The sac contains the food for the embryo. A thick dry skin covered with scales made of a protein called Keratin . Reptiles molt as they grow. Reproduction is sexual with internal fertilization. Most are oviparous, while some snakes and lizards are either ovoviviparous or viviparous .
Respiration occurs through well developed lungs, and in some turtles a moist cloacal surface.
Reptiles are ectotherms. Being ectothermic enables an organism to survive on much less food than an endothermic organism. Most reptiles contain a 3 chambered heart with a partially divided ventricle. Crocodiles contain a 4 chambered heart. Reptilian Orders .