A phylum of invertebrates comprising the segmented worms (e.g. the earthworm). Annelids have cylindrical soft bodies showing metameric segmentation, obvious externally as a series of rings separating the segments. Each segment is internally separated from the next by a membrane and bears stiff bristles (see chaeta). Between the gut and other body organs there is a fluid-filled cavity called the coelom, which acts as a hydrostatic skeleton. Movement is by alternate contraction of circular and longitudinal muscles in the body wall. Traditionally the phylum contained three classes: Polychaeta, Oligochaeta, and Hirudinea. However, recent molecular studies have shown that this taxonomy does not reflect the annelids’ evolutionary relationships, although alternative schemes are contentious. One proposal is that the annelids constitute two main clades—the Errantia and the Sedentaria—with each containing representatives from one or more of the traditional classes. Molecular systematics now places the annelids in the clade Lophotrochozoa, along with other wormlike groups and the molluscs.

http://www.ucmp.berkeley.edu/annelida/annelida.html A basic round-up of evolution, life history, systematics, and morphology