Figure 33.12: A Hydrostatic Skeleton Alternating waves of muscle contraction move the earthworm through the soil. The red dot enables you to follow changes in the position of one segment as the worm moves forward. Red arrows show forward extension of the body when contraction of circular muscles pressurizes the worm and the interior pressure forces the anterior end to extend forward. Blue arrows show trailing parts of the body being pulled forward while the anterior is anchored.