Explanation

Complete Circulation When there is complete separation of oxygenated and deoxygenated blood in the heart, it is called complete circulation, $e.g.,$ birds and mammals Incomplete Circulation When there is mixing of oxygenated and deoxygenated blood in the circulation via heart. This happens due to the absence of separate chambers in the heart for oxygenated and deoxygenated blood$,\ e.g.,$ amphibian, reptile and fishes