How many gill slits are present in Cyclostomata?

$A$ jawless fish,which lays eggs in fresh water and whose larvae after metamorphosis return to the ocean is

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

$Petromyzon$ is:

Match the name of the animal (Column-$I$) with one characteristic (Column-$II$), and the phylum/class (Column-$III$) to which it belongs.

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