44 



ANIMAL LIFE 



concavity. The tail is provided with a fin, shaped 

 like the propeller of a steamer, by which its grip of 

 the water is increased. In forward movement the 

 body and tail of the fish are thrown into two or more 

 curves, and as the body straightens, and before it 

 takes an opposite curvature, the pressure of the tail 

 on the water at the hollows, and especially the grip 

 of the tail-fin, is greater than when the reverse curvature 

 is obtained. The straightening stroke is more power- 

 ful than the bending one. Moreover, the tail does 

 not merely undulate, but twists about its own axis, 

 meeting the water first with one of its flat surfaces 

 as it straightens, then feathering as it prepares for the 

 next stroke, and again straightening out with full 

 force of the opposite surface and the expanded fin. 

 The swirl helps the fish to gain additional purchase, 

 the eddy helps it to elude the water and eases the 

 feathering stroke. 



In order to render these undulations more effective 

 the swimming-muscles that run down the body start 

 from an elastic spinal rod, which gives them a central 

 origin, and form not a continuous sheath, but a 

 segmented mass of bundles, each bundle being bent 

 into a V or W shape, with the points turned towards 

 the head. The skin into which these muscle-bundles 

 are inserted is elastic and lubricated ; and if scaly, 

 the points of the scales project backwards, so as 

 not to impede the forward movement. Moreover, the 

 amount of muscular tissue is increased by reducing 

 the space in which the internal organs are packed to 



