312 COMPARATIVE ZOOLOGY. 



' polished, overlapping scales, as in the Perch. Earely, 

 as in the Sturgeon, it is defended by bony plates, or by 

 minute, hard spines, as in the Shark. Scales with smooth, 

 circular outlitie are called cycloid; those with notched or 

 spiny margins are ctenoid. Enameled scales are ganoid, 

 and those with a sharp spine, like those of the Shark, are 

 placoid. 



The vertical fins (dorsal, anal, and caudal) are peculiar 

 to Fishes. The dorsal vary in number, from one, as in 

 the Herring, to three, as in the Cod ; and the first dorsal 

 may be soft, as in the Trout, or spiny, as in the Perch. 



Pio. 284.— Blue-fish {Teimnodon mUatm). All seas. 



If the dorsals are cut off, the Fish reels to and fro. The 

 caudal may be homocercal, as in ordinary species ; or het- 

 erocercal, as in Sharks. In ancient heterocercal Fishes, 

 the tail was frequently vertebrated. The pectoral and 

 ventral fins stand for the fore and hind limbs of other 

 Vertebrates. As the specific gravity of the body is greater 

 than that of the water, most Fishes are provided with 

 an air-bladder, which is an outgrowth from the oesopha- 

 gus. This is absent in such as grovel at the bottom, as 

 the Eays, and in those, like the Sharks, endowed with 

 compensating muscular power. 



Fishes have no prehensile organ besides the month. 

 Both jaws are movable. The teeth are numerous, and 



