Q.How do you justify the statement "heart in fishes is a branchial heart"?

Think about how different animals get around — a fish glides through water, a frog hops between pond and land, a bird flies, a mammal runs. Their bodies demand different amounts of energy, and energy needs oxygen. The heart is the pump that delivers oxygen-carrying blood, and across the vertebrates it becomes steadily better at keeping fresh, oxygen-rich blood separate from stale, used blood. That gradual improvement in heart design across groups is what we mean by vertebrate heart evolution.

Every vertebrate has a muscular, chambered heart, but the number of chambers is not the same in every group. The pattern runs like this:

• Fishes have a two-chambered heart — one atrium and one ventricle.
• Amphibians and reptiles (crocodiles aside) have a three-chambered heart — two atria but only a single ventricle.
• Crocodiles, birds and mammals have a four-chambered heart — two atria and two ventricles.

The number of chambers matters because it decides how well oxygenated and deoxygenated blood are kept apart. In a fish, the heart handles only deoxygenated blood, which is sent to the gills to pick up oxygen. In amphibians and reptiles, oxygenated and deoxygenated blood both empty into the single ventricle and mix there, so the blood pumped to the body is a blend of the two. Only in birds and mammals do two separate ventricles keep the two streams completely apart, so pure oxygenated blood reaches the tissues. …