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Show 1868.] DR. F. DAY ON INDIAN FRESHWATER FISHES. 275 carrying on the respiratory process with unusual activity." This occasional swallowing of atmospheric air is also perceptible when the water becomes foul, or when the amount of air in it appears to be almost exhausted by its inhabitants ; for instance, in three days in May 1867, the heat of the atmosphere and of the water in an aquarium at Madras was as follows:- Mav 26th ,1 27th „ 28th 8 A.M. Air. Water. • • . • 89° 87° 90° 87° 12 A.M. Air. Water. 98° 97° 91° 89° 92° 89° 2 P.M. Air. Water, 90° 88° • • • • * • • • Yet notwithstanding this heat the contained fishes did well, although they swam near the surface to obtain more oxygen, and sometimes appeared even to take in atmospheric air directly. Native fishermen maintain that the middle of the day is the best time for netting freshwater species, as they are then near the surface, no doubt from the heat of the water, and its consequent deficiency of air. In India, instead of discharging the previously inhaled air by the anus, the fish appear to do so by the mouth; at all events the compound breathers adopt this process. Sometimes they do not emit any bubble, but seem only to take in air, whilst at other times the bubble is discharged at the bottom of the water ; this is especially apparent after they have taken food. The purely water-breathers, if the term is admissible, can live without rising to the surface, unless under peculiar circumstances, whilst the compound breathers, as already mentioned, expire in a longer or shorter period if unable to reach the atmospheric air. The difference between the respiration of the compound breather and that of the water-breather is very apparent when they are lying side by side on the moist sand at the bottom of an aquarium. For instance, the Macrones tengara keeps its gills in constant excited motion, while the Ophiocephalidae scarcely move theirs, but at intervals rise to the surface, open their mouths, and take in air. This latter phenomenon, viz. breathing the air pure, and not subsequent to its solution in the water, is especially apparent in some species, such as the Polyacanthus cupanus, Cuv. & Val., which dart up suddenly to the surface, descending again as rapidly into the rockwork in which they delight to make their home, and where they keep all other species at bay, and even fight amongst themselves for the possession of a coveted corner. Loaches (of the genus Platacanthus) also ascend in a similar manner to obtain air. The earliest observation on this peculiar mode of respiration which has fallen under m y notice is made by Dr. Garden*, M.D., F.R.S., in a letter from Charles Town, South Carolina, dated August 14th, 1774, where he states, respecting the Electrical Eel, that its pectoral fins seem to be chiefly used for supporting and raising the fish's head when he wants to breathe, which he does every four or five minutes, by raising his mouth out of the water. With reference to * Philosophical Transactions of the Boyal Society of London, 1775, vol. Ixv. p. 102. |