Reptiles are ectothermic animals that rely on the environment to regulate their body temperature. They lay amniotic eggs that have a protective shell and membranes, allowing embryonic development on land outside of water. Reptiles have key adaptations for terrestrial living including lungs, a four-chambered heart, efficient excretion, limbs for movement, internal fertilization, and shelled eggs. The four main groups of modern reptiles are lizards and snakes, crocodilians, turtles and tortoises, and tuatara.