FISHES 127 



95. The locomotion of fishes. — Many fishes, like the 

 goldfish and perch, are able to maintain a given position in 

 the water while at rest. This is made possible by means 

 of an internal organ known as the swim bladder (Fig. 98). 

 The swim bladder may be compressed, permitting the fish 

 to sink, or it may be expanded, causing the animal to rise. 

 Since, therefore, the fish is poised in a liquid medium, it is only 

 necessary to overcome the resistance of the water about it in 

 order to move in any given direction. This resistance is more 

 easily overcome, first, because the head is somewhat pointed 

 like the prow of a boat, secondly, because the overlapping 

 scales point backward, and third, because the whole body is 

 covered with a slimy mucus. 



One who is at all familiar with a canoe knows that it is 

 impossible to propel it by the use of a slender rod. One 

 must have a paddle with the lower end broad and flat so that 

 sufficient force may be exerted against the water to propel 

 the canoe. Now, in swimming, the fins of a fish act more or 

 less like paddles. Their broad, flat surfaces press against 

 such an amount of water that the fish is enabled to exert 

 enough force to push its body in any desired direction. 



If one watches a goldfish swimming slowly about in an 

 aquariiun, one would think that the paired fins, especially 

 the pectoral fins, were the important swimming organs. 

 But careful experiments have shown that this is not the case. 

 When the goldfish has occasion to move more rapidly, the 

 paired fins are not used at all, but are pressed close to the 

 sides, the body being driven through the water by the 

 movement of the tail and tail fin. The paired fins, to- 

 gether with the dorsal and anal fins, seem to be used prin- 

 cipally in steering the fish. The energy necessary for 

 swimming is developed in the powerful muscles of the tail 

 and trunk. 



