The long sac filled with air in the coelom cavity between alimentary canal and kidney of fish is called patka or swimming sac or air sac. This is two types. namely-
1. Physocleistus: Absence of ductus pneumaticus between esophagus and air sac is called physocleistus. For example, kai fish, taki fish etc.
2. Physostomes: Ductus pneumaticus between esophagus and air sac is called physostomes. For example – cotton fish, cuttle fish etc.