Vertebrates like birds and mammals possess a 4-chambered heart with two atria and two ventricles. In these animals, oxygenated and deoxygenated blood are kept separate and flow through two separate circulatory pathways (double circulation). Whales belong to the Class Mammalia. Frogs (Amphibians) and Lizards (Reptiles) have a 3-chambered heart with incomplete double circulation, while Sharks (Pisces) have a 2-chambered heart with single circulation , , .