the outer integument and underlying musculature that together form the saclike wall of a worm’s body.
Leaches swim and earthworms move in the soil by means of wavelike contractions of the musculocutaneous sac. The structure is pronounced in turbellarians and other flatworms, round-worms, acanthocephalids, echiuroids, nemertines, polychaetes, oligochaetes, sipunculids, and priapulids. In some worms (for example, certain annelids) the musculocutaneous sac has separated into individual muscular bundles, owing to the development of appendages, or parapodia. In others (gastrotrichs, rotifers), the sac is only weakly expressed, a result of increased motor function of the ciliate integument and decreased body size.