Framework 6i 



It is obvious that if this gill-current is an obstacle when 

 the fish wants to stand still, it can be an advantage when he 

 wants to move. The flounders and the other flatfishes spend 

 much time lying on the bottom on one side, which brings 

 one gill underneath. One might think that this would be a 

 drawback in breathing, but they turn it to advantage. If 

 they breathed through the upper gill, its movement might 

 betray their presence, thus nullifying all the hard work of 

 the chromatophores to conceal them with protective colora- 

 tion. They get around this by breathing through the hidden 

 lower gill only. They arch the body slightly along the mid- 

 line, and the outgoing current of water passes along this 

 tunnel between the body and the ground, and so out past 

 the tail. Further, and this is the part which has to do with 

 locomotion, when they want to move in a hurry, they shoot 

 a quick jet of water out through the under gill with such 

 force that it lifts them right off the bottom and starts them 

 on their way. This jet propulsion is probably used by many 

 fishes in making a quick start. There is also a theory that in 

 their normal swimming fishes make use of this current from 

 the gills, not so much for its propulsive force as for the effect 

 which a stream of water along the sides has in reducing 

 friction and cavitation. Interesting arguments are advanced 

 to support it, but insufficient experimental evidence. It still 

 remains a theory. 



All that we have said so far about locomotion is true of 

 the great majority of fishes, but there are some exceptions 

 which make little or no use of the body movement for loco- 

 motion. The brilliant parrot-fish of tropical waters, with its 

 large, thick scales, rows itself along with its pectoral fins. 

 The trunk-fish, with its body encased in immovable armor, 

 has to depend on the sculling movement of its free caudal 

 fin, with some help from its pectorals. And the little sea- 

 horse progresses through the water standing on its tail, pro- 



