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, 
Fic. 195.—Scale of a flounder, highly being kept soft and slimy 
magnified. «, natural size. by a secretion under them, 
but principally from that 
which exudes from about the mucous scales (Fig. 194, 
B, ms). 
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 Fisnes (Zv/asmobranchit). 
General Characteristics—In this class, we first meet 
Vertebrates having paired fins and 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 
