The clitellum of an earthworm is located about one-third of the way down its body. Only sexually mature earthworms have clitella. Clitella are usually white or have an orange hue. The clitellum secretes a substance that becomes a hardened cocoon, which houses the eggs and sperm after mating.
When two worms mate, they lie next to each other, head to tail. The clitella line up with one another and the two worms exchange sperm. The number of segments making up the clitellum differ among species and can be an identifying characteristic.
The earthworm lays eggs and then deposits the sperm it retained from its mate into the cocoon structure. The earthworm then withdraws, leaving the cocoon in the soil.