158 



BACKBONED ANIMALS. 



in, and the news is telegraphed to the brain, just as the 

 impression of an object seen is carried by the optic nerve 

 e. The ears, A, ea+ are little cavities placed on each side 



of the head, containing a 

 liquid, in which float two 

 otoliths, or ear-stones. 



Covering. Fishes are 

 covered with scales (Fig, 

 195), that overlap each 

 other like shingles on a 

 roof, and grow, totally un- 

 like those of reptiles, out 

 of little pockets in the skin, 



bein g ke Pt Soft and slim 7 



by a secretion under them, 

 but principally from that 

 which exudes from about the mucous scales (Fig. 194, 



FIG. I9 5-Scale of a flounder, highly 



magnified, a, natural size. 



Development. Most fishes deposit eggs or roe. Some 

 are ovoviviparous, while others, as the Zoarces, Ditrema, 

 etc., are viviparous, or produce their young alive. 



ECONOMIC VALUE. As nearly all fishes are valuable as food, refer- 

 ence to their economic value has not been given in every case. The 

 reports of the United States Fish Commission show the value of this 

 class of vertebrates to man. Artificial fish-hatching establishments have 

 been organized in nearly all the States. American fish-eggs are sent 

 to Europe, and others received in return, and the arrangements for the 

 prevention of the depletion of fisheries are becoming more complete 

 every year. 



Sub-Class I. STRAP-GILLED FISHES (Elasmobranchii). 



General Characteristics. In this class, we first meet 

 Vertebrates having paired fins arid a genuine lower jaw. 

 The skeleton is made up of cartilage ; the ribs are small 

 and often rudimentary. The skull is a simple cartilagi- 

 nous box, and the jaws, and in fact all the parts, are 



