The movement of water across the gills of non-ram- ventilating fishes involves the action of two pumps: a pressure pump that pushes water across the gills from the oropharyngeal to the parabranchial cavity, and a suction

which water is pumped from the oropharyngeal chamber, across the gills and into the parabranchial chambers. At the same time, blood is pumped through the gills, where it picks up oxygen from the water and loses carbon dioxide. The movement of water across the gills involves the action of two pumps: a pressure pump that pushes water across the gills from the oropharyngeal to the parabranchial cavity, and a suction pump that draws water across the gills from the oropharyngeal into the parabranchial chamber (Hughes and Morgan, 1973). A synchrony between these two pumps can theoretically yield a continuous flow of water across the gills in spite of discontinuous flow into the mouth and out of the gill slit(s). The blood flow in the respiratory structures of the gills (secondary lamellae) is in the opposite direction to that of water (Grigg and Read, 1971; Scheid and Piiper, 1976). The continuous flow of water across the gills requires two cyclical pumps; one must be active while the other is refilling. The buccal pressure pump is clearly present in all fishes; however, the suction pump is not. In the bony fishes (Osteichthyes), abduction of the operculum serves as the suction pump (Hughes, 1960b; Hughes and Shelton, 1958). It is less obvious in cartilaginous fishes (Chondrichthyes) where the motive force for the suction pump comes from, although the elastic recoil of the branchial skeleton and small muscles that extend the elements of the gill arch may be responsible (Hughes and Ballintijn, 1965). Flow is difficult to measure inside a fish’s head, so relative pressures in the oropharyngeal and parabranchial (or opercular) cavities have been used as an indicator of flow direction. In experiments on bony and cartilaginous fishes, these measures of pressure have been used to indicate that the dominant flow pattern is a continuous, 1577 The Journal of Experimental Biology 204, 1577–1587 (2001) Printed in Great Britain © The Company of Biologists Limited 2001 JEB3366