Earthworm Biology

It was Charles Darwin who first recognized the earthworm's importance, describing them as 'nature's plow'. Nowadays it is well know what beneficial effects the earthworm has chemically and physically on soil. By burrowing into the soil, the earthworm increases the moisture of the soil by allowing rainwater to infiltrate deeper, and the aeration of the soil thus enabling better root development.

While the droppings of the earthworm, known as castings, help in enriching the soil, due to it being rich in phosphorus, magnesium, calcium, and nitrogen. Since the earthworm increases the fertility of the soil, it is being increasingly introduced in land areas that lack earthworms, in order to ameliorate degraded areas of land and increase the production of plants in agricultural areas.

Scientifically, the earthworm is classified in the phylum Annelida, and they range in size from just a few millimeters to more than 3 feet in length, although most species measure few inches in length.

The Biology of the Earthworm

One of the most obvious features of the earthworm's body is its segmentation, which isn't just an external feature but also occurs through almost all its internal structure. The anterior section of the earthworm, or the head, is made up of the prostomium, which is a lip-like extension situated in front of the mouth, which it uses to force its way into the soil.

Each of the segments in its body has hairy structures, known as setae, which can be extended as well as retracted, and are used for locomotion. The absence of other locomotive structures, apart from setae, enables the earthworm to burrow efficiently into the soil. Plus, there are glands in its skin which secrete mucus which help in keeping the earthworm lubricated, which again helps the earthworm to burrow easily through the soil and also aids in stabilizing casts and burrows.

The earthworm has no eyes; instead, it has cells that sensitive to light on its outer skin. These cells enable the earthworm to detect light as well as the changes in the intensity of light. The cells of the skin are sensitive to chemicals and touch as well.

The earthworm does not have any lungs either, and it breathes by oxygen being diffused into its body through the skin. The highly permeable skin of the earthworm makes it susceptible to drying out easily.

The digestive tract of the earthworm is one of its unique features, being highly adapted according to its activities of burrowing and feeding. The earthworm ingests soil, along with decomposing organic matter in it, which are mixed by powerful muscles and passed through its digestive tract. In the digestive tract, digestive fluids that contain enzymes are released, which are mixed the oil mixture that has been ingested. The digestive fluids help in releasing sugars, amino acids, as well as smaller sized organic molecules. These molecules are then absorbed via the membranes of the intestines, which are used for energy and the synthesis of cells.

The Reproductive System of the Earthworm

Earthworms are hermaphroditic, similar to slugs and snails, having both female and male reproductive parts in its body. Usually, they do not self-mate. Two earthworms mate, and during the mating they exchange sperm mutually. The egg cells, mature sperm, and nutritive fluid are placed in cocoons, which are made by the clitellum, which is a structure like a girdle that is located at the anterior portion of its body. The sperm then fertilize the eggs inside the cocoon, which then is shed by the earthworm, being deposited either on or in the soil. After about three weeks, the eggs begin hatching, with about 2-20 earthworms being produced by each cocoon, the average being 4.