The ventral fins, it will be noticed, are placed well under- 

 neath, and so set that they exert, in a great measure, an 

 upward force, and serve to keep the fish down low when 

 desired. When, however, the creature has set itself in any 

 desired position, it seems that, without the aid of fins at all, 

 it can stay there by adapting its bulk exactly to the density 

 of the surrounding water. 



The pectoral fins are especially used as balancers, and 

 when the fish is maintaining its position in any fixed place 

 it will be noticed that they are constantly being used. These 

 fins, too, like the ventrals, can exert an upward force, and in 

 many ways are most important, for without them the sudden 

 halt, or side turn, could not be made, nor could the creature 

 set itself to different currents as a bird does in the air — at 

 least, those birds that use their wings for floating in the air. 



We can see, then, that, when at rest, or nearly so, a 

 nervous action affects all the fins and tail — that the fish is 

 always balancing itself, its centre of gravity being set so high 

 that it must do so. W 7 e can see, too, that this high centre 

 of gravity concentrates the weight somewhere in the middle 

 of the fish, and that the power applied by the tail is perfectly 

 effectual, and that when high speed is necessary the tail 

 alone is used. 



