FISHES 1339 



like mass. The familiar blennies, found in rock- 

 pools after the tide has receded, are also able to emit 

 a large amount of this glutinous fluid. 



Illustrating the opposite extreme of the develop- 

 ment of scales, we find such fishes as the bony 

 pikes of North American lakes and rivers, the bodies 

 of which are covered with an armor of closely fitting 

 and overlapping scales or plates, named ganoid, 

 from their shining appearance (Greek ganos, splen- 

 dor) . The scales of this fish are said to be employed 

 in the manufacture of the little "mother-of-pearl" 

 buttons, so commonly used. Many fossil fishes were 

 also abundantly provided with these hard, bony 

 plates; and our living sturgeons possess scales of 

 similar nature, although in the latter fishes they do 

 not completely cover the body. The bright silvery 

 scales of the herring and its neighbors are thin struc- 

 tures, and are very easily detached from the skin; 

 and a curious form of scale is seen in the perches; 

 the hinder edge of each scale in the latter case being 

 cut into comb-like teeth. In the sharks, skates, and 

 rays, the scales are small and horny, and are often 

 provided with little spines. If we draw our hand 

 along the back of a dog-fish from tail to head, as 

 when we stroke a cat's back the wrong way, we feel 

 numerous small projecting points, borne on the 

 scales. The rough skin surface thus produced is fre- 

 quently used under the name of "shagreen" in the 

 manufacture of spectacle-cases and like articles, and 

 is also employed for polishing the surface of wood. 



In their general shape the bodies of fishes exhibit 

 a great compression from side to side, a rounding 



