In computing the number of invertebrates normally present in a given type of soil, the method adopted consists of making individual counts of all such organisms as occur in each sample of a series taken over a period of twelve months. This method considerably reduces errors due to season and to the possible deviation of one or more samples from the average. If the total number of these organisms is known for the samples taken, it becomes a simple procedure to arrive at their approximate numbers per acre. TABLE XIV. (Based on Morris, 1922 A.) +------------------------------+---------------+-------------+ | |Unmanured Plot.|Manured Plot.| +------------------------------+---------------+-------------+ |Insects | 2,474,700 | 7,727,300 | +------------------------------+---------------+-------------+ |Larger Nematoda and Oligochæta| | | | Limicolæ | 794,600 | 3,600,400 | +------------------------------+---------------+-------------+ |Myriapoda-- | | | | Diplopoda | 596,000 | 1,367,000 | | Chilopoda | 215,400 | 208,700 | | Symphyla | 64,000 | 215,500 | | +---------------+-------------+ | Total | 875,400 | 1,791,200 | +------------------------------+---------------+-------------+ |Oligochæta (Terricolæ) | 457,900 | 1,010,100 | +------------------------------+---------------+-------------+ |Arachnida-- | | | | Acarina | 215,400 | 531,900 | | Areinida | 20,200 | 20,200 | | +---------------+-------------+ | Total | 235,600 | 552,100 | +------------------------------+---------------+-------------+ |Crustacea (Isopoda) | 33,700 | 80,800 | +------------------------------+---------------+-------------+ |Mollusca (Pulmonata) | 13,500 | 33,700 | +------------------------------+---------------+-------------+ | Total Invertebrata | 4,885,400 | 14,795,600 | +------------------------------+---------------+-------------+ [Illustration: FIG. 20.--Distribution in depth of the more important groups of soil invertebrates in the manured and unmanured (or control) plots at Rothamsted. (From Morris, “Annals of Applied Biology,” vol. ix., nos. 3 and 4, Cambridge University Press.)]