The earthworm is a creature of the dark. It cannot see, but it is sufficiently sensitive to light to avoid the sun, the rays of which would shrivel up its moist skin. Having no lungs or gills, the worm uses the skin as the breathing organ; and it must be kept moist in order to serve its important use. This is why earthworms are never seen above ground except on rainy days, and never in the top soil if it has become dry. In seasons of little rain, they go down where the earth is moist, and venture to the surface only at night, when dew makes their coming up possible.
Earthworms have no teeth, but they have a long snout that protrudes beyond the mouth. Their food is found on and in the surface soil. They will eat scraps of meat by sucking the juices, and scrape off the pulp of leaves and root vegetables in much the same way. Much of their subsistence is upon organic matter that can be extracted from the soil. Quantities of earth are swallowed. It is rare that an earthworm is dug up that does not show earth pellets somewhere on their way through the long digestive canal. The rich juices of plant substance are absorbed from these pellets as they pass through the body.
Earthworms explore the surface of the soil by night, and pick up what they can find of fresh food. Nowhere have I heard of them as a nuisance in gardens, but they eagerly feed on bits of meat, especially fat, and on fresh leaves. They drag all such victuals into their burrows, and begin the digestion of the food by pouring on it from their mouths a secretion somewhat like pancreatic juice.
The worms honeycomb the earth with their burrows, which are long, winding tubes. In dry or cold weather these burrows may reach eight feet under ground. They run obliquely, as a rule, from the surface, and are lined with a layer of the smooth soil, like soft paste, cast from the body. The lining being spread, the burrow fits the worm's body closely. This enables it to pass quickly from one end to the other, though it must wriggle backward or forward without turning around.
At the lower end of the burrow, an enlarged chamber is found, where hibernating worms coil and sleep together in winter. At the top, a lining of dead leaves extends downward for a few inches, and in day time a plug of the same material is the outside door. At night the worm comes to the surface, and casts out the pellets of earth swallowed. The burrow grows in length by the amount of earth scraped off by the long snout and swallowed. The daily amount of excavation done is fairly estimated by the castings observed each morning on the surface.
One earthworm's work for the farmer is not very much, but consider how many are at work, and what each one is doing. It is boring holes through the solid earth, and letting in the surface water and the air. It is carrying the lower soil up to the surface, often the stubborn subsoil, that no plough could reach. It is burying and thus hastening the decay of plant fibre, which lightens heavy soil and makes it rich because it is porous. Moreover, the earthworms are doing over and over again this work of fining and turning over the soil, which the plough does but seldom.
By the continuous carrying up of their castings, the earthworms gradually bury manures spread on the surface. The collapse of their burrows and the making of new ones keep the soil constantly in motion. The particles are being loosened and brought into contact with the soil water, that dissolves, and thus frees for the use of feeding roots, the plant food stored in the rock particles that compose the mineral part of the soil.
The weight of earth brought to the surface by worms in the course of a year has been carefully estimated. Darwin gives seven to eighteen tons per acre as the lowest and highest reports, based on careful collecting of castings by four observers, working on small areas of totally different soils. In England, earthworms have done a great deal more toward burying boulders and ancient ruins than any other agency. They eagerly burrow under heavy objects, the weight of which causes them to crush the honeycombed earth. Undiscouraged, the earthworms repeat their work.
"Long before man existed, the land was regularly ploughed, and continues still to be ploughed 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 as have these lowly organized creatures."
After years of study, Charles Darwin came to this conclusion. The more we study the lives of these earth-consuming creatures, the more fully do we believe what the great nature student said. The fertile soil is made of rock meal and decayed leaves and roots. Only recently have ploughs been invented. But the great forest crops have grown in soil made mellow by the earthworm's ploughing.