Non-beating cilia are the candidate sensors that trigger particle capture. (A) DIC micrographs of live, wild-caught advanced-proboscis-stage pilidium (likely of Lineus flavescens or a close relative thereof) at two different focal planes to show the non-beating cilia (arrowheads) and their posture in the inner ciliated band of the lappets (la) and the single ciliated band of the anterior lobe (al). Insets double the magnification for inner band of the lappet (bottom left) and anterior lobe (top right). (B) Unconstrained pilidium (same kind, developmental stage, and plankton sample as panel A) viewed askew into the lappets, showing the stationary cilia (arrowheads) amidst the metachronal wave of the primary ciliated band (pcb). Note also that the inner ciliated band (icb) lies nearly flat against the inside wall of the lappet. (C) Equivalent view of the lappet margin by SEM; four circles identify anemone-shaped uniciliate collar cells, which correspond to the non-beating cilia on panels A and B, and to the phalloidin-stained collars shown in Figure 3. (D) Video sequence of capture in which a cell passes the lappet margin nearly in focus (see Additional file 5: Video 5). The larva is in frontal view. Times are relative to the first frame in which the lappet moves. A non-beating cilium (arrowhead) is in focus. The cell to be captured pauses just beyond the position of this non-beating cilium immediately before the lappet moves to suck the cell in. (E) Kymograph of the straightened path indicated in the first frame of (D). This depiction provides a readout of the captured cell’s speed, as indicated by triangles: 1 = 0.63 mm/s; 2 = 1.7 mm/s; 3 = 2.9 mm/s. A non-beating cilium is indicated by the black arrowhead, and the cell exhibits a pause of ~20 ms as it passes this point. Note that this cell is as much as 10 μm out of focus, and thus surely interacts with a different non-beating cilium than the one visible.