The heart among the vertebrates show different patterns of evolution. Different groups of
animals have evolved different methods for this transport. All vertebrates possess a muscular chambered heart.
(i) Fishes have a two-chambered heart with an atrium and a ventricle. The heart pumps out deoxygenated blood which is oxygenated by the gills and supplied to the body parts from where deoxygenated blood is returned to the heart.
(ii) Amphibians and the reptiles (except crocodiles) have a three-chambered heart with two atria and a single ventricle. The left atrium receives oxygenated blood from the gills/lungs/skin and the right atrium gets the deoxygenated blood from other body parts. However, they get mixed up in the single ventricle which pumps out mixed blood.
(iii) Crocodiles, birds and mammals possess a four-chambered heart with two atria and two
ventricles. Oxygenated and deoxygenated blood received by the left and right atria respectively passes on to the ventricles of the same sides. The ventricles pump it out without any mixing up, i.e., two separate circulatory pathways are present in these organisms, hence, these animals have double circulation.