2. DERMAL SYSTEM.
a. Branchiæ or Gills.
1909. The external wall, being constantly exposed to the air that is in the water, can adopt no other than the aerial character, and is thus like the leaf of the plant. The skin is the organ of evaporation, and with this of oxydation also.
1910. A self-oxydizing integument is called a Branchia or gill.
1911. The skin is essentially nothing else than a gill; and, if it subsequently appears as anything else, this happens only through a higher state of perfection being attained by its branchial function.
1912. The lowest animals, such as most of the worms, molluscs, and snails, breathe through the external integument; even the gills of fishes are none other than a piece of skin.
1913. Thus gills and intestine would be the first two organs, which are developed out of the tegumentary system by the antagonism of air and water. Through the gills, air, and through the intestine, water enters the body. The gill is the atmosphere of the animal, the intestine is its sea.
b. Tracheæ, or Air-tubes.
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.