III. The bionomics of certain air-breathing fishes of India, together with an account of the development of their air-breathing organs

It has long been known that certain species of fresh-water fish, inhabiting the rivers, lakes and ponds of India, are in the habit of leaving the water and making considerable excursions over the adjacent marshes and meadows, and some have been credited with the power of climbing trees. The older authors (Cuvier, Owen and Günther) explained the power which these fishes possessed of sustaining life outside the water by supposing that they carried with them, in reservoirs at the sides of the head, supplies of water by which the gills were kept moist. On the other hand, Taylor (1831) and specially Day (1808) and Hyrtl (1863) showed, as the result of close observation, that the reservoirs in question contained little or no water, and that the fish, when kept in tanks or globes, could be seen to come to the top at intervals and emit air-bubbles. It was, therefore, clear that the reservoirs in question contained not water but air, and that the fish must be regarded as true air-breathers. The fact that certain fish, such as Protopterus in Africa, Lepidosiren in S. America and Ceratodus in Australia, possess organs for breathing air as well as gills for respiring in water is well known; the structure and habits of these “double-breathers” (Dipnoi) form an integral part of the instruction given in every elementary course of Zoology. But the air-breathing organs of the Dipnoi are homologous with the lungs of the higher vertebrates, and the Dipnoi must be regarded as comparatively unchanged survivors from the time when the Devonian fish were making their first attempt to invade the land, at a period when no land-inhabiting air-breathing vertebrates existed.