$A$ jawless fish,which lays eggs in fresh water and whose larvae after metamorphosis return to the ocean is

$Petromyzon$ is:

Which of the following animals belong to the class Cyclostomata?

Which animal has a circular and sucking mouth located on the anteroventral side?

$A$ group of researchers procured some fish-like animals and upon investigation the following characters were observed: $A$. Endoskeleton was made of cartilage. $B$. Ectoparasitic; as they were found attached on fish skin with their circular sucking mouth. $C$. Paired fins and scales were absent, but $7$ pairs of gill slits were present. Which of the following species of animals did they consider to fit best with these characters?

Write a short note on class Cyclostomata.