1899. When the animal organization, however, ranks upon a higher stage, light, or even air, operates more upon its external wall, but upon its internal, water; thus the antagonism of the two walls is carried out to the utmost degree. 1900. Through the different, ay, opposed processes, the two walls finally adopt another structure. The external becomes denser and harder, on account of the decomposition by light and dessication by air; the internal, however, retains its original structure and consistence. Soft, aqueous, indifferent, and constantly absorbent, it is only a viscous mucus. 1901. In place of an integument of similar tissue throughout, one will originate, whose external tissue is dense and oxydized, but whose internal is loose or spongy, and indifferent. The previously uniform integument will now separate or fall into two distinct layers; into a soft muco-cellular layer, and into a tough coriaceo-cellular layer. 1902. With the last attainable antagonism the layers finally separate; two bladders or cysts, disunited from each other, originate; of these the internal is the _mucous_, the external the _coriaceous cyst_. 1903. Now the internal cyst alone is the _intestine_, the external the _cutis_ or _skin_. 1904. Intestine and cutis belong to _one_ formation, or to the integument. They pass directly into each other at the mouth and anus. Their structure also is wholly similar. 1905. They are merely distinguished by darkness and light, but more closely by water and air. The intestine is the _water-tegument_, the skin the _air-tegument_. 1906. The functions of both are therefore co-related like dissolution is to combustion, along with which the evaporation has been bestowed. 1907. Intestine and skin stand in antagonism with each other. 1908. The first animal, as being sentient integument, is a sac; the first skin is also a sac; an animal around the animal. 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._