In which organisms can double circulation be found?

Double circulation is a circulatory system in which blood passes through the heart twice during each complete circuit through the body. This system is characterized by two distinct circuits: the pulmonary circuit, which transports blood between the heart and the lungs, and the systemic circuit, which delivers oxygen-rich blood to the rest of the body.

In amphibians, reptiles, mammals, and birds, double circulation is present and allows for a more efficient delivery of oxygen to tissues. In amphibians, the heart has three chambers which partially separates oxygenated blood from deoxygenated blood, enabling some degree of double circulation. Reptiles, although not entirely separated (with the exception of crocodilians), do have a more advanced circulatory system than fish. Mammals and birds possess a fully developed four-chambered heart that provides complete separation of oxygenated and deoxygenated blood, thus maximizing their metabolic efficiency.

In contrast, fish possess a single circulatory system where blood flows from the heart to the gills for oxygenation and then to the rest of the body without returning to the heart, which supports less metabolically active lifestyles. Therefore, the correct answer reflects the presence of double circulation as observed in amphibians, reptiles, mammals, and birds.