Some aspects of the anatomy, histology and physiology of the swimbladder of Notopterus have been studied, with special reference to its behaviour in aerial respiration.

The large swimbladder with a wide pneumatic duct is reported to function as an accessory respiratory organ. Circulation of blood is modified to discharge this function. The network of blood capillaries covered by a single layer of epithelium apparently facilitates the diffusion of gases between the blood and the swimbladder.

Notopterus is able to survive for long periods in foul water, if allowed to reach the surface to take in air. There is a regular exchange of gases between the cavity of the swimbladder and the exterior.

The frequency and extent of aquatic and aerial respiration in Notopterus under different conditions have been determined. They throw considerable light on the double respiratory mechanism in the fish.

Under normal condition, the mean value of oxygen is 11 per cent and that of carbon dioxide 3 per cent in the “alveolar air” of the swimbladder. The result of analysis of gases from the swimbladder of Notopterus under adverse experimental conditions clearly establishes the remarkable capacity of the swimbladder of the fish to function as an efficient respiratory organ.

The presence of a high percentage of carbon dioxide (6.5 per cent) in the swimbladder is of interest and suggests high resistance of the blood of the fish to carbon dioxide.

The air-breathing habit of Notopterus is probably a secondary adaptation to its life in ponds and puddles under tropical conditions.