Heterocercal tail is found in

Which of the following statements are not true about shark?
$a$. The skin is tough and covered with placoid scales
$b$. They possess a swim bladder which regulates buoyancy
$c$. The tail of shark is heterocercal
$d$. Ampulla of Lorenzini present in the snout of shark is a thermoreceptor.
$e$. It has five pairs of gill slits covered by operculum

Which of the following animals possesses open gill slits?

Statement-$I$: Teeth of cartilaginous fishes are modified placoid scales which are backwardly directed.
Statement-$II$: Jaws of cartilaginous fishes are very powerful.

The heart of fishes is:

Cartilaginous fishes do not have