AQUATIC VERTEBRATES 



371 



under the pectoral fins. Internally, this is accompanied by a complete 

 disappearance of the pelvic girdle. 



The skeleton is composed of bone, a fact which hardly needs men- 

 tioning- since everyone who has eaten fish knows this fact from experi- 

 ence. Another noteworthy internal structure is the swim bladder, an 

 air-filled bladder in the dorsal region of the body cavity which serves to 

 make the fish more buoyant. By regulating the amount of air in this sac 

 the fish is able to maintain itself at a desired level in the water. In some 

 fish this bladder has a connection with the pharynx, and air can be gulped 

 down into it. In these forms it serves as an accessory organ of respira- 

 tion, for oxygen can be absorbed through its walls when the oxygen 



DORSAL FINS 



NOSTRIL 



—TAIL 

 OR CAUDAL 

 FIN 



GILL COVER 

 (OPERCULUM) 



PECTORAL FIN 



Biology, the Science of Life, McDougall and Hegner, McGraw-Hill Book Company, Inc. 

 Fig. 25.4. External features of a fish. 



content of the water is reduced. The alligator gar is such a fish, and ob- 

 servation shows that it can live in foul water long after other fish have 

 died due to lack of sufficient oxygen. The swim bladder, thus, has a sig- 

 nificance as a possible forerunner of the lungs of the land vertebrates. 



Protection among Fish. Life in the water is rugged and dangerous ; 

 murder and starvation are found on every hand; it seems as if every 

 animal is trying to eat every other animal that is small enough to go in 

 its mouth. There are numerous methods of protection used by fish in 

 an effort to survive under such conditions. One of the most universal is 

 protective coloration. Fish are so colored that they are hard to see in 

 their natural environment. The dorsal surface is colored darker than 

 the ventral surface. An enemy swimming over them would see them 

 against the dark bottom of the body of water, but while swimming under 

 them would see the light under surface of the body outlined against the 

 light upper surface and sky and could overlook them in either case. 



