Earthworm

I

Introduction

Earthworm, name given to more than 1000 species of worms in a class of the annelid phylum. The earthworm has a cylindrically shaped, segmented body that tapers off at both ends. Minute bristles called setae project from the body. Although a difference in shading exists between the upper and under surfaces and between different parts of the body, earthworms in general are uniform in color, usually pale red, but varying from dull pink to brown. Many species grow to a length of only a few centimeters, but some tropical species attain a length of up to 3.3 m (11 ft).

Earthworms play an important role in soil ecology. By being continually loosened, stirred up, and aerated by the action of earthworms, soil is made more fertile. Earthworms also form a source of food for many animals, and constitute the principal food of moles and shrews.

II

Behavior

Earthworms must live in moist soil containing organic matter. They usually live in the upper layers of the soil, but in winter they penetrate more deeply to escape frost. During unusually hot weather they also penetrate downward to avoid dehydration. Earthworms shun daylight but frequently come to the surface of the soil at night to feed and to throw off their castings. In the daytime they appear upon the surface of the soil only under unusual conditions, such as the flooding of their burrows by excessive rainfall.

Earthworms are capable of burrowing with considerable speed, especially in loose soil; the bristles along the sides of the body are of great assistance in their movements. In burrowing, they swallow large quantities of earth that often contain considerable amounts of vegetable remains. They are able to digest the nutritive matter of the soil, depositing or casting out the remains on the surface of the earth or in their burrows.

---

---

III

Structure and Life Cycle

The muscular system of the earthworm consists of an outer series of circular or transverse muscle fibers that girdle the body and an inner series of longitudinal muscle fibers employed in moving the setae. The circulatory system consists of a prominent dorsal blood vessel and at least four ventral blood vessels, running longitudinally in the body and connected with one another by a regularly arranged series of transverse vessels. The dorsal vessel is provided with valves and is the true heart. Most of the pumping of blood, however, is performed by general muscular movements. The central nervous system consists of a pair of suprapharyngeal ganglia, often called the brain, and a ventral cord that lies beneath the alimentary canal with ganglia in every segment. Earthworms have no sense organs other than those of touch. The digestive system consists of a muscular pharynx, a slender esophagus, a thin-walled crop or food receptacle, a muscular gizzard used for grinding ingested earth, and a long, straight intestine.

Earthworms are hermaphrodites, with each worm having both male and female reproductive organs. Mutual cross-fertilization usually takes place. The eggs, containing considerable yolk, are buried in the earth in capsules formed from secretions of the clitellum, a thickened portion of the body wall. The capsules protect the young until they hatch as small, fully developed worms. Some species live for ten years or longer.

Scientific classification: Earthworms belong to the class Oligochaeta of the phylum Annelida. They make up five families: Lumbricidae, in North America, Europe, and northern Asia; Moniligastridae, inhabiting India, Sri Lanka, Malaysia, and the eastern part of Africa; Megascolecidae, inhabiting India, Australasia, Africa, and South America; Eudrilidae, inhabiting central parts of Africa; and Glossoscolecidae, inhabiting South and Central America, Africa, and southern Europe.

	More from Encarta
	•  Presidential Myths Quiz •  Coffee break: Recharge your brain