The head or first segment of a worm is called the peristomium, which contains the prostomio or mouth. The pharynx is found from segments one to six, while the esophagus, which allows the eaten organic material to reach the intestines, is located in segments six to 13. A cerebral ganglium connected to nerves is responsible for the sensation of touch, heat and pain, and encircles the anterior part of the pharynx, between segments two and three.
Middle segments host important internal organs, responsible for circulation and reproduction. Annelids have one or more pairs of hearts, linking the dorsal and ventral vessels. Worms propel the blood by contracting their segments in a wave-like fashion when they move. Most segmented worms are hermaphrodites, containing both male and female organs, but they form mating pairs to exchange sperm. Sexual organs are often found from segments 10 to 15.
Clitellum is an external organ only present in sexually mature worms, which extends from segment 33 to 37. Present in earthworms and leeches, it secrets mucus that keeps the worms together during sperm exchange. The clitellum also produces a mucous sac, where the eggs are deposited after the fecundation. The clitellum is often lighter in color than the rest of the body.
The posterior segments of an Annelid host organs of the digestive system, such as intestines and the anus, as well as the posterior part of the central nerve cord, which is connected to the the cerebral ganglium found in the anterior segments. The last segment, which contains the anus, is called pygidium (Greek for "little tail"). The growth zone, where new segments are formed, is located just ahead of the last segment.