A HISTORY Of VERTEBRATES: FISHES 



437 



mouth is often buried in the sand or mud, and water for respiration 

 enters the pharynx via the pair of enlarged spiracles. A spiracular 

 valve in each one is then closed, and the water is forced out the typical 

 gill slits. Most skates and rays have crushing-type teeth and feed upon 

 shellfish, but others are adapted for other methods of feeding. The 

 Sawfish (Pristis) has an elongated, blade-shaped snout armed with 

 toothlike scales. By thrashing about in a shoal of small fishes, it can 

 disable many and eat them at leisure. As in the sharks, the largest mem- 

 bers of the group (the devilfish, Manta) have reduced teeth, and are 

 plankton feeders. Some devilfish have a "wing spread" of 20 feet and 

 can easily upset small boats. Harpooning these is an exciting sport! 



200. Lungs and Swim Bladders 



While early sharks were becoming dominant in the ocean, another 

 offshoot of the placoderms, the bony fishes of the class Osteichthyes, 

 became dominant in fresh water. They subsequently entered the ocean 

 and became the most successful group there as well. Most of the familiar 

 present-day fishes (gar, herring, minnows, perch, cod, lungfish) belong 

 in this group. 



The Osteichthyes resemble the (^hondrichthyes in being evolution- 

 ally advanced fishes with efficient paired appendages and jaws (Fig. 22.9). 



Ye.llov\r perch 





iP^lteSSS^S' 



Austrsd.ia.-a lundf'is'h 



Figure 22.9. Representative bony fishes. A, The yellow perch, Perca flavescens, is 

 a member of the ray-finned group of bony fishes; B, the Australian lungfish, Epicemto- 

 dus, belongs to the fleshy-finned group. {A, After Hubbs and Lagler; B, after Norman.) 



