Physostomes:
1. pneumatic duct connecting swim bladder to alimentary canal
2. allows the swim bladder to be filled or emptied via the mouth
3. fill bladder by gulping air, but can also to rapidly ascend in the water without the swim bladder bursting
4. utilised by bichirs, gars, some carps, trouts, herrings, catfish, eels and lungfish
5. some physostomes can use gas bladder as a lung, allowing them to live from atmospheric oxygen in conditions where aquatic oxygen levels are low