Annelid worms - polychaetes, earthworms, and leeches - are distinguished by linear series of external ringlike segments; the grooves between segments coincide with internal compartments, often separated by transverse sheets of tissue (septa), containing serially repeated nervous, muscle, and excretory systems. Anterior segments bear jaws, eyes, and cirri (singular: cirrus, a slender appendage) in some species; the terminal segment may bear a cirrus. Annelids have spacious, mesoderm-lined coeloms - except for leeches, in which tissue packs the coelom - and their coeloms are important in excretion, circulation, and reproduction. Chitinous lateral bristles called setae on each segment are used for locomotion or to anchor the annelid in substrate or burrow; leeches lack setae. Parapodia are unique to polychaetes; these thin, fleshy flaps protrude laterally from each body segment. Chitinous cuticle covers the entire body.