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longitudinal muscles

The longitudinal muscles, which run the length of the body, are the inner layer of the muscular series and cradle the coelom (Noble). The length of the earthworm's body depends on whether or not the longitudinal muscle series is contracted (making the body shorter) or relaxed (making the body longer). The longitudinal muscles also house the roots of the setae, the bristle-like hairs important for traction. The two muscular series, longitudinal and circular, work in conjunction for locomotion. When one series is relaxed, the other is usually contracted. This alternation of roles explains why, when moving, a worm may be long and thin at one moment and relatively short and fat the next.