For whatever reason, the old earthworm seems to pop up now and again....

The earthworm moves by coordinated waves of muscle contraction (peristalsis) from anterior to posterior. Each wave consists first of an elongation and thinning followed by a shortening and thickening of the body. The earthworm body is divided into about 100 segments, each of which is filled with incompressible coelomic fluid. Each segment, for the most part, is isolated from adjoining segments so that, in theory, it could expand or contract independently of the others. This, however, does not happen. The nervous system coordinates movement so that segment expansion or contraction occurs in peristaltic waves along the length of the body. Circular muscles, under the epidermis, run around the animal while longitudinal muscles run the length of the worm.
http://www.bigelow.org/mitzi/images/teacher%20resources/Phylum%20Annelid.pdf..