Teeth are modified . . . . . . scales which are backwardly directed.

$Pristis$ belongs to the class:

Which fish has an electric organ?

Which one of the following is a cartilaginous fish?

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.

Pharyngeal gill slits are found in