Beat of preoral cilia and particle paths were filmed for veligers of Crassostrea gigas, Tritonia diomedea, Nassarius obsoletus and an unidentified prosobranch. Particle captures were filmed for the three identified species. 2. Clearance rates per unit length of velar edge are estimated from the equation (L-R)WPF/2C, where L is cilium length, R a correction for recovery stroke, W angular velocity, C/P the ratio of velocities of cilium and particle, and F the fraction of particles captured. The clearance rates are in rough agreement with Bayne's values for veligers of Mytilus edulis. 3. In the three identified species, longer preoral cilia clear particles at a higher rate but with less efficiency. Since veligers from larger eggs generally have both longer preoral cilia and a longer velar edge, a larger egg generally produces a veliger with a higher maximum clearance rate when the veliger begins to feed. 4. Angular velocities increase with cilium length in the three identified species of veligers but the larger unidentified species did not continue this trend. 5. Preoral cilia in their effective strokes move 1 to 3 times faster than particles travelling in about the same arc with a mean of about 1.5 times the speed of the particles. In mid effective stroke, the ratio of velocities of cilia and particles is not significantly different for captured and non-captured particles, nor does the ratio vary significantly with angular velocity of cilium. The ratio does vary significantly among species. 6. Particles passing closer to the base of the preoral cilia are more likely to be captured. 7. We hypothesize that suspended particles are concentrated when they are overtaken by preoral cilia in their effective stroke, weakly adhere to the preoral cilia, and are pushed faster than the water. Capture is completed when particles are drawn into the food groove, probably by the action of the recovery stroke of preoral cilia, the current from postoral cilia, or both.
