There is considerable interspecific variation in the detailed structure of the intersegmental septa of polychaetes. In a few worms, septal muscles are specially developed to produce dorso-ventral flattening of the body, or to provide horizontal bracing of the thin intersegmental lateral body-wall. In the majority of species examined, the septa are thin and the disposition of the muscle fibres in them suggests that they serve as water-tight diaphragms which impede the transmission of fluid pressure changes along the worm. The types of movement in which such diaphragms are necessary or useful are discussed. Often, the septa are incomplete but can still probably seal off the coelom of each segment under certain circumstances. Septa appear to be severely reduced or completely lost in errant species in which the extrinsic parapodial musculature is especially efficiently disposed for moving the parapodia (a relatively unusual situation), as well as in sedentary forms.