Earthworms are soil-dwelling animals that have elongate cylindrical segmented bodies. Adult earthworms have a distinctive thickened band, known as the clitellum, about one third of the way down the body from the head end, this is part of the reproductive system of the worm.
Earthworm activities are usually beneficial in gardens, for both soil structure and in nutrient recycling. Earthworms feed on dead plant material and in doing so ingest a certain amount of soil. As a consequence, an earthworm's excrement has a muddy consistency and appearance. Most earthworm species void waste material underground but a few species deposit casts on the surface. In most cases worm casts can be tolerated and brushed off when dry. Some gardeners collect these casts and use them as a potting medium. Worm casts may however be undesirable by some gardeners trying to achieve fine low cut lawns, particularly if the casts get squashed and spread over the surface by trampling feet or lawnmowers. This can create places where mosses and lawn weeds can establish.
Worm casts can be distinguished from soil brought up to the surface by other lawn animals, such as ants, mining bees and moles, by the fine muddy nature of the excreted soil. Freshly deposited worm casts often have a coiled appearance.
Find out more about earthworms and earthworm research at Earthworm Watch and the Earthworm Society of Britain (link to external websites).