There are about a million described animals in the world. To communicate and study living organisms scientists have found it a necessity to name living organisms and classify them into groups. This classification is made chiefly on the basis of structures. Organisms with certain structures in common are placed in one group and those with other structures are put in other groups. From the most fundamental unit (species) to the largest group (Kingdom) these groups are an ascending series of ever increasing inclusiveness and are termed the hierarchical system of classification.