ADAPTATION OF EXTREMITIES FOR SWIMMING. 501 
far backwards, so as almost to occupy the position of the tail. 
In the Whale and its allies, on the other hand, the posterior 
extremities are almost entirely wanting, and the tail is greatly 
prolonged and expanded at its extremity (fig. 241). This 
Fig. 241. —Skeleton of Dugong. 
expansion, however (which is in the horizontal direction, fig. 
242), is not supported by bones, except in its centre; but it 
consists internally of cartilages and tendons, which last are 
prolonged from a set of very powerful muscles that are at¬ 
tached to the spine, and give to this organ an enormous force 
and great variety of motion. The texture of the portion of it* 
by which the blow is usually given, is such that it can hardly 
be injured ; it is so tough that it cannot be torn, and so free 
from feeling, that a stroke of it against a hard substance gives 
no pain to the animal. If it strike a boat across the middle 
with its edge, the boat is cut asunder as clean and suddenly 
as if by one stroke of a giant axe; whereas, if it strike with 
