The yolk sac which contains the food for the embryo. They are covered by a thick dry skin covered with scales made of a protein called Keratin . Reproduction is sexual with internal fertilization. Most are oviparous , while some snakes and lizards are ovoviviparous. Respiration occurs through well developed lungs, and in some turtles a moist cloacal surface.
Reptiles are ectotherms.
Most reptiles contain a 3 chambered heart with a partially divided ventricle. Crocodiles contain a 4 chambered heart. Turtles are members of this order. They developed during the Mesozoic era. They show very little change from their ancestral form.