Place a worm on damp paper and touch it gently to make it move, while the children watch. Get them to draw quick sketches of the worm's body several times over the next ten minutes, showing how it stretches out and bulges in different places. Use magnifying glasses if possible, to show detail.
Arrange the drawings in a shallow diagonal procession across a tabletop to give an impression, as if in animation frames, of how the worm changes shape as it moves.
Explain how movement depends on two layers of muscle: circular and long. Draw a diagram on the whiteboard to show longitudinal stripes -- representing the long muscles -- down the length of a worm.
Draw hoops around the worm, to represent the circular muscles. Describe how the muscles work in turn - the circular muscles squeezing and the long ones pulling - to make the worm change shape and move
Explain that the squeezing and pulling seems to come in waves, because the worm's body is made up of segments, which work one after the other.
Use magnifiers to let the students observe this in worms from the worm jar. Choose particularly long worms, to show how the worm can bulge in more than one place at once.
Ask the children to move like worms -- wriggling their way across the floor by stretching out then bunching up their body. They can do this individually, then in groups, each child representing one segment and each group representing one worm. The "segments" stretch out and bunch up one after the other to simulate the wave effect they saw in the real worm. Hold "worm races" between competing groups, or line up the whole class to make a giant worm.
Talk about how the way worms move makes them good at tunneling. Discuss how important earthworms are in recycling leaf litter and aerating soil. This makes healthy soil that grows healthy plants.