The bristle-worms. This class of worms has bunches of bristles on both sides of each segment of the body, which serve as organs of locomotion, or bristle-feet. The bristles emanate from outgrowths of the body known as /parapodia/, which are practically limbs. The parapodia are sometimes divided into distinct lobes or branches. The bristles are of various shapes and often of brilliant color. They are usually horny, sometimes simple, sometimes divided into joints, and vary in shape in different genera. The parapodia have, besides the bristles, a second set of [pg172] hair-like appendages, called /cirri/. These are sense-organs and also have a respiratory function. The cirri too vary in shape in different genera. Generally they are hair-like, but sometimes conical; again they are broadened into scales, as in the /Aphroditidæ/. The body-cavity in chætopod worms is divided into a series of chambers or segments. These divisions seem on the exterior like constrictions, and give the body the appearance of a series of rings. The internal organs are repeated in each segment, so that each of the latter contains a portion of the alimentary canal, a pair of /nephridia/, a pair of nerve-ganglia, and blood-vessels which connect the main blood-vessels running along the dorsal and ventral surfaces. The alimentary canal runs through the body, but is constricted at each joint. The nephridia are curved tubes, which are excretory organs, opening to the outside and carrying off the waste products which have passed into the fluid of the body-cavity. The chætopod worms have a well-developed nervous system, beginning with a two-lobed ganglion forming a brain and head, then extending through the worm in a double ventral chain and series of ganglia. They have also a blood-vascular system. The blood is either colored or colorless. Sometimes it is a bright red or green, and is often visible through the body-wall. The circulation is effected, not by means of a heart, but by wave-like (peristaltic) contractions of the dorsal blood-vessel. The body is cylindrical, but in many cases is somewhat flattened. There are two layers of muscles, one of which encircles the body, while the other extends parallel with its length. Respiration is effected by /gills/, which may be simple, hair-like appendages to the parapodia, or branched, or comb-like in form. Sometimes they are confined to the middle segments, as in /Arenicola/; sometimes to the segments near the head, as in /Tubicola/; or they may be extended over the whole dorsal surface. SUBCLASS =POLYCHÆTA= ("/Many-bristled/")