Describe the evolutionary change in the pattern of heart among the vertebrates.

(N/A) All vertebrates possess a heart,which is a hollow muscular organ composed of cardiac muscle fibres. The primary function of the heart is to pump blood to all parts of the body. The evolution of the heart is driven by the need to separate oxygenated blood from deoxygenated blood for more efficient oxygen transport.
$1$. Pisces (Fishes): The heart is two-chambered,consisting of one auricle and one ventricle. It receives only deoxygenated blood,which is pumped to the gills for oxygenation. It also contains accessory chambers like the sinus venosus and conus arteriosus.
$2$. Amphibians: They possess a three-chambered heart with two auricles and one ventricle. The auricles are separated by an inter-auricular septum,but the ventricle remains undivided,leading to the mixing of oxygenated and deoxygenated blood.
$3$. Reptiles: Most reptiles have an incomplete four-chambered heart (except for crocodiles,alligators,and gharials). The ventricle is partially divided by an incomplete septum,resulting in some mixing of blood.
$4$. Birds and Mammals: They possess a complete four-chambered heart with two atria and two ventricles. $A$ complete muscular septum prevents the mixing of oxygenated and deoxygenated blood,ensuring highly efficient circulation.