In vertebrates, the interaction of oxygenated and deoxygenated blood is a crucial process for efficient oxygen delivery to the body. While most vertebrates have a fully separated circulatory system, there are certain exceptions. Option 2, Amphibians, is the correct answer because they have a partially mixed circulation. This means that oxygenated and deoxygenated blood can mix partially in their circulatory system. This occurs mainly due to the presence of a non-divided ventricle in their heart, which allows some mixing of oxygenated and deoxygenated blood. However, it is important to note that option 1, Fish, also has a partially mixed circulation system, but it is not the correct answer in this case. Options 3 (Bird) and 4 (Mammal) have completely separated circulatory systems and do not mix oxygenated and deoxygenated blood.