Solution

Heart is a hollow muscular organ present in all vertebrates which pump blood to whole of the body. Evolutionary change in the heart is observed on the basis of separation of oxygenated and deoxygenated blood for efficient oxygen transport to the body.
Fish heart- It is made up of 2 chambers- one auricle and one ventricle. Since there is no division of auricles and ventricles, deoxygenated blood passes through it which enters the gills for oxygenation. It has two additional chambers such as sinus venosus and conus arteriosus.
Piscean heart

Amphibian heart- It has 3 chambers- 2 auricles and one ventricle. The auricles are divided by a septum while ventricle is undivided. Additional chambers such as sinus venosus and conus arteriosus are also present. The deoxygenated blood from whole body enters the right auricle while oxygenated blood enters the left auricle from the lungs and ultimately two types of blood get mixed up in ventricle.

Amphibian heart

Reptilian heart- It is incompletely 4 chambered except in crocodiles, alligators and gharials. They have only one accessory chamber called sinus venosus. The reptilian heart also shows mixed blood circulation.

Reptilian heart

Avian and mammalian hearts- It is made up of four chambers- 2 auricles and two ventricles. These two pairs of chambers separate oxygenated and deoxygenated blood. The upper two chambers are called atria and the lower two chambers are called ventricles. The chambers are separated by a muscular wall that prevents the mixing of the blood rich in oxygen with the blood rich in carbon dioxide.
Avian and mammalian hearts