Any of nearly 2000 species of terrestrial worms which act as one of the main agents by which plant litter, humus, and minerals are incorporated and mixed in soil. Earthworms are responsible for the maintenance and stability of various types of soil, especially the brown forest soils. The character of a soil may change markedly if the plant litter made by the vegetation changes to a kind which is unpalatable to earthworms. The effects of earthworm sorting may be seen on archaeological sites in the blurring of layers and the development of worm-sorted layers in the top of buried soils. Earthworms usually remain near the soil surface, but they are known to tunnel as deep as 6 feet during periods of dryness or in winter. Indirectly they provide food for man by aerating the soil, promoting drainage, and drawing organic material into their burrows where it decomposes faster, thus producing more nutritive materials for growing plants.