TUNNIES AND THEIR RELATIONS 



to the ship's engines, the propeller of the fish benefits 

 by the accumulated force of all the muscles which 

 make up the flesh of the trunk. Now we understand 

 the reasons for the swiftness of the tunny, a swiftness 

 greater than that of most other fishes. Its body seems 



Fig. io. — Transverse section of a Tunny's trunk, showing the muscular 

 segments like fitted discs which form the flesh. In the centre is 

 the vertebra (in white), and, below, the abdominal cavity (in black). 

 The shaded zones are those which, since they have a considerable 

 wealth of blood-vessels, are distinguishable from the rest by their 

 dark red, almost black colour. 



corpulent, but this is due to the greater size of the 

 muscles which produce the motive power; the calibre 

 of these muscles being greater, the power they develop 

 is greater also. In the tunny we may see more clearly 

 than in any other fish the action of this tail-swimming, 

 which begins with a tremendous muscular effort and 

 ends in the oscillation of a propelling scull as stiff as 

 a flail. Certain complementary features in these fishes 



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