In earthworms, when the ovaries mature, the clitellum secretes a viscous substance in the form of a girdle. This girdle hardens in to a cocoon around the clitellum. The ova are discharged into cocoon. When this cocoon moves over the spermathecae openings, the sperms which are already collected from another worm and stored in the spermathecae are discharged into the cocoon. The cocoon finally, comes out of the body and fertilization takes place in the cocoon. As it is taking place outside the body, it is called as "external fertilization".