Which of the following characteristics does not apply to a hagfish?

Animals of this class are marine,but migrate to freshwater for spawning.

Lampreys are

Petromyzon belongs to

$Petromyzon$ is:

$A$ 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?