How many gill slits are present in Cyclostomata?

Cyclostomata belongs to which division?

$Petromyzon$ is:

The correct classification of $Myxine$ is:

Select the incorrect statements with respect to Cyclostomes:
$(a)$ They lack scales and paired fins.
$(b)$ They have a circular mouth with jaws.
$(c)$ They bear $6-15$ pairs of gills.
$(d)$ They migrate to deep sea for spawning.
Choose the most appropriate answer from the options given below:

Which of the following sets of animals belong to the class $Cyclostomata$?