater,but migrate to the ocean for spawning.
$VI$ - They show direct development.