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.