Annelida: More on Morphology

Annelids are coelomate animals; they have a fluid-filled body cavity in which the gut and other organs are suspended. Oligochaetes and polychaetes typically have spacious coeloms; in leeches, the coelom is reduced to a system of narrow canals, and archiannelids may lose the coelom entirely. The coelom is divided into separate compartments by partitions called septa, which give the "segmented worms" their segmented appearance.