TUNNIES AND THEIR RELATIONS 



horizontal, whereas that of fishes is vertical. The 

 hind limbs are atrophied, and the bulky masses of 

 muscle, the true tail, at the end of the trunk actuate 

 this tail-fin and supply a propelling power seldom 

 possessed by the terminal appendages of other animals. 



When we examine one of these creatures out of the 

 water we are surprised at its shape. It looks like a 

 fish; its body is spindle-shaped with tapering surfaces, 

 but it is thickly built, broad, and provided with a 

 thick cushion of fat under the skin. We hardly 

 expect to find a creature like this, built almost clumsily, 

 instead of the slender, swift-looking creature which its 

 great speed gave us every reason to look for. It is 

 very like the greater cetaceans, the whales. But when 

 we examine its construction more carefully we cease 

 to be astonished. The plump body, buoyed up by its 

 fat, has the rounded contours which let the water slip 

 past; and the two paddles in front serve to maintain 

 the balance. The chief propeller, the broad, powerful 

 tail with its fin, beats the water with an up and down 

 movement, and thrusts the animal forward. Each 

 beat forces the body upwards and forwards in such a 

 way as to produce a series of successive bounds, 

 sometimes very close together, sometimes at a greater 

 distance. 



Most fishes cannot swim so swiftly. They have a 

 caudal fin, often, in comparison with their size, as 

 large as that of the Cetacea, but it does not often 

 have so much muscular power in front of it. More- 

 over, the blood, which conveys to the muscles the 

 oxygen they need to produce their energy, is not so 

 rich. The fishes belong to that category of animals 

 called ' cold-blooded ', or of variable temperature, in 

 which the respiratory oxidations which stimulate vital 

 activity remain on a lower level. The tail with its 

 caudal fin, set vertically, is able to effect a rapid burst 

 of speed which allows the fish to make a dash after 

 its prey, but it cannot keep up this speed for long. 



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