The Food Tube and its Glands.--The mouth leads like a funnel into a short tube, the _gullet_. On the lower floor of the mouth can be seen the slitlike _glottis_ leading to the lungs. The gullet widens almost at once into a long _stomach_, which in turn leads into a much coiled intestine. This widens abruptly at the lower end to form the _large intestine_. The latter leads into the _cloaca_ (Latin, _sewer_), into which open the _kidneys_, _urinary bladder_, and reproductive organs (_ovaries_ or _spermaries_). Several _glands_, the function of which is to produce digestive fluids, open into the food tube. These digestive fluids, by means of the ferments or enzymes contained in them, change insoluble food materials into a soluble form. This allows of the absorption of food material through the walls of the food tube into the blood. The glands (having the same names and uses as those in man) are the _salivary glands_, which pour their juices into the mouth, the _gastric glands_ in the walls of the stomach, and the _liver_ and _pancreas_, which open into the intestine. Circulation.--The frog has a well-developed heart, composed of a thick-walled muscular ventricle and two thin-walled auricles. The heart pumps the blood through a system of closed tubes to all parts of the body. Blood enters the right auricle from all parts of the body; it then contains considerable carbon dioxide; the blood entering the left auricle comes from the lungs, hence it contains a considerable amount of oxygen. Blood leaves the heart through the ventricle, which thus pumps some blood containing much and some containing little oxygen. Before the blood from the tissues and lungs has time to mix, however, it leaves the ventricle and by a delicate adjustment in the vessels leaving the heart most of the blood containing much oxygen is passed to all the various organs of the body, while the blood deficient in oxygen, but containing a large amount of carbon dioxide, is pumped to the lungs, where an exchange of oxygen and carbon dioxide takes place by osmosis. In the tissues of the body wherever work is done the process of burning or oxidation must take place, for by such means only is the energy necessary to do the work released. Food in the blood is taken to the muscle cells or other cells of the body and there oxidized. The products of the burning--carbon dioxide--and any other organic wastes given off from the tissues must be eliminated from the body. As we know, the carbon dioxide passes off through the lungs and to some extent through the skin of the frog, while the nitrogenous wastes, poisons which must be taken from the blood, are eliminated from it in the kidneys. Change of Form in Development of the Frog.--Not all vertebrates develop directly into an adult. The frog, for example, changes its form completely before it becomes an adult. This change in form is known as a _metamorphosis_. Let us examine the development of the common leopard frog.