OCEANS AND LAKES 



259 



FIG. 277. Globigerina ooze. Magnified 

 about 20 diameters. (Murray and Renard.) 



genus of Foraminifera, microscopic animals of extremely 

 simple structure. Oozes of this class cover nearly 50,000,000 

 square miles of the 

 ocean bed. The 

 largest area is in the 

 Atlantic Ocean (Fig. 

 276). Radiolarian 

 ooze is also made up 

 of the remains of a 

 group of tiny, one- 

 celled animals, but is 

 composed of silica 

 instead of calcium 

 carbonate. It is con- 

 fined, so far as known, to the Pacific and Indian oceans (Fig. 

 276). Diatom ooze is a siliceous deposit composed of the 

 cases of minute plants known as diatoms. The largest de- 

 posit is in the Antarctic Ocean (Fig. 276). Various other 

 deposits of similar origin receive special names. 



Coral deposits. Reef-building corals are limited to ocean 

 waters whose temperature does not fall below 68 Fahrenheit. 

 They are restricted also to places where the water is clear and 

 not over 100 feet deep. This confines them to the shallow 

 areas of tropical seas, and prevents their growth off the 

 mouths of large rivers where the waters are muddy. They 

 thrive best where the water is agitated vigorously by waves 

 and currents. This insures a continual supply of food, oxygen, 

 and calcium carbonate, and removes the carbon dioxide. 

 Reef-building corals live in colonies, many of which look like 

 stubby plants. Each polyp consists of a fleshy, cylindrical 

 sac with an opening at the top which serves as a mouth, and is 

 surrounded with armlike feelers. Each polyp absorbs cal- 

 cium carbonate from the sea water, and builds it into the 

 stony framework which supports the colony. 



Coral reefs are of several classes. Those extending along 

 the shore and attached to it are fringing reefs. Those which 



