70 THE OCEAN. 



an internal 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 tlie direction of 

 the line of the body, but cannot, by its means, ascend 

 or descend. Tlie direction of a fish's motion is 

 governed 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, in a 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 

 ^laterial change in their depth of water, have the 

 ventral fins placed far back on the belly, as the 



