Economic Importance of Animals 573 



Class Cyclostomata (Cyclostomes) 



The lampreys ("round mouths") feed on blood, mucus, and internal 

 organs of fishes and crustaceans which they attack with their rasping 

 mouth (Fig. 143). The flesh of certain lampreys is used as food. 



Class Elasmobranchii (Sharks) 



Several species of dogfish sharks destroy lobsters and fishes. Oil and 

 fertilizers are manufactured from sharks. The skin of the dogfish shark 

 is used as leather and shagreen. The teeth of sharks are used as 

 weapons by certain people (Fig. 144). 



Class Pisces (True Fishes) 



Fishes (Figs. 145 to 147) furnish an important article of food. Cod- 

 liver oil and halibut-liver oil are valued because of their high vitamin 

 contents. Caviar is prepared from the salted roe of the sturgeons. Fish 

 scales are used for ornamental purposes. The swim bladder of codfishes 

 is used in the making of isinglass. Fishes are frequently used as ferti- 

 lizer, the early settlers of this country many times placing a fish with the 

 seeds they planted. 



Class Amphibia (Frogs and Toads) 



Frogs are used extensively as food, the breeding of large specimens 

 for human consumption having become quite a business in itself. Frogs 

 and toads destroy large numbers of harmful insects. Frogs are used 

 extensively for laboratory studies in dissection and physiology. 



Class Reptilia (Reptiles) 



Reptiles are frequently of considerable benefit because they kill large 

 numbers of obnoxious insects and other pests. Turtles and tortoises are 

 used as food. Certain lizards (Iguana of tropical America) are used as 

 food. The skins of crocodiles and certain snakes are used for manufac- 

 turing bags, boots, and cases. Tortoise shell, especially that from the 

 horny covering of the carapace of the Hawk's bill turtle, is used in 

 manufacturing combs and similar articles. There are only a few species 

 of poisonous snakes in the United States, while the venomous types of 

 the tropics cause a larger number of human deaths than any other 

 group of tropical animals. The oils of the boa, rattlesnake, and copper- 

 head are used for medicinal purposes. Musk, leather, and oils are 

 secured from alligators (Figs. 151 to 153). 



