166 ] Fish and Their Fabulous Neighbors 



What Fins Are For 



As you watch your goldfish swimming you may get the idea 

 that its fins are an important element in its forward movement. 

 And you are right, though nowadays scientists do not attach as 

 much value to the locomotion value of fins as they once did. 

 Experiments have shown that a fish can navigate even without 

 its fins. 



You will note seven fins on the common goldfish. Just behind 

 its gill covers it has a pair of fins called the pectorals. Farther back 

 is another pair called the ventrals or, if quite far forward, called 

 the pelvics. On its back the fish has a dorsal fin, which it some- 

 times lifts and shuts down like a fan. On the underside, toward 

 the tail, is the anal fin. Finally, at the end of the tail is the caudal 

 fin; we often call it the tail fin. 



How FINS HELP FISH To SWIM 



How the different kinds of fins aid a fish's movement de- 

 pends on their shape and location. The caudal or tail fin helps 

 the fish propel itself as it presses its tail against the water first to 

 one side then to the other. The shape of this fin seems to be re- 

 lated to the swimming speed of different species. On swift swim- 

 mers, such as the trout, the caudal fin is strongly forked or 

 moon-shaped; on slow swimmers this tail fin is blunt or rounded. 



The dorsal fin acts as a keel: It prevents rolling. The anal fin 

 serves the same purpose and in some species it is also used to give 

 the fish a powerful upward sweep. 



Pectoral fins appear to serve chiefly as brakes for fish with bony 

 skeletons this is particularly true of perchand these fins also 

 have a slight balancing effect. In fish with other than bony 

 skeletons and this makes an interesting contrast the pectorals 

 have a powerful balancing action but are of little use as brakes. 

 Sharks, for example, are apparently unable to make a sudden 

 stop. As for the ventral fins, they contribute further to keeping 

 the fish evenly balanced. 



The fins of the common goldfish are by no means standard 

 equipment. Many fishes do not have ventrals. Some, like the cod, 

 have three dorsal fins; others have two; others, one. Some have 

 two anal fins; others, one. 



