Annelida is a phylum made up of segmented worms ranging from earthworms to leeches. Its body segments give it great advantages overt other types of worms. The pattern of linear segments or metamerism allows the worm to contract its muscles and move better. It also allows the worm to have a greater complexity in body structure and organization. Another distinct characteristic that defines the structure of annelids are the muscle fibers that can be both circular and longitudinal throughout the body. These muscles help the body of the annelids in movement and in day- to -day functions. A critical part in the development of the muscular system is hydrostatic pressure that is maintained throughout each segment. This pressure allows the body to keep its rigidity and allows the muscles to contract fluently without collapsing the annelida's body.
http://animaldiversity.ummz.umich.edu/accounts/Annelida/
http://animaldiversity.ummz.umich.edu/accounts/Annelida/