Hemicyclaspis belongs to the class

What is found in the skin of Cyclostomata?

Which animals live in the sea but migrate to freshwater for breeding?

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.

Petromyzon is a connecting link between

What type of endoskeleton is found in cyclostomes?