The annelids include earthworms, polychaete worms, and leeches. All members of the group are to some extent segmented, in other words, made up of segments that are formed by subdivisions that partially transect the body cavity. Segmentation is also called metamerism. Segments each contain elements of such body systems as circulatory, nervous, and excretory tracts. Metamerism increases the efficiency of body movement by allowing the effect of muscle contraction to be extremely localized, and it makes possible the development of greater complexity in general body organization.