500 ADAPTATION OF EXTREMITIES FOR SWIMMING. 
the spreading-out of the fingers, and their union by means of @ 
fold of skin which is stretched over them ; as the web of a 
swimming Bird is stretched over its toes, so as to make an 
oar or paddle of sufficiently wide surface. This is the case, 
for example, in the Ornithorhyncus of Australia, and in the 
Otter of our own country. When the members are intended 
Fig. 240.—SKELETON oF SEAL. 
ve, cervical vertebrz ; vd, dorsal vertebre; vl, lumbar vertebra ; ws, sacral vertebra; 
vg, caudal vertebra ; 6, pelvis; s, sternum; h, humerus; 7, radius; ca, carpus; 
mc, metacarpus ; ph, phalanges ; o, scapula; ¢, ribs; f, femur; 7, patella; #, tibia; 
ta, tarsus; mt, metatarsus; ph, phalanges. 
exclusively for swimming, however, they undergo more con 
siderable modifications in structure. The parts corresponding 
with the arm and fore-arm are very short, and the movements 
of the hand are thus limited, but they can be accomplished 
with all the more force. But the bones of the hand are large 
and spread asunder, and are enclosed in a firm integument 
which may even cover their extremities. Sometimes the 
number and arrangement of these bones are precisely the 
same as in the hand of Man; this we see in the Seal (fig. 240), 
where their extremities are furnished with separate claws that 
project. beyond the integument. Sometimes the number 0 
phalanges in the fingers is considerably increased, as in the 
Whale ; and in other instances, the fingers are replaced by a 
multitude of small rods of bone, enclosed within a continuou 
skin, such as we see in the fins of Fishes (fig. 243). 
665. In the Seal, which does not depart widely in i 
general construction from land quadrupeds, the hind feet are 
formed upon the same plan as the fore; but they are carrie 
