_Mollusca._--The terrestrial Mollusca are included in the sub-order _Pulmonata_ of the _Gastropoda_. These organisms, which include the snails (_Helicidæ_) and slugs (_Limacidæ_), regularly deposit their eggs in moist earth. Slugs adopt the soil as a frequent habitat, only leaving it for feeding purposes in the presence of sufficient moisture. They are frequent consumers of vegetation, with the exception of _Testacella_, which is carnivorous. _Crustacea._--The few species of Crustacea inhabiting the soil belong to the order _Isopoda_, family _Oniscidæ_, which are popularly referred to as “woodlice,” “slaters,” etc. _Myriapoda._--The _Diplopoda_ or millipedes include enemies of various crops and are common denizens of the soil. The _Chilopoda_ or centipedes are usually less abundant and are carnivorous. The minute _Symphyla_ are often evident but are of minor importance. _Insecta._--Insects form the dominant element in the invertebrate fauna. Phytophagous species devour the subterranean parts of plants, and notable examples are afforded by the larvæ of _Melolontha_, _Agriotes_ and _Tipula_. Saprophagous forms are abundantly represented by the _Collembola_, and by numerous larval _Diptera_ and _Coleoptera_. Predaceous species preying upon other members of the soil fauna are exemplified by the _Carabidæ_ and many larval _Diptera_. Parasitic species pass their larval stages on or within the bodies of other organisms. The groups of _Hymenoptera_, and the dipterous family _Tachinidæ_, which exhibit this habit, constitute, along with predaceous forms, one of the most important natural agencies controlling the multiplication of insect life. There are also insects (ants, and other of the aculeate _Hymenoptera_) which utilize the soil as a suitable medium wherein to construct their habitations or brood chambers, without necessarily deriving their food from the soil. Lastly, there are many insects, notably _Lepidoptera_, which only resort to the soil for the purpose of undergoing pupation. The insect fauna is, therefore, a closely inter-connected biological complex; for a discussion and an enumeration of its representatives reference may be made to papers by Cameron (1913, 1917), and Morris (1921, 1922 a). _Arachnida._--The two principal classes represented in the soil are the _Areinida_, or spiders, and the _Acarina_, or mites, and ticks. The _Areinida_, which are well-known to be carnivorous, are an unimportant constituent of the fauna. _Acarina_, on the other hand, are abundant, and exhibit a wide range of feeding habits; most of the soil forms are probably carnivorous, and either free-living or parasitic. NUMBER OF ORGANISMS PRESENT AND THEIR DISTRIBUTION IN DEPTH.