Amphibia and Reptilia have three chambered heart. Vertebrates, or animals with backbones, consist of several distinct classes: fish, amphibians, reptiles, mammals and birds. While similar in many ways, the hearts of different vertebrate classes possess different numbers of chambers. These chambers determine how efficiently the heart separates the flow of oxygenated blood and de-oxygenated blood throughout the body. Vertebrate hearts can be categorized by the number of chambers they have: → Two chambers: one atrium and one ventricle (fish) → Three chambers: two atria and one ventricle (amphibian and reptile) → Four chambers: two atria and two ventricles (bird and mammal)