Short rearward-pointing bristles on the body of an earthworm make rectilinear motion possible by grabbing the ground as the worm slides.
"The basic worm trick consists of stretching and squeezing alternate parts of a long, cylindrical trunk, moving each region of stretch or squeeze rearward, as in figure 24.7a. Despite the rearward progression, the scheme can't do much without one more component. Thus, a worm on a smooth and lubricated surface makes negligible progress--we're not looking at an analog of anguilliform (eel-like) swimming. The trunk needs some device so it slides more easily forward than rearward. For earthworms, setae, short rearward-pointing bristles, provide that crucial asymmetry." (Vogel 2003:488)
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