Earthworm - Reproductive System
Earthworms are hermaphroditic, but cannot simply impregnate itself. During reproduction, two worms come together in opposite directions with their clitellums in contact with segments 9-11 of their mating partner. The clitellum produces a mucus which holds the two worms together for sperm exchange. Sperm are made in the seminal vesicles, which are large organs on either side of the esophogus. Sperm is stored, once it is received, in the seminal receptacles (Loretto).
Picture retrieved from http://members.aol.com/camusurca/earthworms/reproduc.htm
The clitellum, the area where the actual fertilization takes place, is about a third of the way down the worm's body. The oviduct opens on the 30th segment and the average earthworm has 100 segments (Musurca). When eggs are made, they are stored in the oviduct. Some time after two worms have exchanged sperm, they will (on their own) secrete a ring of mucus around the clitellum that will allow it to slide off. When the clitellum slides off, it takes sperm from the seminal receptacles and eggs from the oviducts. The clitellum is now a cocoon for the new baby worms and has enough nutrients for the baby worms for a little while (Musurca).