1886. Both walls are opposed to each other like light and darkness, like air and water. The external is the light-and air-wall, the internal the darkness-and water-wall. 1887. It is consequently only the internal wall that stands in the same relation as the root. The internal is "par excellence" root, and is thus a main organ of absorption. 1888. The external wall comes under the idea of the stem-bark, and in so far only as this has a root-nature in itself, is it likewise absorbent. 1889. As upon the external animal wall the light and air constantly operate--for without light indeed no animal originates--so is this wall more and more removed from the idea of the root, and becomes, by virtue of the influence of light and air, instead of an absorbent organ, an organ rather of decomposition--an _evaporation's organ_. 1890. As a cyst the internal wall incloses the nutritive matter, which originates from the mucus, and thus from the organic water. The internal wall is therefore constantly immersed in the water, and is consequently in every respect a root. 1891. As the animal is only developed in light, so must the function of the root languish in the external wall, and decay, because it is devoid of the earth, which protects it from or against the light. This deficiency is compensated for in another way, or by the formation of a cavity, into which the media of nutrition enter, and which is dark like the earth. 1892. Internal and external wall stand also opposite to each other, like water and air. The one is the water-, the other the air-wall. 1893. The nutritive matters are not decomposed upon the internal wall by extraneous influences, but they remain identical; ay, they become indifferent, because they enter into darker and warmer water. 1894. On the other hand such nutritive matters are decomposed upon the external wall; and there here therefore gradually originates, instead of the chemicalizing root-process, the polarizing process of air. 1895. In a perfect light-animal it is only the internal wall that is chemicalizing; the external has become oxydizing. The internal is a mucus-wall, but the external, on account of the decomposition of mucus, an oxygen-wall. _Division._ 1896. The more an animal has been exposed to the air and light, by so much the greater is the antagonism between its internal and external wall. In aquatic animals the antagonism is at its minimum, because externally and internally there is water; both parietes are therefore mucus-walls. The external wall of the fishes secretes an abundance of mucus, as does that also of the worms, snails, and molluscs. 1897. But an internal wall is still the more mucous of the two, because it is darker and warmer. 1898. At first the animal is content with the antagonism of the walls; especially so long as it remains occluded in dark and deep water, or within other animals. Many intestinal worms, polyps, and even Acalephous animals, are but simple sacs.