Another unknown organism is an agent partly responsible for breaking down the cellulose of dead plants in the soil. The mold, Aspergillus fumigatus, world-wide in its distribution, does this in the laboratory. Nowhere, however, is it found in nature in sufficient numbers to accomplish the titanic job attributed to it. The great, invisible jungle, of course, must eat to live. Some organisms demand fresh food and are responsible for root rot in plants. The majority, however, find their sustenance in the enormous mass of dead and dying roots of annual vegetation. Decomposition of annuals is an explosive process involving the development of countless billions of bacteria. _The Self-Perpetuating Sponge_ Close to primaeval chaos is the sponge—lowliest of animals. It is an animal without a brain, nervous system, heart, lungs, stomach, muscles or blood. But it has an _I Am_. The sponge is in essence an anarchical horde of numberless cells. When the conglomeration is split up as can be done by a technique of squeezing through fine-meshed silk gauze, the cells continue to live as individuals. They crawl about. They take nourishment. But when a few thousands of them are thrown together into a tank of sea water they will conglomerate again, apparently into the same sponge that existed before the disintegration. If sponge animals of two different species are mixed in the tank they will combine into two sponges, duplications of the conglomerations from which they came. If cells of two sponges of the same species are mixed it may be that they will recombine into the two original individuals—but this experiment never has been tried and would be quite difficult to interpret. The sponge is the simplest, most primitive of metazoa, or many-celled animals. It acts as an individual, although there is apparently no central government, like a brain, controlling the behavior of the millions of individuals constituting the conglomeration. It ranges in size from organisms a fraction of an inch long, by far the most numerous, to masses several feet in diameter. Various species present about all the colors of the rainbow. There are red, scarlet, green, yellow, blue and violet sponges, especially in shallow, tropical waters. Abysmal species tend to be a drab brown. The living sponge when taken from the water is a slimy, rather repulsive mass which has the general appearance of a piece of raw beef liver perforated with holes and canals. The commercial sponge is merely the skeleton, the supporting framework of the gelatin-like tissues, which is composed of a substance similar in chemical and physical properties to silk, horn and the chitin which forms the shells of insects and crabs. This material is distributed in a fibrous network the pattern of which varies for each species.