Annelid worms—polychaetes, earthworms (oligochaetes), and
leeches (hirudineans)—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 (Figure A). 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.