FISHES AND FISHERIES 245 



trout or the supply may be increased in the regions where too 

 constant fishing has depleted the numbers. 



With certain modifications of this general plan, necessitated 

 by the structure and habits of the fish, the black and striped 

 bass, the whitefish, shad, pike, codfish, mackerel and others are 

 also artificially propagated in great quantities. 



The federal government now operates thirty-six permanent 

 hatcheries, besides nearly a hundred auxilliary stations. In 

 these more than forty species of the best food and game fishes 

 are handled. These hatcheries are distributed over thirty- 

 three states, and some of these states themselves maintain 

 other hatcheries which handle even more fish than the govern- 

 ment hatcheries. New methods and improved appliances are 

 constantly being adopted, wonderfully increasing the usefulness 

 of these establishments. In this way the United States 

 Bureau of Fisheries and the various State Fish Commissions 

 are doing a valuable work in economic zoology and one that 

 can be appreciated by all citizens. The same Bureau and 

 Commissions are at work also on similar problems in con- 

 nection with the lobster, oyster, crab, shrimp, clam, mussel 

 and other invertebrate animals that are considered as a part 

 of our fishery resources. 



Classification. The class Pisces may be divided into four 

 sub-classes, namely: the Elasmobranchii, including the sharks, 

 skates, torpedoes, etc.; iheHolocephali, including the chimaeras, 

 a few strange-bodied forms; the Teleostomi, including nearly 

 all of the other fishes, as the sturgeons, catfish, bass, salmon, 

 trout, cod, mackerel, herring, etc.; and theDipneusti, or lung- 

 fishes, represented by only a few genera whose members have 

 lungs in addition to gills. 



The Sharks, Skates and Ray. (sub-class Elasmobranchii}. 

 These differ from the bony fishes in several important respects, 

 and some icthyologists raise the sub-class to the rank of a 

 class. They have a skeleton composed of cartilage, there is 

 no operculum, and no true scales. Their teeth are distinct, 

 often large and highly specialized. All the members of the 

 group are marine. 



The fierce, carnivorous and voracious sharks live in the 



