Pressure measurements made at various points inside the mantle show that the ventilatory stream of the resting animal is driven by very small (often less than 0.5 cmH2O) pressure differences. Inspiration occupies less than onethird of the total cycle time, while flow across the gills is evidently continuous, since there is always a pressure differential between the prebranchial and postbranchial parts of the mantle cavity. The fact that branchial heartbeats do not correlate with ventilatory movements is further evidence that water flow through to the gills is both steady and continuous.

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