Question

what is double circulatory system and what are its advantages

Answer 1

Double circulation refers to the circulation of blood twice through the heart. In human boy, Double circulation takes place, and it can be easily characterized by the presence of septum that divides the right part of the heart from the left part, and thus double circulation carries on. It comes from lungs carrying oxygen, to left artrium [a] then to left ventricle [v]. From there it goes to aorta to supply oxygenated blood to all body parts and returns to right a and then to right v  [this is where the same blood re-enters the heart] and then, goes to lungs for exchange of gases. 
Its advantages :-
It is required in organisms that need high energy demand. And there is no 'casual' mixing of deoxygenated with the oxygenated blood.