96 



ANIMAL LOCOMOTION. 



semi-rigid condition, are incapable of expansion. They there- 

 fore present their maximum and minimum of surface by 

 a partial rotation or tilting of the pinion, as in the walrus, 

 sea-bear, and turtle. The feet, which are moved with less 

 vigour, are, on the contrary, rotated or tilted to a very slight 

 extent, the increase and diminution of surface being secured 

 by the opening and closing of the membranous expansion or 

 web between the toes. In this latter respect they bear a cer- 

 tain analogy to the feet of the seal, the toes of which, as has 

 been explained, spread out or divaricate during extension, 

 and the reverse. The feet of the penguin entirely differ 

 from those of the seal, in being worked separately, the 

 foot of one side being flexed or drawn towards the body, 



Fig, 48.— Swan, in the act of swimming, the right foot being fully expanded, 

 and about to give the effective stroke, which is delivered outwards, down- 

 wards, and backwards, as represented at r of fig. 50; the left foot being closed, 

 and about to make the return stroke, which is delivered in an inward, up- 

 ward, and forward direction, as shown at s of fig. 50. In rapid swimming 

 the swan flexes its legs simultaneously and somewhat slowly; it then 

 vigorously extends them. — Original. 



while its fellow is being extended or pushed away from it. 

 The feet, moreover, describe definite curves in opposite direc- 

 tions, the right foot proceeding from within outwards, and 

 from above downwards during extension, or when it is fully 

 expanded and giving the effective stroke ; the left one, which 

 is moving at the same time, proceeding from without in- 

 wards and from below upwards during flexion, or when it is 

 folded up, as happens during the back stroke. In the acts of 

 extension and flexion the legs are slightly rotated, and the 



