The lamprey $(Petromyzon)$ is included in the same taxonomic class as the

What is the number of gill slits in a hagfish?

Petromyzon belongs to which of the following groups?

Which of the following animals are ectoparasites on fish in their adult stage?

$A$ 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.