470 



THE ANIMALS OF THE EARTH 



In most vertebrates, except fish, the waste product, carbon 

 dioxide, is discharged through the lungs ; in fish it is discharged 

 through the gills. As water flows over the gills it absorbs car- 

 bon dioxide from the blood and carries it away with it through 

 the gill openings on the side of the head. 



The absorption of oxygen through the gills into the blood, 

 and the discharge of carbon dioxide through the gills into the 

 water keeps the blood of the fish pure, and the gills serve the fish 

 as effectively as lungs serve the other vertebrates. 



Oysters. Next to farm animals and their products, and fish, 

 oysters are the most important animal food. Oysters live in the 



shallow water near 

 ocean coasts, at- 

 tached to stones, 

 rocks, and shells. 

 They are found in 

 the United States 

 along both the Atlan- 

 tic and the Pacific 

 coasts. In Long Is- 

 FIG. 340. The inside of an oyster. (From Brooks's land Sound, Chesa- 



peake Bay, and the 



Gulf of Mexico immense quantities of them are obtained by 

 dredging. The edible part of the oyster lies between two rough, 

 thick shells of limestone which are hinged together at one end 

 (Fig. 340), When the oyster is undisturbed, the upper shell or 

 lid is raised and water enters ; when the oyster is disturbed 

 or threatened by enemies, the lid is drawn down by a strong 

 muscle and fits tightly against the lower shell. The tough por- 

 tion which we notice when eating an oyster is the muscle which 

 opens and closes the shell. 



Reproduction in oysters is similar to reproduction in fish: 

 females discharge eggs and males discharge milt. The 



