In which class of animals do the larvae return to the sea after metamorphosis?

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

Jawless vertebrates are placed in the group .........

$A$ 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?

Which one of the following pairs of animals comprises 'jawless fishes'?

Which of the following serves as the respiratory organ in animals belonging to the class $Cyclostomata$?