Why does the body of the worm, during movement, either stretch and become thin, then shorten and thicken?

The worm moves in the ground due to the movement of the skin-muscle sac. First, it wedges into the soil, then, thickening the anterior segments, it pumps the cavity liquid into them. It alternately contracts and relaxes the muscles. With the contraction of the annular muscles, his body becomes longer and thinner. With the contraction of the longitudinal muscles, the body of the worm is shorter and thicker.

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