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Show 1874.] RESPIRATION OF FRESHWATER FISHES. 313 The following experiments were undertaken with the view of extending our knowledge in this direction, by determining what other species are compound breathers, and in what degree the direct use of atmospheric air is necessary for their existence. The manner of conducting the experiments was similar to that adopted by Mr. Francis Day *, to whose investigations these notes may be regarded as addenda. The specimens experimented upon were recently taken and quite uninjured. The temperature of the water during the period occupied in making the following observations varied very slightly, from about 87° to 91°Fahr. Exp. 1.-A specimen of Plotosus canius was placed in a large cylindrical glass jar (of equal diameter throughout) nearly filled with water, and having a metal plate, pierced throughout with large openings, fixed 1 inch beneath the surface of the water. The metal plate was fixed in this position immediately after the fish had been observed to rise to the surface of the water. Twelve minutes afterwards the fish again rose and pushed violently against the plate, but not succeeding in getting to the surface it sank slowly to the bottom of the jar. Having remained thirteen minutes at the bottom rapidly moving the gill-covers, it made a second attempt to reach the surface, and dashed so strongly against the plate as to force it upwards, and so obtained access to the air. On sinking down again, a large number of air-bubbles passed upwards from the gill-openings. The fish was then placed for the night in a large earthenware basin, with a specimen of Ophiocephalus punctatus about the same size. Next morning the latter had eaten the greater portion of the barbels of the former, and had torn away the skin of the abdomen. The greater portion of the anal fin was detached from the body and eaten, but the fish seemed as lively as before. Two other fish of the same species had leaped out of another basin, and were found dead on the floor. The Plotosus was again placed in the same glass jar, having a diaphragm of net fastened 1| inch below the surface of the water. It remained inactive for nearly half an hour, then sought the surface, but being repulsed by the net struggled violently against it, during which time several air-bubbles passed from the gill-openings to the surface of the water. It died in one hour from the time of being first placed in the jar, and in half an hour from first commencing its struggles. Exp. 2.-Placed four specimens of Macrones tengara at 10.40 A.M. in the glass jar, the netted diaphragm being fixed as in Exp. 1. In another jar without a diaphragm two specimens were also placed. At 3 P.M. the respiratory action of the fish in the jar with the diaphragm became much excited; at 3.40 the largest individual, which had been for some time swimming about, rose to the net and pushed against it several times; at 4.40 all the fish had almost * Loc. cit. p. 278, exp. 1 & 4. PROC. ZOOL. Soc-1874, No. XXI. 21 |