Clitellata is a class of Annelid worms, characterized by having a clitellum - a collar that forms a reproductive cocoon during part of their life cycle. The clitellates comprise around 8,000 species.  All clitellata are hermaphrodites. During reproduction, the clitellum secretes a coat which hardens. The worm then creeps out backward from the coat and deposits either fertilized zygotes or both ova and sperm into the coat, which is then packed into a cocoon. The zygotes then evolve further directly in the cocoon without passing through a larva stadium (as opposed to other annelids, e.g. polychaeta.)   Description accessed from WikiPedia 3rd August 2008, and modified.