SWIMMING 



to these limbs is not approximately equal to the mass posterior thereto, 

 but very much less, and there must therefore be specialized apparatus 

 for equilibration, which exists in both the hind feet and the tail. 



But there are different methods of swimming by the anterior limbs. 

 Thus the platypus apparently pushes the water directly backward with 

 the palm and partly clenches the manus during recovery. The marine 

 turtle holds the axis of the manus almost parallel with the body axis 

 (at least in the more specialized sorts) and pushes the water to the rear 

 by a vertical motion of the arm. The sea-lion also employs an oblique 

 movement of the flipper, but by pushing the water from the ulnar 

 border of the manus by adduction of this member in the transverse plane. 



There are two features of interest illustrated by mammals which swim 

 by kicking the hind feet in alternation. In one, encountered in almost 

 all aquatic rodents and insectivores, as the hind feet are kicked the tail 

 is involuntarily moved from side to side by the wriggling motion of the 

 hinder end of the body proper. If the tail be of considerable length 

 this member will at first effect all the equilibration that is necessary, 

 and later, as the tail becomes more speciaHzed, it will take over a sub- 

 stantial part, if not the whole, of the function of propulsion. If the 

 hind limbs be relatively small their action in swimming may be assisted 

 by the fore limbs, or if the tail be too short to act efficiently as an 

 equilibrator the pectoral limbs may be kicked in alternation chiefly as 

 balancing agents, just as a man swings his arms when walking. 



It is easily seen that if an animal swim by alternate strokes of the 

 hind limbs alone there will be no serious disturbance of equilibrium 

 providing the legs be close together. If the animal be of a tubby shape, 

 However, with hind legs far apart, each kick will throw the body to the 

 side unless there be some separate provision for maintaining equilibra- 

 tion. This may be nicely illustrated by the common mud turtle. If 

 one bind down the fore limbs of one of these animals by means of ad- 

 hesive tape and then place it in a pool of water it is found that the 

 turtle is very greatly handicapped thereby and will progress by a series 

 of erratic jerks, first to one side and then the other (fig. 1) . The reason 

 for this is that in vertical dimension the body is very thin, and in trans- 

 verse very broad, so that the four legs appear almost as though attached 

 to the corners of a rectangle. If one foot only is kicked it will turn the 

 rectangle, and to prevent such turning there would either have to be a 

 heavy tail, which the animal lacks, or a compensating kick by the oppo- 

 site limb of the other pair. Thus the mud turtle in reality trots through 

 the water. As specialization advances and the feet become paddles in 



[19] 



