[65] Waksman, S. A., “Soil fungi and their activities,” Soil Sci., 1916, 2. [66] Waksman, S. A., “The influence of available carbohydrate upon ammonia accumulation by micro-organisms,” Journ. Amer. Chem. Soc., 1917, 39. [67] Waksman, S. A., “Proteolytic enzymes of soil fungi and _Actinomycetes_,” Journ. Bact., 1918, 3. [68] Waksman, S. A., “On the metabolism of _Actinomycetes_,” Proc. Soc. Amer. Bact. Abstract Bact., 1919, 3. [69] Waksman, S. A., “The influence of soil reaction upon the growth of _Actinomycetes_ causing potato scab,” Soil Sci., 1922, xiv. [70] Waksman, S. A., and Cook, R. C., “Incubation studies with soil fungi,” Soil Sci., 1916, 1. CHAPTER IX. THE INVERTEBRATE FAUNA OF THE SOIL (OTHER THAN PROTOZOA). The micro-organisms of the soil have been fully discussed in the preceding chapters of this volume. There now remains to be considered the fauna of invertebrate animals, other than protozoa, which inhabit that same medium. In the first place, it is necessary to define what groups of invertebrate animals are to be regarded as coming under the category of soil organisms. The latter expression has rather a wide application and, for the present purpose, is held to mean any organism of its kind which, in some stage or stages of its life-cycle, lives on or below the surface of the soil. It will be obvious that, with so comprehensive a definition, the intimacy of the association of these animals with the soil will vary within very wide limits. Some animals pass their whole life-cycle in the soil; others are only present during a limited phase, and not necessarily in a trophic condition, but since their occurrence is constant, they cannot be entirely omitted from consideration. Unlike the groups of organisms which have been dealt with in the foregoing pages, the invertebrates of the soil do not admit, as a rule, of investigation in culture media. It is, in consequence, much more difficult to achieve in the laboratory the same control over their environmental conditions. This fact in itself largely explains why the interpretations of field observations in animal ecology have not usually been subjected to the test of laboratory experimentation. The study of animal ecology, in so far as the denizens of the soil are concerned, is of very recent birth. It has not, as yet, passed the preliminary stage of cataloguing empirical data, and much spade work will be necessary before the various factors controlling the phenomena actually observed are understood. Owing to the paucity of information available, this chapter is essentially based upon observations conducted at Rothamsted. Its object is not so much to attempt to evaluate the invertebrate fauna of the soil, as to suggest a line of ecological work demanding investigation on land of many different types. METHOD OF INVESTIGATING THE SOIL FAUNA.