A very curious object is, The Air-Bladder in Fishes.- There is no doubt that fishes extract air from water by means of their gills, since it is through them that they renew the air of their air-bladder. This bladder is an oblong bag, consisting of two or three membranes easily separated; sometimes it has only a single lobe or cavity, as in the case of pikes, whitings, trouts, etc.; at other times it has two lobes, as in the case of barbel and carp; three, as in that of the sea tench; or four, as in the Chinese gold fish. It is by expanding or compressing this bladder, that the fish occupies more or less space in the water, becomes more or less heavy, and ascends ordescends as it chooses. The division of the bladder into different lobes has proceeded from-a very sufficient reason. When the bladder has only one cavity, as in the case of fishes of prey, the motion of ascent or descent takes place slowly, and without a break; because, as they compress the whole bladder at once, the whole body is moved horizontally, upwards or downwards, as the case may be; a circumstance which has the effect of lessening, in consequence of the resistance of the water, the swiftness of those tyrants of the deep. When the bladder has two lobes, as in the case of the carp, which lives on insects, that fish, by expanding the anterior and compressing the posterior lobe, rises rapidly with the head foremost to the surface of the water, or sinks to the bottom with equal expedition, by compressing its two lobes in different ways. The consequence is, an increased promptitude of movement, and additional means of escaping from its enemies. When the bladder has four lobes, as in the case of the gold fish, that fish is thus enabled to vary greatly its contractions and expansions. It rises, sinks, bends, erects, or turns itself in a thousand ways, and plays in the water, like a bird in the air. It displays all the richness of the colours of gold, silver, or purple, with which Nature has adorned it. Its attitudes are so graceful, and its movements so varied, that the Chinese, from whom we originally received it, are said to pass whole days in looking at it, in the basins of the fountains in their gardens, or in crystal vessels. It is evidently indebted for the ease and grace of its motions, to the modulations consequent on the four divisions of its air-bladder.