فیزوستوم

thumb|right|The ductus pneumaticus from the gas bladder to the gut in a [[common rudd.]] Physostomes are fishes that have a pneumatic duct connecting the gas bladder to the alimentary canal. This allows the gas bladder to be filled or emptied via the mouth. This not only allows the fish to fill their bladder by gulping air, but also to rapidly ascend in the water without the bladder expanding to bursting point. In contrast, fish without any connection to their gas bladder are called physoclisti.