do with the decomposition of organic substances. The fungi, algæ and bacteria are amongst the lowest forms of plant life, and hold somewhat the same position in the plant kingdom as the protozoa do among animals; they are, especially the fungi and bacteria, of primary importance in the maintenance of soil fertility. The role of algæ lies mainly in increasing the organic content of the soil, and they are invaluable in developing favourable conditions for the establishment of vegetation on purely mineral soils. The fungi and bacteria are responsible for setting up the intricate reactions involved in the decomposition of organic matter, the bacteria being concerned in practically all of the chemical processes going on in the soil. Both fungi and bacteria are of two kinds: those that bring about decomposition, and those that live in a reciprocal relationship with plants upon the roots of the latter. Such relationship, which benefits both organisms and plants, is called symbiosis, the fungi being known as mycorrhiza, while the bacteria form nodules on the roots of such plants as the legumes. CHAPTER III. Structure of Insects. Although insects present a great variety of forms, they nevertheless agree in general features; thus by studying the structure of some generalised species, which will give a broad idea of the main characteristics, one is enabled to recognise different structural modifications assumed by various species. For this purpose a weta, grasshopper, or cockroach may be taken as a type. Just as in the case of the crayfish, so the body of an insect is completely covered and protected by a continuous “shell,” very solid in some insects, more or less pliable in others, but even in the most delicate forms tending to become rigid and brittle after death. This shell acts as a skeleton and as a very effective armour-plating, protecting and supporting the soft body within. Unlike the shell of the crayfish, which is mainly calcareous, that of insects consists of a horny substance called _chitin_, secreted by the underlying skin, and constitutes what is known as a _cuticle_. It is due to this horny cuticle or shell that the form and colour of most insects are preserved after death, though the enclosed body tissues decay unless preserved in some suitable medium. The cuticle, though forming a complete covering, does not enclose the body in an inflexible shell; flexibility is allowed by the cuticle being formed of a segmented series of strongly-chitinised sections alternating with skin-like, feebly-chitinised, and very elastic sections; this arrangement gives freedom of movement to the enclosed body, as is readily seen in the movements of a caterpillar.