Many useful bacteria have the power of so acting on dead organic bodies as to decompose them, the three most conspicuous end-products of this decomposition being water, carbonic oxide and ammonia. When the dead bodies are decomposed in the soil there are other bacteria, in addition, that have the power of further acting on the ammonia, causing its oxidation and producing nitrous and nitric acids and their salts. The unicellular plants that bring about these changes are the _nitrifying bacteria_. Conspicuous illustrations of the functional activity of these little naked pieces of protoplasm are seen in the immense saltpeter beds of Peru and Chili, where, from the enormous fecal accumulations of sea-fowls, the immense quantities of nitrates are produced that supply the commercial world. [Illustration: FIG. 5.—Difflugia Pyriformis.] =Arcella=, of which there are many species, is a unicellular animal whose protoplasmic body has secreted from its surface an enclosing “test” that is composed of a _horny membrane_, resembling very much in constitution the chitin which gives firmness to the integuments of insects. This creature is commonly discoidal in shape, with one face arched and the other flat, an aperture being situated in the center of the flat side through which the creature may thrust its pseudopodia or withdraw them. The surface of the testaceous covering is often marked with a regular but minute and attractive pattern. In =Difflugia= (Fig. 5), the test is somewhat pitcher-shaped, and is mostly made up (by the constructive activity of the protoplasm) of exceedingly small particles of shell and gravel cemented together. Many testaceous amœbans form tests of singular beauty and remarkable regularity. In some of the animals the minute plates of which the tests are formed have been picked up from the surface over which the animals crawl, and are cemented into various charming patterns; and in other cases they are formed by secretion from their own bodies. In =Quadrula symmetrica= the protoplasmic body has constructed a pear-shaped testaceous covering, of complete transparence-like glass, composed of a great number of square plates touching each other by their edges. The protoplasmic body of the animal does not entirely fill the test, the intervening space being occupied by a clear liquid and traversed by bands of protoplasm. A clear, large spherical nucleus is seen in the part farthest from the pseudopodia. It contains a dark and well-defined nucleolus. In front of the nucleus two contractile vesicles are to be observed. The pseudopodia in these creatures, it must be remembered, are not appendages, but lobate protrusions of the protoplasmic body, are few in number, rounded, short and broad.