1914. As the intestine, and in general every water-tegument is prolonged into mucous tubes or absorbent vessels; so also, with a more vigorous formation, the branchial membrane is drawn out into tubes, in order to conduct the air or oxygen towards the intestinal vessels, just as the intestine conveys through its absorbent ducts the water to the vessels of the skin. 1915. This saccular inversion of the skin constitutes the tegumentary lymphatic vessels, whose original function has been to transport the oxygen, combined with the water, to the intestine. They are the original respiratory vessels, which in the higher animals become, through the pure influence of air, true _Air-tubes_, like the spiral vessels. 1916. The air-vessels traverse the thickness of the body toward the intestinal membrane, like the mucous vessels do toward the branchial membrane. 1917. Thus an infinite number of air-vessels will and must originate. 1918. The air-tubes are consequently the formation which is properly opposed to the lymphatic vessels. They are for the air or for the skin, what these are for the water or the intestine. Air-vessels are first displayed in insects, then in fishes, reptiles, birds, and Thricozoa or Mammalia. 1919. If the infinitely numerous air-vessels concur to form one stem, they are then called _lungs_, as in the higher animals. 1920. The pulmonary vesicles are nothing but ramified air-tubes, such as the insect has. 1921. The formation of air-tubes is one of a higher character than that of the gills. For in it, indeed, the function has been separated from all other functions. It is simply destined to convey the air without water. 1922. As they pass into the dark, light does not operate upon them; and they will therefore less promote evaporation than mediate unto combustion. 1923. The anatomical idea of the air-vessels, or of the lung, is a saccular inversion of the skin. The skin is prolonged into, and ramifies towards, the body. The intestine is prolonged through the absorbent vessels, as being also small inversions, toward the lung, and becomes a stem--thoracic duct. Then the thoracic duct finally unites with the lungs through the medium of the heart, which is a new formation. 1924. Everything becomes stem which attains a higher grade, and which approximates the air and light. The stem strives to be a centre, but the ramification devolves upon the periphery; the former upon the Solar or Animal; the latter upon the Planetary or Vegetative. 1925. The nobler therefore a formation, by so much the more single and stemmy is it: such is the case with the trachea of the lungs, such with the lymphatic duct. 3. VASCULAR SYSTEM.