Buoyancy disorders are common in goldfish, often having a sudden onset and present floating at the surface or lying on the bottom. In many cases, there were no external lesions and no consistent environmental factors could be identified. In fish, buoyancy is controlled by the amount and distribution of gas within the body. This is primarily enclosed within a gas-filled buoyancy organ, the swim bladder. In some fish, physostomes, there is a patent duct that connects the swim bladder to the anterior oesophagus, which permits air to be swallowed and forced into the swim bladder. In other fish, physoclists, there is no patent connection and the swim bladder is inflated by the release of gas from arterial blood by a vascular rete in the wall of the swim bladder: this also occurs in some physostomes.