82 ‘ THE OCEAN. 
specific gravity than its own body; but in order to 
regulate its own weight, it is provided with an in- 
ternal bladder, filled with air, and furnished with 
muscles for its compression or expansion: by the 
former process rendering its body heavier, and by 
the latter lighter than the water. It is true there are 
many fishes which are destitute of the air-bladder; 
but these are, for the most part, ground fishes, which 
reside habitually upon the bottom, rarely swimming 
to any distance. The tail, as was observed, is the 
grand organ of progression; and most of the muscles 
of the body are so inserted upon the joints of the 
Spine as to give the greatest possible energy to the 
motions of this organ. Its expansion is vertical, and 
its motion is only horizontal, from right to left: so 
that, striking the water on either side with great force, 
the fish shoots rapidly forward in the direction of 
the line of the body, but cannot, by its means, ascend 
or descend. The direction of a fish’s motion is go- 
verned by the pectoral and ventral fins, which aid, 
likewise, in balancing the body, and obviate the 
tendency to turn belly uppermost, a position which 
a dead fish assumes, from the weight of the muscular 
back being superior to that of the hollow and air- 
filled belly. There is considerable diversity in the 
depth of water which different species of fishes habit- 
ually inhabit; and this depends, ina great measure, on 
the position of the ventral fins. Such as mainly reside 
at or near the surface have them so placed that the 
centre of the body shall fall nearly midway between 
them and the pectorals. Those whose habits lead 
them to range to great distances without any material 
