In the simplest forms of life, any part of the soft and delicate surface will do for the blood to reach, in order to throw off its load of carbon "smoke" and take on its supply of oxygen.
In fact, animals like jellyfish and worms are lungs all over.

But as bodies begin to get bigger, and the skin begins to toughen and harden, this becomes more and more difficult, although even the highest and biggest animals like ourselves still throw off a certain amount of this carbon dioxid and other gases through the skin.

Accordingly, certain parts of the surface of the body are set apart specially for this business of breathing; and as we already have an opening into the body provided by the mouth and food tube, the simplest thing to do is to use the mouth for taking in air, when it is not being used for taking in food, and to set aside some part of the food tube for breathing purposes.
[Illustration: DIAGRAM OF THE AIR TUBES AND LUNGS The arrows show the direction of the incoming air.] The lungs sprout out from the front of the gullet, just below the root of the tongue, in the days when we are getting ready to be born.

The sprout divides into two, forming the beginning of the pair of lungs.
Each lung sprout again divides into two, and each of the two smaller buds again into two, until finally we have the whole chest filled up with a "lung-tree" whose trunk stems and leaves are hollow.

The stem of the tree or bush becomes the windpipe (_trachea_).