Q.What is meant by "branchial heart" in fishes?

Cartilaginous fishes (Chondrichthyes) have a cartilage skeleton and no swim bladder; bony fishes (Osteichthyes) have a bony skeleton and a swim bladder — along with several other consistent differences.

Class Pisces (fishes) is divided into two major classes based fundamentally on the composition of the skeleton:

Cartilaginous fishes (class Chondrichthyes) — e.g. sharks, rays, skates:

• Endoskeleton made entirely of cartilage, never ossified into true bone.
• Exclusively marine in distribution.
• Body covered by placoid scales (small, tooth-like scales with an enamel-dentine structure, giving sandpaper-like skin).
• No swim bladder — buoyancy is instead helped by a large, oil-filled liver and constant swimming; without a swim bladder these fishes must keep swimming or sink.
• Mouth typically ventral (on the underside of the head).
• Gill slits are separate and exposed (typically 5-7 pairs), without a covering operculum.
• Fertilisation is mostly internal, and many species are ovoviviparous or viviparous.
• Persistent notochord, often persisting alongside the cartilaginous vertebral column.

Bony fishes (class Osteichthyes) — e.g. Catla, Labeo, Rohu, tuna:

• Endoskeleton made of true, ossified bone.
• Found in both marine and freshwater habitats.
• Body covered by thin, overlapping cycloid or ctenoid scales.
• Possess a gas-filled swim bladder (air bladder), which provides buoyancy control, letting the fish maintain depth without constant swimming.
• Mouth typically terminal (at the front tip of the head). …