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A number of articles I have visited indicate that some sharks, e.g., nurse sharks can ventilate their gills by some mechanism which is hinted at but not well described.
I have observed that, in bony fishes, gill ventilation is accomplished by alternate expansion and contraction of the buccal (mouth) cavity aided by a flexible fringe on the operculum which prevents back flow as the mouth opens and the cavity expands, and by a cresent shaped membrane on the roof of the cavity near the mouth which prevents back flow when the mouth shuts and the cavity contracts.
I presume that shark gill slits act as a back flow valves in the same manner as the opercular fringe in bony fishes, preventing back flow as the buccal cavity is expanded and that the spiracle acts to prevent back flow as the cavity is compressed. Is there an expert out there who is willing to confirm this? Don Seib 02:20, 31 August 2009 (UTC)