Earthworms are classified as Annelida. Annelida mean little rings which refers to the many segments in their body. The structure of an earthworms body is made up of more than one hundred segments separated by partitions that divide the coelum. All segments are identical except by the anterior and posterior ends. The anterior segments reflect the cephalization that is an adaption of burrowing. The head of the earthworm contains the sense organs. The muscle lines that make up the interior body wall are circular and longitudinal. Earthworms move by anchoring some segments by their setae and contracts the circular muscles in front of those segments, producing fluid pressure in the anterior coelom cavities. The anterior setae grip the ground, the longitudinal muscles contract pulling the posterior along. Earthworms burrow and feed on soil and organic matter at the same time. They digest the organic matter and eliminate wastes and undigested matter as dirt and feces called castings. Earthworms are good for the soil because they sucked up soil into the by the muscular pharynx.
The soil then passes through a tubelike esophagus to a temporary storage called a crop, and from there to the gizzard. The gizzard walls grind the soil, releasing and breaking up organic matter. Through the earthworms body via a closed circulatory system it transports oxygen, nutrients, and wastes. The blood travels from the anterior to the posterior through ventral blood vessel and then forward through a dorsal vessel. Aortic arches are five tubes that link the major vessel near the anterior. Smaller vessels branch into each segment of the body. Earthworms have no respiratory system or no gills. Carbon dioxide and oxygen diffuse directly across the skin. This process can only happen if the skin is moist. Earthworms secretions of mucus and a thin cuticle help keep the skin moist.
Through a long tube called nephridia the earthworms eliminate nitrogenous wastes. Earthworms are sensitive to touch, light, vibrations, moisture, chemicals, and temperatures. Other sense organs and the nerves that control individual muscle contractions are present in each segment. Earthworms are hermaphrodites, but one worm cannot fertilize it own eggs. When earthworms are join to head to tail its called mating. Together they form a mucus around each other. They both inject sperm into the mucus. One of their sperms goes to a pouchlike seminal receptable. After a several days a mucus and chitin sheath is secreted by the clitelllum a swelling around the sex organs. When the worm wriggles to slip the sheath off its body, eggs, and sperm are joined and fertilization occurs.