Earthworms are one of many types of worms, including those of the flat and round species. Over a century ago, Charles Darwin spent 39 years studying earthworms and wrote The Formation of Vegetable Mould Through the Action of Worms with Observations on Their Habits, an entire book that described his research on earthworm behavior and intelligence and further explained how important earthworms are to agriculture. Long before [the plow] existed, he wrote, the land was, in fact, regularly plowed and still continues to be thus plowed by earthworms. It may be doubted whether there are many other animals which have played so important a part in the history of the world. Darwin was referring to the way that earthworms naturally mix and till soil, while both improving its structure and increasing its nutrients. As they tunnel in the soil, earthworms open channels that allow in air and water, improving drainage and easing the way for plants to send down roots; they also carry nutrients from deep soils to the surface. Earthworms eat plant material in the soil, decaying leaves, and leaf litter, and their own waste provides nourishment for plants and other organisms. Slime, a secretion of earthworms, contains nitrogen, an important plant nutrient. It is estimated that each year earthworms in one acre of land move 18 or more tons of soil.

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