The protective polypide-withdrawal reflex ofBowerbankia imbricata (Bryozoa, Ctenostomata) was released by controlled oscillations of the medium (water). The range of effective oscillation frequencies extends from 10 to 200 Hz. The displacements at the threshold of the reflex are 120 μm (peak to peak) at 10 Hz, an average of 30 μm in the range 40-80 Hz, and 150 μm at 200 Hz. The thresholds of oscillation velocity remain within the limits of 0.5 to 1.0 cm/s throughout the frequency range 10-110 Hz, which suggests that this parameter is involved in stimulus reception. The "all-or-none reflex movement" of the animal was employed as an indicator of the position of a defined amount of displacement of the medium beneath a capillary surface wave; this result was compared with calculated values
