502 PROPULSION OF WHALES AND FISHES BY TAIL. 
the flat surface, the boat is driven to the depth of many fa- 
thoms with the swiftness of an arrow. Hence this tail is a 
most efficient instrument for the propulsion of the bulky 
body of the Whale through the water ; and it is, in fact, its 
principal organ of locomotion. The paddles formed hy the 
fore-feet are placed near the centre of gravity of the whole 
mass ; and thus can readily exert their peculiar action, which 
is that of changing the direction of the movement, and espe- 
cially of raising and lowering the body. 
_ 666. The propulsion of the body by the stroke of the tail 
in Whales and Fishes, is effected precisely in the same manner 
as the urging-forwards a boat through the water, by the 
Fig. 248.—SKELETON OF PERCH. 
lateral strokes of an oar at the stern, in the mode commonly 
termed sculling. The expansion of the Whale’s tail-fin being 
horizontal, its stroke is vertical, and may thus readily bring 
the animal to the surface of the water for occasional respira- 
tion, as well as propel it forwards; but that of the Fish’s 
body and tail being vertical, its stroke is horizontal, and its 
action will simply be to urge the body through the water. 
The power of ascending and descending, as well as of changing 
the direction of the motion, is principally due to the side-fins, 
which represent the arms and legs. The direction of the 
surface and stroke of these side-fins varies in different species. § 
In the Cod, Halibut, and others, their action appears to be § 
principally directed towards keeping the body in its right’ 
position in the water; since, without such an action, the body 
would be liable to turn over, in consequence of the position 
