Earthworms: Oligochaeta

The segmented bodies of earthworms measure up to 19.68 feet (6 meters) and resemble a tube within a tube. The outer body wall is made up of two muscle layers. The outer layer is made up of a series of circles wrapped around the body, while the inner layer of muscle runs along the length of the body. This inner layer shortens or extends the length of the body. Inside the body is the digestive tract, a tube that runs from the mouth, where food is taken in, to the anus (AY-nuhs), where waste, undigested food, and other particles leave the body. Between the muscular body wall and the digestive tract is the body cavity, where all the other organs are located. These organs are usually organized into body segments, just like the outer body. Earthworms do not have flaplike structures to help them move. All but the first body segment is covered with small, stiff bristles, or chaetae (KEY-tee), that help earthworms to hold position as they burrow through the soil. Toward the front of the body is a swollen, collarlike band called the clitellum (KLAI-teh-lum). Special tissues in the clitellum produce a collarlike egg case called a cocoon. These tissues also produce food for the eggs as they develop.