CLASS FISHES. 2QI 



the salmon, the herring, the bass, the mackerel, the eel, 

 etc., include about 10,000 species, or two-thirds of the 

 whole group. Few others come under the observation of 

 the student. They have a bony skeleton in the adult, 

 have the mouth at or near the end of the head, and have 

 symmetrical tail fins. 



Sturgeons and gar-pikes (ganoids) illustrate a group of 

 fishes not very large at the present time, but of great 



FIG. 114. Brook trout (Salvelimus fontinalis). U. S. Fish Commission. 



geological importance. They were the dominant fishes 

 of the so-called geological "age of fishes." They are 

 covered with bony plates which give them complete pro- 

 tection. The internal skeleton is usually cartilaginous, 

 though it may be bony. These forms have the air-bladder 

 connected with the gullet in such a way that they can 

 actually take in and give out the air by bringing the mouth 

 to the surface of the water. The air-bladder is well 

 supplied with blood vessels, and thus almost deserves 

 the name of lung. They include the sturgeons, the gars, 



