Which fish has an electric organ?

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.

The electric ray fish is a...

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

Which fish gives birth to young ones?

In which of the following animals,the heart does not have the left and right auricles?