The Book of Household Management by Mrs. Beeton

motion, are their air-bladder, fins, and tail. By means of the air-bladder they enlarge or diminish the specific gravity of their bodies. When they wish to sink, they compress the muscles of the abdomen, and eject the air contained in it; by which, their weight, compared with that of the water, is increased, and they consequently descend. On the other hand, when they wish to rise, they relax the compression of the abdominal muscles, when the air-bladder fills and distends, and the body immediately ascends to the surface. How simply, yet how wonderfully, has the Supreme Being adapted certain means to the attainment of certain ends! Those fishes which are destitute of the air-bladder are heavy in the water, and have no great "alacrity" in rising. The larger proportion of them remain at the bottom, unless they are so formed as to be able to strike their native element downwards with sufficient force to enable them to ascend. When the air-bladder of a fish is burst, its power of ascending to the surface has for ever passed away. From a knowledge of this fact, the fishermen of cod are enabled to preserve them alive for a considerable time in their well-boats. The means they adopt to accomplish this, is to perforate the sound, or air-bladder, with a needle, which disengages the air, when the fishes immediately descend to the bottom of the well, into which they are thrown. Without this operation, it would be impossible to keep the cod under water whilst they had life. In swimming, the _fins_ enable fishes to preserve their upright position, especially those of the belly, which act like two feet. Without those, they would swim with their bellies upward, as it is in their backs that the centre of gravity lies. In ascending and descending, these are likewise of great assistance, as they contract and expand accordingly. The _tail_ is an instrument of great muscular force, and largely assists the fish in all its motions. In some instances it acts like the rudder of a ship, and enables it to turn sideways; and when moved from side to side with a quick vibratory motion, fishes are made, in the same manner as the "screw" propeller makes a steamship, to dart forward with a celerity proportioned to the muscular force with which it is employed.