The soil is the Natural habitat for animals of many kinds.  Prominent among them are the segmented worms, called annelids, which form over half of the weight of soil animals. Earthworms are world-wide in distribution, excluding only the Arctic and Antarctic regions. There are about 1,500 species, ranging in size from less than 1 mm long to the gigantic Australian worm which attains a length of 10  to 11 feet.  Our common earthworm, Lumbricus terrestris, described here, rarely exceeds 9 or 10 inches in length. It is abundant in moist, rich soil.
 
Habits. If a lively worm is dropped onto a well-watered lawn, it will disappear from sight with surprising speed.  It burrows head first into the soil to a depth of several inches, then tunnels to the surface again in another spot. It makes its burrow partly by using the pointed head end to push soil particles aside, partly by eating the soil,  which passes through the body and is left as castings at the entrance to the burrow.   In the winter earthworms retreat below the frost line, where they lie coiled up in an enlarged soil chamber.  Earthworms are nocturnal feeders, and spend the day quietly with the head end just below the soil surface, often with a  pebble or piece of leaf drawn over the opening of the burrow for protection.  At night they come part way out of the ground,  but leave the tail end anchored in the burrow to ensure a hasty retreat. In the circular area within their reach, they feed on grass cuttings, bits of  leaf and scraps of organic matter of almost any kind.  They also digest the humus from the soil they eat. Since the head end is light-sensitive, the first light of dawn sends the worm below ground again.
 
Body structure.  The long, slender body of the worm, is cylindrical except for being a  little flattened on the underside.  The body is divided into about 150 ring-like segments.   The head end tapers to a point and the tail end is capable of being broadened as a means of gripping the sides of the burrow.   Along the back, the dorsal blood vessel shows through as a darker red line.  On the ventral side may be  found the bristle-like setae (sing., seta) used  in locomotion, the pair of encircled openings of the sperm ducts, and the tiny pair of oviduct openings on the segment just ahead.  Mature worms show a swollen band called  the clitellum, which produces the egg capsule at breeding time.
The body is built on the plan of "a tube within a tube."   The inner tube is the straight-line digestive tract, extending from the slit-like mouth on the first segment to the anal slit on the last segment. The outer tube, forming the body wall, consists of two layers of muscle, the moist skin, a cuticle, and a secretion of slimy mucus.  The fluid-filled cavity between the two tubes contains a well-developed circulatory system,  a nervous system,  numerous thread-like kidneys, and the reproductive organs.  In fact, the worm is exceptionally well organized,  more so than one would imagine from its lowly position in the animal world. 

Life activities.  If we place a worm on a table we may watch it crawl. The first ten to fifteen segments reach forward, grip the table,  then contract, pulling the next few segments forward. These, in turn, do the same and so on, section by section, until the whole body has advanced.  Meanwhile,  the front end is pushing ahead again to repeat the cycle.  This locomotion is performed by the layers of muscle in the body wall,  the circular bands of muscle which squeeze a portion of the body to lengthen it, and the longitudinal muscles which contract to shorten it.  The setae, pointed backward, hold on to any advantage gained.  If the direction of the setae is reversed, a similar series of muscular actions will send the worm backward.
    The earthworm breathes through its moist skin, which must not be allowed to become dry.   Blood in the skin absorbs oxygen,  which is distributed by an intricate blood-tube system to all parts of the body. The blood also distributes the products of food digestion absorbed from the intestine. Waste carbon dioxide is given off through the skin, while nitrogenous waste is eliminated  through fine, tubular kidneys, a pair per segment throughout most of the body  length.  To coordinate the worm's activities,  especially its muscular actions, there is a  nerve cord the full length of the body  branches, segment by segment, leading to the muscles and organs.
    It might be said that a worm is both male and female, since both kinds of reproductive organ are present in each individual.   Nevertheless, it is necessary for two worms  to exchange sperm cells before each can lay  fertilized eggs. The clitellum secretes a  mucus band which slips forward,  receiving  eggs on passing the oviducts, and then the sperm from receptacles nearer the head end.  The egg capsule which slips off the head may develop several tiny worms, but with usually only one survives, the rest being eaten by the first to hatch.

Classification. The earthworm is a member of the phylum Annelida (Latin,annellus, a little ring), the "segmented worms."  Most of its members live in the sea. The leech or "blood-sucker" is a well-know fresh-water relative. White Enchiytrae worms (one inch long) are often cultured as a food for aquarium animals,  and the bright red Tubifex  worms frequently may be seen on the mud in polluted waters.