Electric organs are found in

Polymorphism occurs in

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.

Anadromous fishes move

Identify the figure.

Statement $X$: Boat-shaped body,skin covered with placoid scales.
Statement $Y$: Mouth is generally ventral,jaws are generally toothed.