Which of the following groups has no member having gliding or flying appendages?

$A$ 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?

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

Which among the following is the only vertebrate osmoconformer?

Petromyzon is a connecting link between

What type of scales are present in $Cyclostomata$?