Lateral line system is present in

Which of the following characteristic features always holds true for the corresponding group of animals?

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

Choose the correct option with regards to $Chondrichthyes$.

Which fin is heterocercal (asymmetrical) in cartilaginous fishes (Chondrichthyes)?

At present,the Dipnoans (lungfishes) are distributed over which of the following regions?