migrate thousands of miles in a season, without the
slightest indications of languor or fatigue.

201. THE PRINCIPAL INSTRUMENTS EMPLOYED BY FISHES to accelerate their
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.

202. THE BODIES OF FISHES