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Annelida is the group of segmented worms including earthworms and leeches. Repetition of body units supports comparison of internal and external organization, although segmentation can be modified or obscured in particular lineages.

An earthworm is a terrestrial segmented member of Annelida whose burrowing and feeding can reorganize soil. Its body moves through coordinated muscular action against the surrounding material; ingested soil and organic matter are deposited as casts. Small repeated actions can consequently change a landscape over long intervals.

Charles Darwin investigated the amount of fine earth brought to the surface and the progressive burial of objects in pasture. In one long-observed field, a stone-free surface layer accumulated above stony material, while casts provided a visible transport mechanism. ^[ref] The inference combines observed activity with measurements of cumulative change, rather than treating an earthworm merely as a conspicuous resident of pre-existing soil.