TUNNIES AND THEIR RELATIONS 



handle this terminal fin, we understand the greatness 

 of the effort it is capable of making. 



Everything in these animals seems to have been 

 designed systematically for its one purpose. The 

 caudal fin, a perfected scull, is supplemented by a 

 system of balancing fins set in front of it along the 

 medial line of the back and of the belly. The first 

 dorsal is rigid; the second, more supple, is succeeded 

 by a series of miniature fins called finlets. The anal 

 is stiff like the first dorsal, and is also succeeded by 

 ventral pinnules like those of the back. Altogether 

 we have a system of stabilising keels, made up of 

 several parts, some on the top running along the 

 trunk, the others below, all regulating and maintaining 

 the attitude during swimming. 



When we look at this caudal fin we still ask our- 

 selves how, despite its size and solidity, it is able to 

 propel a body of such thickness at such a speed. 

 Where is its motor, powerful enough to produce such 

 a result? Again the comparison with a ship forces 

 itself upon us. In a ship the screw is small in relation 

 to the hull, more so even than the tunny's screw in 

 relation to its body; yet despite its minute dimensions, 

 it is able to move the ship by the turning of its blades 

 in the water. The essential factor is the power behind 

 this action, that is, the efficiency of the engine which 

 produces it. We find that the engine takes up a 

 great deal of room in the ship, and that its elaborate 

 machinery of polished metal occupies a considerable 

 space in the ship's body. The enormous power which 

 it develops is communicated to a horizontal shaft, 

 makes this revolve at a great speed; and this shaft, 

 connected in its turn to the screw whose shaft it is, 

 makes it turn also at an equal rate of speed. The 

 little screw gathers up in itself all the power of the 

 enormous machine which acts as a motor, and so 

 pushes the ship along. 



It is the same with the tunnies. Their propeller 

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