The Annelida are segmented worms in which the body wall, coelom (body cavity), epidermis, circular muscle, longitudinal muscle, and peritoneum are arranged into a longitudinal series of rings or segments. True segmented animals exhibit metamerism, a repetition of a structure or organ from segment to segment. Each segment has the same fundamental structures as all the others.With the exception of the digestive system, the major organ systems of the annelids are metameric in structure. Young annelids generally have few segments, but as they grow, new segments are formed by the division of the terminal segment. Annelids represent the most highly organized animals capable of complete regeneration.
General Characteristics of the Annelids
The mouth lies between the first and second segments and forms one segment called the prostomium. In leeches, the mouth contains suckers for attaching to the body of a host. The brain originates in the prostomium and develops a pair of circumpharyngeal nerve rings that reach around the pharynx to form the ventral nerve cord, which appears as a chain of ganglia, one pair in each segment. In the Polychaetes, a pair of swimming or crawling parapodia are located on most of the segments. Both the Polychaetes and Oligochaetes contain external setae to assist in locomotion. The annelid body is covered with a thin cuticle. Each segment has a ring of circular and longitudinal muscles that contract to either elongate or shorten the segment. Aspacious coelom, divided by septa, lies between the body wall and an internal digestive tract. The coelom is filled with fluid and serves as a hydrostatic skeleton in all annelids except the leeches. The coelom also contains the circulatory and excretory systems. A system of large vessels (hearts) pump blood through a ventral vessel into capillary beds that invade all of the tissues. The blood is returned to the hearts via the dorsal vessel. Each segment, except the first and last, contains a pair of nephridia, which collect wastes and deliver them to the outside.
Thanks for description - Animal life club