(_b_) In their relation to the nitrogen cycle (_vide_ p. 174), the activities of the soil invertebrates may be expressed diagrammatically, as a side-chain in the process (Fig. 23). The proteins, elaborated by plants, are utilised as nitrogenous food by the phytophagous animals present. The waste products of the latter, which contain the nitrogen not used for growth or the replacement of loss by wear and tear, are returned to the soil. Here they disintegrate, and are ultimately converted into ammonium salts, mainly by bacterial action. The dead bodies of these animals are also broken down by various means, becoming eventually chemically dissociated and available as plant food. Animal (and plant) residues serve, however, as food for the large number of saprophagous invertebrates present in the soil. In this event the nitrogen contained in such residues becomes “locked up,” as it were, for the time being in their bodies. Both saprophagous and phytophagous animals are preyed upon by carnivorous species, but ultimately the nitrogen is returned to the soil upon the death of those organisms. The amount present in the bodies of the whole invertebrate fauna has been calculated by Morris (1922) upon analyses furnished by chemists at Rothamsted. It is estimated that the fauna of manured land contains about 7349 grm., or 16·2 lb. of nitrogen per acre, and that of untreated land, 3490 grm., or 7·5 lb. per acre. These amounts are equal respectively to the nitrogen content of 103·6 lb. and 48 lb. of nitrate of soda. The primary question affecting agriculture is, whether any notable loss of nitrogen is occasioned by the presence of these organisms in the soil. It has been mentioned that their nitrogenous waste material, and their dead bodies, ultimately undergo disintegration; any loss, if any, takes place during the latter process. With the more complex compounds it probably consists in the production of amino-acids and their subsequent hydrolysis or oxidation. During this process an appreciable loss of nitrogen in the gaseous form occurs. This loss, which is discussed on p. 173 would represent the net deficit occasioned by the incidence of invertebrates in the soil. Against this loss must be placed the beneficial action of such organisms as earthworms, which, in all probability, more than counterbalances it. (_c_) Many soil insects, on account of their phytophagous habits, are well-known to be some of the most serious enemies of agriculture. Certain of these, and also other classes of invertebrates, which are likewise directly injurious, have been instanced in the earlier pages of this chapter. Detailed information on this subject will be found in textbooks of economic zoology, notably the volume by Reh (1913). LITERATURE REFERRED TO. ADAMS, C. C., “An Ecological Study of Prairie and Forest Invertebrates,” Bull. Illin. St. Lab. Nat. Hist., 1915, xi.