Reptiles are cold-blooded vertebrates which generally have scales, four limbs with five toes each, and hearts with three or four chambers. Most also lay eggs with leathery shells, which are protected from dessication by an amniotic sac. Reptiles are a diverse group, however, and there are many exceptions to these general characteristics, including species without scales, without limbs or which give live birth rather than laying eggs.
Reptiles include the major groups of turtles, lizards and snakes, and crocodiles and their relatives. Many scientists also include birds as reptiles. Indeed, molecular evidence shows that crocodiles and birds are more closely related to one another than crocodiles are to other reptiles. The relationship of turtles to other reptiles is also controversial.
Turtles are easily identified by their unique shells, which are actually built from their vertebrae and ribs. They are toothless with keratin-based beaks. Snakes and lizards constitute the majority of conventional reptile species and are typified by their flexible jaws with an ability to manipulate food items. Crocodilians include crocodiles, alligators, caimans and the gharial. These are large reptiles with elongated snouts and strong jaws. Their eyes and nostrils are located on top of their heads to aid them in their partially aquatic lifestyles.