Another group of fishes, also inhabiting India, and possessing powers of existing “out of water,” is the Ophiocephalidæ (“snake-headed”); a family allied to the Mullet group. It would appear, from some observations on these fishes, that they are enabled not only to live, like the climbing perch, out of water, but that they die if kept below the surface of the water even for a comparatively short time. Thus when an Ophiocephalus and a carp were placed together in a vessel of water, a net being placed about two inches from the surface, the carp swam, as might be expected, freely and continuously below the surface, while the Ophiocephalus made vigorous efforts to attain the surface, for the purpose of inhaling air directly from the atmosphere. When not allowed to reach the surface, the Ophiocephali died, suffocated, in periods varying from twenty minutes to two hours. The explanation of the power possessed by the latter fish, of being able to live out of water, resides in the fact that these fishes possess two cavities in the throat, in which blood is purified by the inhalation of atmospheric air. Thus the Ophiocephalus not only can exist out of water, but escape from that medium must, in fact, be viewed as an absolute necessity for the normal life of the animal. The climbing perch appears also to exhibit this latter peculiarity of requiring to escape periodically from the water, for this fish, like the Ophiocephalus, may be actually drowned, if kept from obtaining a supply of atmospheric air. The curious Lepidosirens or mud-fishes, which occur in the Gambia of Africa and the Amazon of South America, exhibit a greater peculiarity of structure, which still more completely fits them for living out of water. In the great majority of fishes, a curious sac or bag known as the _swimming_ or _air bladder_ is found. The use of this structure in ordinary fishes is to alter the specific gravity of the animals; and, by the compression or expansion of the air or gases it contains, to enable them to sink or rise in the water at will; but it would also appear that indirectly it may aid in the breathing of all fishes which possess the organ. In the mud-fishes, however, the air-bladder becomes divided externally into two sacs, while internally each sac exhibits a cellular structure resembling that seen in the lungs of higher animals, with which structures, in fact, the swimming-bladder of fishes actually corresponds. Then also this elaborate air-bladder of the mud-fish communicates with the mouth and throat by a tube, which corresponds to a windpipe. The nostrils of the mud-fishes further open backward into the mouth; while, as already mentioned, in all other fishes, save one genus, the nostrils are simple, closed, pocket-like cavities. And it may lastly be noted that the Lepidosirens are in addition provided with true gills, like their ordinary and more commonplace neighbors.