A group of researchers procured some fish-like animals and upon investigation the following characters were observed: A. Endoskeleton was made of…

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 $\displaystyle 7$ pairs of gill slits were present. Which of the following species of animals did they consider to fit best with these characters ?