Cilia are non-motile and are oriented perpendicular to the direction of water flow in the osculum. a. Serial longitudinal sections (86 nm apart) show each cilium arises just above the cell nucleus (n) from simple basal bodies (bb); no links between the bases of the ciliary pair were found. b. In cross-section the cilium lacks a central microtubule pair in contrast to the cross section of a flagellum from a choanocyte chamber. c. Cilia pairs are aligned parallel to the long axis of the cells in the osculum, and both the cilia pairs and the cells’ long axes lie perpendicular to the direction of water flow (shown by the blue arrow) at 345.12 ± 4.72° (mean ± SE) (rose diagram: HA:0°; V = 0.841; p < 0.001; n = 49). d. Still images from high-frequency time-lapse imaging of live cilia (arrows) labeled with FM1-43 (see Additional file 2: Movie S1). Scale bars: a, 500 nm b, 100 nm c, 10 μm d, 20 μm.