Anadromous fishes move

Which of the following statements are not true?
$a$. Dermal scales of fishes form the exoskeleton.
$b$. Cycloid and ctenoid scales have distinct lines of growth which aid in determining the age of the fish.
$c$. Fish scales are never shed.
$d$. The skin in elasmobranchs is covered with placoid scales.
$e$. The development of a placoid scale is not like the development of a tooth.

Which fish has the ability to breathe air?

Which of the following characteristic features always holds true for the corresponding group of animals?

In $Chondrichthyes$,the heart possesses:

Which class of animals lacks an exoskeleton?