In which of the following organisms does the heart pump only deoxygenated (impure) blood?

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 is the most venomous fish?

The heart of fishes is:

The thermoreceptor organs in $Scoliodon$ are ........

In rays (cartilaginous fish),the mouth is located on which side?