The mechanism which synchronizes the beating of the hundreds of thousands of long cilia making up a ctenophore comb plate was investigated by microsurgical experiments on single comb plates of Mnemiopsis and Pleurobrachia. Comb plates of lobate ctenophores (e.g. Mnemiopsis) are triggered to beat by ciliated grooves which run between the centres of adjacent plates. By creating gaps or introducing mechanical barriers between two parts of a plate, or by severing the cells at the base of a plate, it was shown that physical proximity of cilia, not tissue continuity, is required for synchronization of beating. In Pleurobrachia only the first comb plate of each row is activated by a ciliated groove, and similar experiments to those done on Mnemiopsis gave identical results. Although adjacent comb plates in Pleurobrachia are triggered mechanically by movements of the preceding plates without the need for an intraplate synchronizing mechanism, unilateral amputation of a plate showed that cilia within these plates may also be synchronized by mechanical coupling. Therefore, in cases where the beating of a comb plate is triggered by a ciliated groove - either at the head of a comb row (in all ctenophores) or along the row (lobates only) - the cilia within the plate are synchronized by hydrodynamic coupling forces between them, not by electrical coupling between their cells as assumed previously.

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