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

Skin without scales and presence of unicellular mucous glands are characteristics of which class of animals?

Assertion: Cyclostomates are marine but migrate for spawning to fresh water.
Reason: After spawning,within a few days they die. Their larvae after metamorphosis return to the ocean.

Which of the following animals lack jaws?

Agnatha includes:

$A$ 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?