TUNNIES AND THEIR RELATIONS 



make this even clearer. The circulation of the blood, 

 a very necessary factor in the case of such active 

 muscles, is extremely rich; the flesh is deep scarlet in 

 colour in consequence of the abundant blood it con- 

 tains; the muscular zones most closely in contact 

 with the vertebrae are a mass of closely packed tiny 

 blood-vessels and they have a still darker, almost 

 black, colour. The energy put forth during swimming 

 is indicated by an appreciable rise in temperature, 

 particularly in the dorsal muscles, which are most 

 constantly employed. The whole construction of these 

 large scombers is designed to make of them, like fast 

 ships, the greyhounds of the sea. 



Many other fishes have a similar conformation, and 

 use their caudal fins as sculls. But, in most of them, 

 the action is weaker and the power developed less. 

 The fin itself is often, proportionately, larger than 

 that of the mackerel and the tunnies, but it is not so 

 rigid or so strong. It bends and waves in the water, 

 and the zone in which it is actively effective is not so 

 great. The trunk muscles have their four groups of 

 motors with segments and rings; but they are not so 

 thick; the blood does not circulate so thoroughly 

 through them and feed them; the work they do 

 produces inferior results. The body may be more 

 slender; it may appear better adapted for rapid 

 swimming, but actually it is slower, for lack of power 

 in the motor and because of the limited capacity of 

 the propeller. 



Usually, these fishes use their propelling scull only 

 when they have to make a hasty burst of speed. They 

 generally prefer to use their two pectoral fins, which 

 they employ as oars with great rapidity. The tail 

 becomes a sort of rudder though, when necessary to 

 give a greater speed, it is capable of affording the 

 required impulse. Normally, however, it remains at 

 rest. Such is not the case with the stronger and 

 swifter species which are built to swim at a great 

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