MOLLUSKS AS FOOD 531 



water by means of hanging plates of whalebone or baleen, the slen- 

 der filaments of which form a sieve from the top to the bottom of 

 the mouth. 



In a balanced aquarium the plants furnish food for the tiny 

 animals and some of the larger ones, for example, the snails. The 

 smaller animals are eaten by the larger ones. The waste matter 

 given off by the animals and their death and decay furnish the 

 plants with the required nitrogen and other material. Thus we see 

 the aquatic world is a great balanced aquarium. Man disturbs 

 this ecological balance when, as in the Illinois River, he dumps his 

 untreated sewage and factory wastes into the stream near its source. 

 The immediate result has been the destruction of fish life for a dis- 

 tance of about 100 miles. It has been estimated by Professor Forbes 

 that the Illinois River, before it was polluted by the Chicago drain- 

 age canal, produced annually over 150,000,000 pounds of fish food. 

 On the other hand, diluted sewage in a river may be utilized by the 

 bacteria which in turn are used by microscopic animals and these in 

 turn by crustaceans and snails which form the food of fishes. 



Practical Exercise 10. Explain how living things in any body of water in 

 your locality indirectly produce food for man. 



Direct use of animals as food ; lower forms. The forms of life 

 lower than the mollusks are of little use directly as food, although 

 the Chinese are very fond of sea cucumbers (page 227), which are 

 preserved by drying and are called trepang. Sea urchins are eaten 

 in the West Indies, under the name of " sea eggs." 



Mollusks as food. The oyster. The oyster industry is very 

 profitable. Hundreds of boats and thousands of men are engaged 

 in dredging for oysters. Three of the most important of our oyster 

 grounds are Long Island Sound, Narragansett Bay, and Chesa- 

 peake Bay. The western coast also produces oysters, but they are 

 inferior to those of the eastern coast. 



Oysters are never found in muddy water, for they would be 

 quickly smothered by the sediment. They cling to stones or 

 shells or other objects which project a little above the bottom. 

 Here food is abundant and oxygen is obtained from the air in the 

 water surrounding them. Oyster raisers usually throw oyster 



