AbstractThe internal architecture of the limbate setae ofSabella penicillus(Sabellidae) is described, based on transmission and scanning electron microscopy. The setae are composed of a number of cylindrical channels (mean= 877). The number of channels is less than one third of the number of spines on the distal swollen part of the setae. In the proximal shaft, channels are hollow and cylindrical. Towards the expanded distal part of the setae, the lumen gradually disappears, and the shape of the wall of the channels changes from circular to hexagonal. With no lumen, the term microrod is suggested as more descriptive than ‘channel’ for the units in this part of the seta. The solid microrods change from a strictly hexagonal shape to a more irregular indented shape, and gradually taper off towards the tip region. The cortical layer of the expanded part contains numerous irregular cavities. Morphometric data on the organization of the internal components are presented. When tested for flexibility, the setae broke when bent to a mean of 93.6° (±9°). Calculations based on morphometry show that the bending stiffness is up to 19 times greater in the swollen part than in the shaft, and it is reduced to about 1/50 in the dista