In which class of animals do the larvae return to the sea after metamorphosis?

What type of scales are present in $Cyclostomata$?

Which one of the following pairs of animals comprises 'jawless fishes'?

Statement $- I :-$ All living members of class $Cyclostomata$ are ectoparasites on some fishes.
Statement $- II :-$ $Cyclostomata$ have a sucking and circular mouth without jaws.

Which of the following pair of animals are jawless fishes?

Which of the following are animals belonging to the class $Cyclostomata$?