In which of the following fishes is the dorsal fin modified into a sucker?

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

Which of the following marine cartilaginous fishes produces an electric current?

Which fish has the ability to breathe air?

Which of the following structures form the copulatory organs in sharks and rays?