Segmented worms, such as earthworms and leeches, have bodies made up of connected sections. To move, they send waves of contractions down their body through the sections. Circular muscles contract to stretch part of a worm forward, followed by longitudinal muscle contractions that shorten the worm, bringing another part along with it. Suckers or stiff hairs keep each section of the worm anchored so that it does not move backwards as the contractions passes through it. The nervous system of a worm includes long nerve cords running its length, interacting with clusters of nerve cells (ganglia) in each section. This long system of nerves, with instructions from two ganglia in the worm's head, controls the muscle contractions that cause movement. Each body section is full of fluid that makes it rigid enough for the muscles to change its shape. Some worms also have little appendages they use like paddles to row or walk themselves along.