Petromyzon is a connecting link between

Which group of chordates possess sucking and circular mouth without jaws?

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.

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

Ammocoete larva occurs in the life-history of

Write a short note on class Cyclostomata.