The heart had to deliver greater amounts of oxygen required by walking muscles. Techniques had to be developed to prevent the body from dessication. Modern amphibians are characterized by the presence of legs, cutaneous respiration, lungs, pulmonary veins, and a partially-divided heart. Reptiles also have internal fertilization, a partially divided ventricle in the heart, and are cold-blooded. The egg is an amniotic egg, which protects the embryo from drying out, nourishes it, and enables the development of the embryo independent of free water.
Nutrients are in the yolk, and wastes are excreted into the allantois and stored until hatching.
Respiration is through the shell and membranes, which are permeable to oxygen and carbon dioxide. Birds differ in that their forelegs have evolved into wings and their bodies are covered with feathers. Feathers are derived from scales. Amphibian legs jut out from the sides of the body, reptilian legs are straightened and tentlike, and mammalian legs are located directly beneath the body. The mammalian arrangement of legs facilitates running.