348 ECONOMIC IMPORTANCE OF ANIMALS 



plankton. It has been estimated by Professor Forbes that the 

 Illinois River, before it was polluted by the Chicago drainage canal, 

 produced annually over 150,000,000 pounds of fish food. Almost 

 all fish that do not take the hook and that travel in schools or com- 

 panies, migrating from one place to another, live largely on plank- 

 ton or on smaller fish which feed on plankton. Some fishes as the 

 menhaden ^ (bony, bunker, mossbunker of our eastern coast), 

 the shad, and others, are provided with gill rakers by means of 

 which they strain these minute organisms from the water. Other 

 fishes are bottom feeders, as the blackfish and the sea bass, living 

 almost entirely upon mollusks and crustaceans. Still others are 

 hunters, feeding upon smaller species of fish, or even upon their 

 weaker brothers. Such are the bluefish, squeteague or weakfish, 

 and others. The whale, the largest of all mammals, strains proto- 

 zoa and other small animals and plants out of the water by means 

 of hanging plates of whalebone or baleen, the slender 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 nitrogen balance 

 is maintained through the wastes of the animals and their death 

 and decay, furnishing needed materials for the plants. Thus we 

 see the marine world is a great balanced aquarium. 



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 certain echinoderms (page 

 348), sea cucumbers, 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. — Oysters are never found in muddy locali- 

 ties, for in such places they would be quickly smothered by the 

 sediment in the water. They are found clinging to stones or on 

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

 in shallow bodies of salt water. 



' Professor Mead of Brown University has discovered that the increase in star- 

 fish along certain parts of the New England coast was in part due to over-fishing of 

 menhaden, which at certain times in the year feed almost entirely on the young 

 starfish. 



