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. The phylum contains three classes: Polychaeta, Oligochaeta, and Hirudinea. Molecular systematics now places the annelids in the superphylum Lophotrochozoa, along with other wormlike phyla and the molluscs.
http://www.ucmp.berkeley.edu/annelida/annelida.html A basic roundup of evolution, life history, systematics, and morphology
Subjects: Biological Sciences.