THE AIR-BLADDER AS A RESPIRATORY ORGAN.—In certain Fishes the air-bladder may become subservient to the function of respiration. In _Amia_ and _Lepidosteus_ the internally sacculated and vascular air-bladder is obviously adapted for air-breathing, and there are not wanting observations[311] which suggest that the organ is actually used for this purpose after the fashion of a lung. According to Jobert,[312] this is also the case with the sacculated air-bladder of certain Brazilian Teleosts, viz. _Sudis gigas_, _Erythrinus taeniatus_ and _E. braziliensis_, since these Fishes die of asphyxia when the organ is cut off from communication with the exterior by the ligature of its ductus pneumaticus. It is in the Dipnoi, however, that the air-bladder becomes most completely a true lung. In _Neoceratodus_[313] the lung is probably of the greatest use to the Fish when the rivers are low during the hot season and the water is charged with foul gases from decomposing vegetable matter, and possibly also when the water is filled with sediment in the rainy season. In _Protopterus_, and more especially in _Lepidosiren_, the partial atrophy of the gills renders it highly probable that the lungs are the principal breathing organs at all times. Nevertheless, it must be emphasised that in all these Fishes respiration by means of the air-bladder {292}necessarily involves a transit of air to and from that organ through the ductus pneumaticus, and at present nothing is known as to the method by which such inspiratory and expiratory currents can be produced.[314] There is also some experimental evidence for the belief that the air-bladder of some Teleosts may be subsidiary to respiration by acting as a reservoir for the superabundance of oxygen which is taken into the blood through the gills, and subsequently reabsorbed into the blood when the Fish is in water containing relatively little oxygen.[315] It is clear, however, that the conditions under which the air-bladder can be used in this way are by no means fully understood, for, under experiment, such Fishes died of asphyxia even though after death the air-bladder still contained upwards of fifty per cent of oxygen. ACCESSORY ORGANS OF RESPIRATION.—In certain Fishes of peculiar habits, or living under special external conditions, accessory respiratory organs are developed.