A comparative study of the structure of heart of vertebrates reveals that there is a gradual development in the structure of heart in the evolutionary line. Among the vertebrates the lowest forms include the fishes which possess 2 chambered heart with an atrium and a ventricle. The next higher form, which is amphibians, possess 3 chambered heart with two atria and a single ventricle. The same structure is seen in reptiles also. In these two groups both the pure blood and impure blood gets mixed in the ventricle. The remaining higher forms which include the aves, the reptiles and the mammals possess 4 chambered heart - two atrium and two ventricle. All these groups exhibit double circulation and hence there is no mixing of oxygenated and deoxygenated blood.