The Significance of Metamerism - The development of repeated parts represents a significant evolutionary innovation that is found in arthropods and chordates as well as in annelids. In locomotion, metamerism provides a more effective system for body movement; the body is divided into a consecutive series of motor units that can be controlled separately or in a wave of contraction.  In annelids, the separation of the coelom by transverse septa means that the forces of muscular contraction are not transferred through the organism.  Thus, the worm can move one part at a time while anchoring other parts to the substrate (which is good for burrowing).  With a repetition of body parts, the possibility also arises for specialization of these body parts into different structures (serial homology); this potential is not realized so much in annelids but will become important in arthropods!

The Classification of Annelids - There are three classes of living annelids:  Class Polychaeta (marine worms), Class Oligochaeta (earthworms and some freshwater forms) and Class Hirudinea (leeches).