(2) Permanent inhabitants that are dependent on the soil for their development and supplies of food, either throughout or during most of their lives--_e.g._, certain insects and spiders, millepedes, earthworms, eelworms, protozoa, fungi, algæ and bacteria. The organisms in the first group play a comparatively minor part in soil development, and influence its fertility to an almost negligible extent, the temporary scavengers, perhaps, being of more importance since they aid in the reduction of vegetable residues. The forms in the second group, however, are invaluable as soil-making agents and in the production of plant food materials, the least important among them being the insects, spiders and millepedes. Many are merely scavengers, but some insects, such as grass-grubs and the caterpillars of certain moths, and millepedes, feed upon living plants and so add organic matter to the soil in their excreta, which also contains quantities of soil swallowed with the food, this latter mechanical action aiding in the pulverising and opening up of the soil; certain eelworms, too, that attack living plants play a somewhat similar part, in that they are primary causative agents in the decay of healthy tissues. Other forms of insects, together with spiders and some eelworms, are predaceous upon their fellows, the remains of the latter being added to the soil residual complex. Apart from the activities of all these organisms, however, it is the earthworms, protozoa, fungi, algæ and bacteria that have the most fundamental influence upon soil fertility. Earthworms may be correctly called the great soil builders; they burrow through it, allowing the free passage of air and water; they swallow large quantities, which they eject on the surface in the form of “worm-casts,” the soil materials being well mixed in the process; they pull underground leaves and other parts of plants from the surface and so increase the supply of organic matter for the action of the micro-organisms that bring about decomposition. Further, by depositing their “casts” on the surface, earthworms soon cover the accumulations of dead vegetable matter, as has been illustrated by Darwin in his classic work on these animals. Without the aid of earthworms--_e.g._, in sour soils in which they do not abound--the plant residues accumulate on the surface, to form a partially decomposed, peaty mass, which only a limited number of plants can tolerate.