Earthworm, a long, slender worm. It is found in damp soils of regions having moderate or warm climates. Earthworms are also called angleworms, because they are used as bait by anglers, or night crawlers, because they crawl about on the surface at night.

Why Earthworms Are Valuable

The earthworm is an aid to farmers and gardeners because its burrowing habits help enrich the soil. An earthworm digs as it ingests dirt for the decayed leaves and other organic matter it contains. The earthworm digests the organic matter and excretes the dirt. Worms benefit plants by ingesting mineral-rich dirt from the subsoil and excreting it in the topsoil, where plants use the minerals as nutrients.

The soil is also enriched by fragments of uneaten food accumulated around the burrows and by worms' waste products. The burrows allow air to penetrate the soil, and provide channels through which water may enter or be drained.

Description and Habits

The common United States earthworm is 6 to 11 inches (15 to 28 cm) long. The giant worms of Australia and South America grow to a length of 11 feet (3.4 m). The earth worm has a pointed head end and slightly flattened tail end. It has soft, somewhat slimy skin that is pinkish-red or brown. Its body consists of a series of segments, called somites, forming a cylindrical tube. The 32nd to 37th segments make up an organ called the clitellum. It plays a role in the reproduction of earthworms, secreting a mucus that holds together two earthworms as they mate, and secreting a cocoon that protects an earthworm's eggs as they develop. The number of segments beyond the 37th varies with individual species of earthworms.

The earthwormThe earthworm consists of a series of segments forming a cylindrical tube.

The earthworm has a digestive tube, which consists of a mouth, a pharynx, an esophagus, a crop, a gizzard, an intestine, and an anus. The pharynx sucks in food and dirt through the mouth. The food passes through the esophagus into the crop, where it is stored temporarily. The gizzard grind the food into small pieces, which are then digested in the intestine. Dirt passes through the entire digestive tube; it and other solid waste are excreted through the anus. Along each side of the digestive tube is a series of nephridia. A nephridium is an organ that draws in liquid waste from the body and discharges it through a pore in the skin.

Beneath the digestive tube is a nerve cord. It enlarges near the mouth to form a large cerebral ganglion, a primitive form of brain. The earthworm has no eyes, but has lightsensitive spots on various parts of its body. A worm's sense of touch is well developed and it has a sense of smell and of taste.

The earthwormThe earthworm has a nerve cord that enlarges near the mouth into a primitive form of brain.

Oxygen passes through the skin of an earthworm into small blood vessels. Blood carries the oxygen through vessels to the rest of the body. The two main blood vessels are the dorsal vessel, above the digestive tube; and the ventral vessel, below the digestive tube. Along each side of the esophagus are five blood vessels called aortic arches, which connect the dorsal vessel to the ventral vessel. The dorsal vessel pumps blood into the aortic arches, which regulate the pressure of the blood. The blood, under pressure, then flows into the ventral vessel. (The aortic arches are commonly called the hearts of the worm.)

Two sets of muscles are located under the skin: longitudinal muscles(arranged lengthwise) and circular muscles(arranged around the body). To move forward, the worm contracts first one set of muscles and then the other. Setae, small retractable bristles on the outside of the body, help provide traction.

Each worm produces both sperm and eggs. The eggs of one earthworm are fertilized by the sperm of another. Fertilized eggs are enclosed in a cocoon, which is deposited below the surface of the ground. The eggs hatch two or three weeks after fertilization. Young earthworms resemble adults except for size.

If certain portions of an earthworm's body are cut off, they will regenerate, or grow again. Usually, all or most of the rear segments beyond the clitellum will grow back again if cut away. Whether the front segments regenerate depends on how many have been cut off, and on the individual earthworm. Ordinarily, the first five segments will grow again. This power of regeneration makes the earthworm a valuable animal for grafting experiments.

A common earthworm is Lumbricus terrestris of the family Lumbricidae, phylum Annelida.